tag:blogger.com,1999:blog-12905674007781131292024-03-22T01:00:28.933+00:00Charles Babbage's Lectures On AstronomyUnknownnoreply@blogger.comBlogger15125tag:blogger.com,1999:blog-1290567400778113129.post-56249060815681911482013-11-11T10:30:00.000+00:002013-11-18T10:35:59.534+00:00Table of Contents<div class="MsoTitle" style="text-align: center;">
<span style="font-size: x-large;"><span lang="EN-GB">Babbage's
Lectures On Astronomy</span></span></div>
<div class="MsoTitle" style="text-align: center;">
<span lang="EN-GB" style="font-size: 12.0pt; mso-ansi-language: EN-GB; mso-bidi-font-size: 10.0pt;">Given at The Royal Institution, 1815</span><br />
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjH4h0paTHc2hgD_zMYh9-bQfC073_0CKT1J6O15oQzJSnVVM0ybun-NCMbFDCeuJFoAOrYbBac67xtQmwL_F8_qe-C6F4HxqnLmFrA0gtrRHoXvxIpa6sRzEmN9R0UwIYX9gYKLCrFhzg/s1600/Babbage+Best.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="320" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjH4h0paTHc2hgD_zMYh9-bQfC073_0CKT1J6O15oQzJSnVVM0ybun-NCMbFDCeuJFoAOrYbBac67xtQmwL_F8_qe-C6F4HxqnLmFrA0gtrRHoXvxIpa6sRzEmN9R0UwIYX9gYKLCrFhzg/s320/Babbage+Best.jpg" width="242" /></a></div>
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<span style="font-size: large;"><a href="http://babbageastronomy.blogspot.co.uk/2013/11/table-of-contents.html" target="_blank">Table of Contents</a></span></div>
<ul class="posts">
<li><a href="http://babbageastronomy.blogspot.co.uk/2013/11/preface.html" target="_blank">Preface</a></li>
<li><a href="http://babbageastronomy.blogspot.co.uk/2013/11/synopsis.html">Synopsis</a></li>
<li><a href="http://babbageastronomy.blogspot.co.uk/2013/11/lecture-1-introduction-to-and-history.html" target="_blank">Lecture 1: Introduction to and History of Astronomy, from Thales to Copernicus</a></li>
<li><a href="http://babbageastronomy.blogspot.co.uk/2013/11/lecture-2-on-astronomical-instruments.html" target="_blank">Lecture 2: On Astronomical Instruments</a></li>
<li><a href="http://babbageastronomy.blogspot.co.uk/2013/11/lecture-3-observing-and-cataloguing.html" target="_blank">Lecture 3: Observing and Cataloguing the Heavens</a></li>
<li><a href="http://babbageastronomy.blogspot.co.uk/2013/11/lecture-4-ascertaining-figure-of-earth.html" target="_blank">Lecture 4: Ascertaining the Figure of the Earth</a></li>
<li><a href="http://babbageastronomy.blogspot.co.uk/2013/11/lecture-5-on-moon.html" target="_blank">Lecture 5: On the Moon</a></li>
<li><a href="http://babbageastronomy.blogspot.co.uk/2013/11/lecture-6-on-sun.html" target="_blank">Lecture 6: On the Sun</a></li>
<li><a href="http://babbageastronomy.blogspot.co.uk/2013/11/lecture-7-inner-planets.html" target="_blank">Lecture 7: The Inner Planets</a></li>
<li><a href="http://babbageastronomy.blogspot.co.uk/2013/11/lecture-8-on-minor-planets-asteroids.html" target="_blank">Lecture 8: On the Minor Planets (Asteroids) and also the History and Development of the Reflecting Telescope </a></li>
<li><a href="http://babbageastronomy.blogspot.co.uk/2013/11/lecture-9-outer-planets.html" target="_blank">Lecture 9: The Outer Planets</a></li>
<li><a href="http://babbageastronomy.blogspot.co.uk/2013/11/lecture-10-on-theory-of-universal.html" target="_blank">Lecture 10: On the Theory of Universal Gravitation</a></li>
<li><a href="http://babbageastronomy.blogspot.co.uk/2013/11/lecture-11-on-comets.html" target="_blank">Lecture 11: On Comets</a></li>
<li><a href="http://babbageastronomy.blogspot.co.uk/2013/11/lecture-12-beyond-solar-system.html" target="_blank">Lecture 12: Beyond the Solar System</a></li>
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Unknownnoreply@blogger.comtag:blogger.com,1999:blog-1290567400778113129.post-81645191549691810372013-11-11T10:21:00.003+00:002013-11-12T07:55:50.902+00:00Lecture 12: Beyond the Solar System<h1>
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Lecture
12</span></h1>
<h1>
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Beyond the Solar System*</span></h1>
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<span lang="EN-GB" style="mso-ansi-language: EN-GB;">[script
of lecture missing]</span></div>
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<span lang="EN-GB" style="mso-ansi-language: EN-GB;">* Note:
conjectured title of lecture.</span><br />
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<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="http://books.google.co.uk/books?id=3PINAAAAQAAJ&pg=PA9&img=1&zoom=3&hl=en&sig=ACfU3U2miFnjLyGnsKWEPQdo7XUEOu9gBA&ci=12%2C12%2C969%2C1556&edge=0" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="320" src="http://books.google.co.uk/books?id=3PINAAAAQAAJ&pg=PA9&img=1&zoom=3&hl=en&sig=ACfU3U2miFnjLyGnsKWEPQdo7XUEOu9gBA&ci=12%2C12%2C969%2C1556&edge=0" width="200" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">Page from the Nautical Almanac and Astronomical Ephemeris for the Year 1815.</td></tr>
</tbody></table>
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Unknownnoreply@blogger.comtag:blogger.com,1999:blog-1290567400778113129.post-52977342176065845362013-11-11T10:20:00.003+00:002013-11-11T23:40:42.132+00:00Lecture 11: On Comets<h1>
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Lecture
11</span></h1>
<div>
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><br /></span></div>
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="http://wordcraft.net/cometstuff/comet1680merian.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="242" src="http://wordcraft.net/cometstuff/comet1680merian.jpg" width="320" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-size: medium;"> German engraving of the great comet of 1680</span></td></tr>
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<div>
<br />
<span style="font-size: x-large;">On Comets</span></div>
<h1>
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"> </span></h1>
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<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Besides the planets which perform
their revolutions in orbits nearly circular and which almost always remain
within our view<span style="mso-spacerun: yes;"> </span>or at least within the
reach of our telescopes there is another species of bodies which only present
themselves to our view at short and distant intervals, which shine with
splendour for a time and then retire into the depths of the heavens. These have
been usually called comets and are distinguished from either stars by a long
train of light which usually accompanies them and which is always situated in a
direction opposite to the Sun and which diminishes in lustre as it recedes from
the body of the comet.</span></div>
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<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Of all parts of philosophy that
which was the latest to receive solid improvement was undoubtedly the theory of
comets. The stars were considered as meteors little different from the
exhalations and luminous appearances which we sometimes behold in the
atmosphere. Some philosophers as Apollonius, Seneca and many of the
Pythagoreans had more correct notions on this subject but these seeds of truth
were extinguished by a weight of prejudice and by the authority of the
Aristotelian school.</span></div>
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<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">From this
opinion it unfortunately happened that the ancients were very careless in
making and transmitting to us observations on these phenomena and we have now
only to regret they were so little enlightened on this subject since from the
want of materials with which they might have supplied us the decision of some
of the most interesting questions of physical astronomy will probably be
postponed some centuries.</span></div>
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<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Until the time of Tycho Brahe we
find concerning comets but few reasonable conjectures. This celebrated observer
began to open the eyes of astronomers to the real nature of these bodies<span style="mso-spacerun: yes;"> </span>by an important discovery. He demonstrated
from the smallness of the parallax of these bodies that they are much more
distant from our Earth than the Moon is. He even endeavoured to represent their
course by supposing them to move in an orbit round the Sun. This however we
should observe was an hypothesis purely astronomical and he by no means supposed
that they were planets of a peculiar nature revolving round the Sun. This
discovery of Tycho [Brahe] was confirmed by the observations of various
astronomers of his time. And at the commencement of the 17th century it
received new illustrations from Galileo and Kepler.</span></div>
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<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">It was
natural as soon as Men were undeceived respecting the situations of these
bodies for Men to endeavour to submit their motions to calculations. Tycho
[Brahe] had set the example and Kepler soon followed it; this celebrated astronomer
imagined he could represent their motions by supposing that they moved in a
straight line. He was obliged however to acknowledge that they did not move
uniformly in this line. This circumstance ought naturally to have led him to
consider their path as curved. But being unwilling to give up the straight line
he was obliged to admit an acceleration and retardment in different parts of
it. It is singular that Kepler who was in other matters so clear sighted, who
possessed a genius peculiarly calculated to penetrate into those causes which
contribute to the order, the harmony and the magnificence of the universe,
should have been little better acquainted with the nature of these stars than
the herd of mankind. He confined himself to supposing that they were new
productions of Nature similar to the meteors which sometimes appear in our
atmosphere.</span></div>
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<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">The
supposition that comets move in a straight line was for a long time the
favourite hypothesis among astronomers. The positions of the comet which appeared
in 1665 were calculated by this method and it had caused some surprise that the
results were not far distant from the truth. We shall however presently see the
reasons of this coincidence. It was however from Cassini that this hypothesis
derived its greatest celebrity. He applied it to the comet which appeared in
1652 and also to several others and his results were sufficiently near the
truth to convince many that he had arrived at the true explanation. It must
however be observed that his hypothesis would not satisfy distant observations
on the same comet and that to a great number of them it was utterly
inapplicable. It will naturally be enquired how it could happen that from a
false hypothesis so many observations should be satisfied as to produce for it
a considerable reputation for several years. The answer to this question is not
difficult. Comets according to more modern observations are found to move in
flattened ellipses. In some cases these ovals are so much elongated that they
approach very nearly in some parts to the nature of a parabola. A parabola is a
curve composed of two branches which at a short distance from the summit
approach very much to straight lines. From this circumstance a comet if seen in
one part of its orbit will seem to be moving in a straight line. A comet when
it is approaching the Sun gradually disappears in its rays and after being hid
during some time is seen moving from the Sun. This can not be reconciled with
the hypothesis of Cassini and in fact he made a singular mistake respecting the
comet of 1680-81. All those who supposed the comet to move in a straight line
imagined that there were two different comets which moved in straight lines
passing very near the body of the Sun, whereas in fact it was one and the same
comet which only disappeared from being lost in the Sun's rays and again became
visible in its recess from that body.</span></div>
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<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">It is
remarkable that the ellipse which this comet really described is very much
elongated so that its two sides approach nearly to straight lines. And it was
principally from the accuracy of his predictions relating to this comet that
Cassini astonished the world and extended the credit of this theory. But the
triumph of this hypothesis was only caused by a fortunate coincidence of circumstances.
It was therefore transient and soon gave place to another incomparably more
accurate. In fact notwithstanding the theory of Cassini it was soon discovered
that the paths of comets are not straight lines, but that they are curved and
that the concave part is directed towards the Sun.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>Helvetius recognised this fact and
Dr. Hooke demonstrated it. He says that we must positively reject the testimony
of observation. It is in contradiction to this. Helvetius imagined comets to be
eruptions projected from the body of the Sun or even from the planets.
"If" said he, "we project a body from the surface of the Earth
it will describe a parabola. Therefore" said he, "these bodies which
are projected from the Sun or planets will also describe parabolas." As to
the physical construction of these bodies he imagined them to be nothing more
than a collection of vapours collected in the atmospheres of the planets which
gradually rose higher and higher till at last they were projected from them and
moved in different curves according to the velocities they had acquired.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Such was
the state of the theory of comets when the celebrated one of 1680 made its
appearance. It was first accurately observed in Saxony on the 4th November. It
was then moving with increasing velocity towards the Sun. About the 30th it
moved at the rate of 5 degrees in a day and shortly after it disappeared. About
22nd December it reappeared moving very swiftly from the Sun and its velocity
gradually diminished until the middle of March 1681 when it was no longer
visible. On its return from the Sun it had a tail or train of light extending
70 degrees, that is it reached much more than one third of the heavens. It was
proved that these two [appearances] were the same comet from the resemblance of
the solid nucleus which presented the same appearance before and after its
passage near the Sun, and also from the direction of its course which was the
same. But the strongest proof was that the calculations which Newton made
respecting this comet and which were founded on this supposition agreed
accurately with observation.</span></div>
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<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">It was a
fortunate circumstance for the progress of Astronomy that the Earth was in a
favourable situation to see both the access of this comet to the Sun and also
its recess from that body. Without this accidental circumstance the true system
of the cometary motions might not perhaps have appeared for a long time. But
this singular coincidence hastened its discovery.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>The first outline of the true
cometary theory came from Germany. A clergyman named Doerfell, the minister of
a small village, had observed the comet with much care. He is an astronomer
very little known and has not received that credit which was due to [him for]
the manner in which he treated a subject which was at that time both new and
difficult. Doerfell proved that the comet which receded from the Sun was the
same as the one which had approached it a short time before. He showed that it
moved in a parabola having the Sun in its focus. He ascertained the distance at
which it passed from the Sun. All these circumstances were published in 1681,
but the language in which it was written and probably the little reputation of
the author caused it to be neglected, and it was not noticed until long after
Newton had established the same truths by other methods.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">The
anticipation of one of the discoveries of our great countryman does not however
in the least denigrate from his glory. It was with Doerfell an astronomical
hypothesis, but with Newton it was a physical truth, a branch of his general
system. In fact it was impossible for our astronomer after having established
the gravitation of the planets towards the Sun and recognising as he did with
the astronomers of his time that comets are not the transient meteor of a
moment, not to suppose them governed by the same laws which regulate the other
bodies of the System. It was therefore necessary to suppose revolving in very
eccentric ellipses to account for their not being constantly visible. But
Newton still further demonstrated the truth of his method by applying them to
the determination of the path of the comet of 1680 and it is remarkable by what
accuracy his calculations of the position of the comet agreed with the
observations of Flamsteed. The greatest difference only amounted to two minutes
of a degree.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">The comet
of 1680 was remarkable for the long period of its revolution, which is 575
years, and also for its near approach to the Sun. According to Newton it
approached so near as to be distant from that body only the ?th part of the
distance of the Earth from the Sun. It must therefore have experienced a heat
26,000 times greater than we ever receive from the Sun's rays and if to obtain
a more elevated point of comparison it is compared to that of red hot iron it
will be found that this comet must have been 2,000 times hotter. From this it
appears that the comet must have been composed of very solid matter not to be
dissipated by such an intense heat and this affords a new proof of the
permanence of these bodies.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>Newton conjectured that this comet
as well as others which like it revolve round our Sun approximate continually
to this body at each revolution and that the[y] ultimate[ly] fall into it, for
the purpose of supplying the loss to which it is continually subject by the emission
of particles of light. But this is purely a matter of conjecture and must not
be ranked with the astronomical discoveries of Newton, but which are not the
less solidly established whatever may be the fate of these conjectures.</span></div>
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<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">With
respect to the tails of comets there have been various opinions. It is almost
needless to refute that entertained by most of the ancients and by some few of
the moderns. They conceived that the tails of comets arose from the refraction
of the rays of light through the nucleus of the comet but this is contradicted
by the fact that the nuclei of comets are evidently opaque. Kepler once
maintained this opinion but in his subsequent writing he gave it up. He then
attributed their tails to their atmospheres and to the evaporation of the more
volatile parts caused by the heat of the Sun. This is nearly the opinion which
Newton embraced and he compared the tails of comets to the smoke which follows
a burning body in rapid motion. Such was the most probable explanation of the
cause of the tails of comets before De Mairan. This eminent philosopher to whom
we are indebted for an explanation of the aurora borealis conjectured with some
degree of probability that the tails of comets are formed by the matter of the
solar atmosphere which these bodies attract to themselves on their approach to
their nearest distance from the Sun, and to account for their tails always
appearing on the opposite side to the Sun he supposes that this is the effect
of the impulsion of the rays of light. There are some circumstances which
render this explanation at least probable. It may be remarked that comets do
not begin to exhibit a tail until they have approached nearer the Sun than the
semidiameter of the earth's orbit and this is supposed to be about half the
extent of the Sun's atmosphere. Those comets on the contrary which have not
approached so near to the Sun such as those which appeared in the years 1585,
1718, 1729 etc. have been seen without any tail. The ingenious work of De
Mairan in which he establishes this opinion contains several other proofs by
which this opinion is<span style="mso-spacerun: yes;"> </span>establish</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">ed. These
appendages to cometary bodies present various appearances according to the
positions in which they are perceived. If a comet is moving in a direction
nearly at right angles to the path of our Earth it will appear to have a tail
in the direction opposite to the Sun, but if the comet is moving almost
directly towards us or directly from us it will appear to be surrounded with a
nebulosity, and in particular cases is said to be bearded.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>After Newton no one contributed more
to the improvements of this branch of Astronomy than Dr. Halley. This learned
astronomer presented to the Royal Society in 1705 a treatise on Comets in which
he applies the principles taught by Newton to the determination of the orbits
of comets and he formed tables of their motions similar to those of the
planets. Towards the conclusion however he gave other methods of his own on the
more accurate supposition of their revolving in ellipses. This was the most
valuable and interesting part of the curious communication of the author.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">He
calculated the orbits of the comets and formed them into a table in order to
compare them. By this means he had the satisfaction of verifying the opinion of
those who supposed these stars [were] subject to periodical return. In fact
from the inspection of the tables he found that the comets which appeared in
1531, 1607<span style="mso-spacerun: yes;"> </span>and 1682<span style="mso-spacerun: yes;"> </span>had very nearly the same orbit and the
intervals between their appearances were nearly 75 years. From this he
concluded with a very high degree of probability that it was one and the same
comet whose period of revolution is about 75 years. He found that the
inclination of all the three orbits was about 18 degrees and that, if the mean
distance of the Earth be supposed to be 100, then the least distance of the
comet of 1531 was 57, that of 1607 was 58 and that of 1682 was 58. This
difference is very small when we consider the imperfection of practical
astronomy at the time the observations on which these calculations were made
depended.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">These
were strong reasons for presuming on the identity of the three comets but
further circumstances rendered it still more probable. In counting back from
1531 75 or 76 years we find other comets. Thus in the years 1546, 1380 and 1305
[also in 1230, 1155, 1080 and 1006] there appeared other comets. In fact no
astronomer has transmitted to us observations by which we may determine
decisively their orbits, but by comparing their appearance and motions as
transmitted to us by historians with those of the comet we are considering and
allowing for the different positions of the Earth Dr. Halley found they agreed
very well. Thus assured of its revolution in 75 years he ventured to predict
its return in the year 1758 or 59. This is the first prediction that was ever
made of the appearance of a comet and it is well known that it was justified by
the event. Dr. Halley remarked that the comet observed in 1661 by Helvetius and
that of 1532 seen by Appianus were the same. Had this been the case it ought to
have returned in 1780 or 81. This however was not the case. By comparing his
tables of the orbits of comets Dr. Halley conjectured that the brilliant comet
of 1680 had reappeared several time at the distance of 575 years.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">He
founded this opinion on the following circumstances that in the year 1106 there
occurred a beautiful comet whose description much resembled that of 1680. In
the year 531 a similar one appeared, and in the year 46 before the Christian
era appeared that prodigious comet so celebrated by historians and which
followed so nearly the death of Julius Caesar.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>But Dr. Halley went still further in
continuing to retrograde 575 years at each step: he found that this same comet must
have appeared very nearly at the time of the universal deluge, and he formed
the bold conjecture that this was the secondary cause made use of to produce
that horrible catastrophe. This body was accompanied by a tail of prodigious
extent which according to Newton consisted of vapours raised by the solar heat.
Halley supposed that the Earth might have passed through this and that by the
effect of gravity these vapours would have fallen on its surface and thus
produce the immense body of water by which our globe was inundated. The
celebrated Whiston has supported this explanation of the deluge with much
ingenuity and seems by his zeal to have acquired the title of its author
although it was undoubtedly Halley's. It may be observed that it is scarcely probable
that such an effect would result from our globe passing through the tail of a
comet. Vapours rarefied to such a degree as these must be even if they exceeded
our globe many times in volume would form but an inconsiderable quantity of
water insufficient for the ravages of which the traces still remain. It would
be easy to prove this from considering what has been demonstrated by Newton
that a cubic inch of air would if carried to the distance of the Earth's
semidiameter from its surface be rarefied to such a degree as to fill the whole
space from the Sun to the orbit of Saturn.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">This same
comet has been employed by another celebrated writer to explain another point
of history. He conjectured that this same comet appeared about the time of
Ogyges and that it gave rise to the singular phenomenon which has been
mentioned by historians with astonishment. They relate that 40 years before the
deluge of Ogyges, the planet Venus was seen to quit its ordinary course and to
be accompanied by a long train of light. Upon which the learned writer observes
that in the infancy of Astronomy men might easily mistake a comet just
disengaging itself from the sun's rays for the planet Venus quitting her usual
course and accompanied by a long tail. But so many other comets might have
given rise to this mistake that we can unfortunately determine nothing certain
as to the date of this deluge from such a circumstance.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>Since the period of Dr. Halley much
more extensive tables have been formed. It appears from them that there are
nearly as many whose motion is retrograde as there as ones whose motion is
direct, and it also appears that their orbits are inclined to the ecliptic at
every possible angle. This is another and powerful argument if any further one
were wanted against the theory of vortices.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">It may
also be remarked that the greater number of comets descend towards the Sun
within the Earth's orbit. Of the 35 whose orbits were calculated by La Caille
there were only six whose least distance from the Sun exceeded the mean distance
of the Earth from that body.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>The comets appear to have no fixed
zodiac. On the contrary there is scarcely any constellation in the heavens in
which some comet or other has not been seen. These different positions and the
different inclinations of their orbits do not appear to be the effect of chance
but rather affords cause for admiration. If they had moved in the plane of the
Earth's orbit or very near to it everytime a comet descended towards the Sun or
ascended from it we should be exposed to the danger of a contact, if
unfortunately our globe should at that time be situated at the intersection.
But by means of this inclination it happens that not one of the orbits of
comets yet known cuts that of the Earth. It would indeed be a very curious spectacle
to see a comet pass at the distance of one or two diameters from our globe.
Possibly these might result [in] some physical changes in our little system
which might not be disadvantageous to us. A celebrated naturalist has supposed
we might acquire a new moon and has attributed a similar origin to the one we
possess. It might however be more happy for us to be deprived of this advantage
then to incur the risk of so near a neighbour. Of all the comets which have yet
been observed that of 1680<span style="mso-spacerun: yes;"> </span>can approach
nearest to the Earth. Dr. Halley found from calculation that on the 11th
November 1680 at one o'clock it was not further distant from the Earth's orbit
than the Moon is from us. But there would have arisen to us no danger from this
circumstance. It would only have afforded us an opportunity for curious
observations if the Earth had been in a convenient part of her orbit.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">It is
true we may not always be so fortunate. According to Whiston this comet [has]
already been the instrument of divine vengeance [and] may at it return involve
us in the burning vapour of its tail, and thus produce a universal
conflagration. We must however always separate these bold and sometimes
fanciful conjectures from the physical theory by which the motions of the
bodies are calculated.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>In consequence of the predictions of
Dr. Halley much more attention was paid to the discovery of comets. In the 120
years which elapsed between 1637 to 1757 on 20 had been seen, but in the next
50 years this number was more than tripled. It was observed that the comet
whose return was predicted by Dr. Halley had alternately long and short periods
and it was therefore supposed that it would return about the year 1759, and the
astronomers of the time prepared very diligently to search for it. It was
necessary to calculate the part of the heavens in which it would reappear but
this and the precise time of its return were alike unknown,<span style="mso-spacerun: yes;"> </span>and this much increased the difficulty of
the search.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">The
astronomer Lalande was very ardent in the search and thus assigns his reasons
for it. "It has already appeared" said he "6 times with very
evident marks of identity particularly the last two or three times. There can
therefore be no doubt of its return. Even though astronomers should not observe
it they would nevertheless be convinced of it. It well known to the them that
its little light and immense distance may possibly hide it from our view. But
the public will hardly believe us and they will rank among the number of vague
conjectures this discovery which does so much credit to modern astronomy.
Disputes will again arise; the terrors of the ignorant will continue and 70
years must elapse before we again have an opportunity of removing these
doubts."</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>The astronomer Messier was actively
employed during 18 months in searching for this comet. This labour was not,
however, unrewarded for in consequence of it he discovered a new comet which he
observed during several months. It was on the 21 June 1759 that he first had
the good fortune of observing the object he was in search of. This day happened
to be very severe and as soon as the stars became visible at sunset he profited
by this circumstance and directed his telescope towards that part of the
heavens in which it was expected to appear. After some time he recognised at
about 7 in the evening a feeble light very similar to the small comet he had
discovered in 1758. He had frequently imagined he had perceived this much
wished for visitor, but he had been deceived by the numerous nebulae which are
scattered in that part of the heavens. After a few observations he found from
its motion that it was the object he was in search of.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Dr.
Halley had observed that the periods of the return of this comet were unequal.
They varied from 75 to upwards of 76 years. He attributed this to the
attraction of the planets. He knew that the motion of Saturn is very sensibly
altered by the attraction of Jupiter and considering that the comet must pass
near Jupiter he imagined this planet might cause the change in its periodical
time. He was not however able to calculate this effect and offered it merely as
a conjecture.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>A little before the reappearance of
this comet Clairaut, who concurred in the opinion of Dr. Halley, undertook the
calculation of the disturbance it must suffer from the action of Jupiter. This
immense undertaking was however beyond the powers of one individual. Clairaut
undertook the discovery of the plan to be pursued in these calculations and
Lalande and Madame Lepante executed the calculations themselves. After 12 months
of fatiguing calculations it was found that the action of Saturn was so
considerable that it could not be neglected. The labour was therefore renewed.
In November 1758 these immense calculations were completed and Clairaut was
able to announce that the period of the comet which was just about to terminate
would exceed the preceding one by 618 days. 500 were caused by the attraction
of Jupiter and 100 were the result of the action of Saturn. He also announced
that the comet would be at it nearest distance from the Sun on the 13 March
1759. It in fact only exceed the specified time by 22 days and arrived there on
the 4th of April. Thus did the most celebrated of all the comets confirm the
theory of Newton and afford a proof that they revolve round the Sun like
planets. It must, however, be observed that this is the only one whose<span style="mso-spacerun: yes;"> </span>periodic time is well ascertained. The
periods of the revolution of many other comets have been conjectured and their
returns expected but with respect to them there is much uncertainty.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">The comet
which appeared in 1264 and 1556 is expected in 1848. It may perhaps be the same
as those recorded to have appeared in 975 and 395. But the observations of 1264
are too imperfect for us to depend much on its reappearance. The comet which
appeared in 1770 occupied considerable attention. It was observed for along
time, nor could any parabolical orbit be found which suited its motions. After
immense calculations it was found by Lexell that it moved in an ellipse and
that its period of revolution was 5 years.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>The terror which was excited by the
comet of 1773 is one of the most singular circumstances in their history. It
was occasioned by a paper of Lalande presented to l'Academie des Sciences,
Paris. This memoir was not read at the time, but a report soon spread that
Lalande had announced the termination of the world. It was supposed that it
would take place in less than a year. As the report spread the time became
shortened to a month and then it soon came to a week. The populace were alarmed
and the police applied to Lalande to contradict the report. His explanation
appeared in the gazette in a few days, but as this was not sufficient to
justify him from the numerous absurdities which had been imputed to him he
resolved to publish whole paper. It consists of an investigation into the
probability of the contact of one of these bodies with the Earth. From a
consideration of the orbits of all the comets which had at that time appeared
he plainly showed the great improbability of such an occurrence. And since the
more recent observation the improbability of the injuring [of] our globe is
most materially increased.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">If a
comet equal in magnitude to our Earth were situated at the distance of about
40,000 miles it would cause a tide of 12,000 [ft.] above the level of the sea,
which would be quite sufficient to overflow in succession the whole globe, but,
if we consider that the comet is in motion and the earth [is also] in motion,
both very rapidly, it will be found that the effect of the comet, during the
very short time it would be at the distance, will be wonderfully diminished,
and, if to this it be added that as yet we have had no instance of any comet
approaching to this magnitude, we shall find that the effect of a comet, even
at this short distance, would be comparatively trivial.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>Of all the comets which have been
observed that of 1770 approached nearest to the earth. Laplace found that by
the attraction of the Earth its time of revolution was diminished two days, and
that supposing it to have been of the same magnitude as our globe its reaction
would have shortened the length of our sidereal year 2 hours 40 minutes. But
according to the most accurate observations the length of the year was
certainly not shortened 3 seconds by this comet, from which we may conclude
that its mass was not equal to the<span style="mso-spacerun: yes;"> </span>?th
of our Earth. These bodies appear in general to exert very little action on the
planets they approach. Their wandering courses do not disturb the harmony of
the system and though frequently announced by the ignorant as the presages of
misfortune they do not appear to have received the power of doing mischief.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">There has
been much question as to the solidity and planetary nature of these bodies. It
has been doubted whether they possess a real nucleus of solid material or
whether their centre may not be the densest and most compact part of their
nebulosity. According to observations on the comets of 1799 and 1807 by
Schroeter and Dr. Herschel it appeared that they possessed solid nuclei of a
round form distinct from the nebulosities which accompany them. This nucleus
was not subject to the same variations as the vapours which surrounded them. It
did not always occupy the centre but generally appeared to incline to wards the
Sun. Dr. Herschel is of opinion that the body of the comet shines by its own
light, for he observed that when from its situation with respect to the Sun we
could not see the whole of its illuminated side, yet the light of the comet
appeared by no means diminished but the whole surface shone with one uniform
light much more resembling by its vivacity irradiance of the stars than the
reflected light of planets and their satellites. This observation however would
be satisfied by supposing the centre to consist of a dense mass of vapours
through which the Sun's rays are refracted.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">In the
comet of 1811 the central body was remarkably distinct from the surrounding
vapour. In its appearance this was one of the most splendid which have been
visible for many years. Its tail or the luminous train which it carried with it
in one part of its orbit was 100 million of miles in length and about 15
million in breadth, yet notwithstanding this enormous atmosphere the solid
nucleus of the comet was according to the observations of Dr. Herschel not more
than 428 miles in diameter. This body would according to the observations made
during it appearance describe an ellipse round the Sun in about 2620 years at
the mean distance of 190 that of the Earth [is] from the Sun. This period might
however be very much altered from the attraction of the planets and if at a
very great distance it should be influenced but in a small degree by any
unknown body it might suffer a total change in its orbit and perhaps never
return to our system.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>Of the numerous comets which have
been discovered during the last half century there is scarcely one which on
whose certain return we can rely. Many have been found to move in curves called
parabolas and hyperbolas, and it is impossible for those again to revisit our
system unless some great change take place in their orbits. Laplace has
calculated that supposing a comet to move in such an orbit it is 57 to against
its being visible to our Earth. On the ground its seems probable that in the
last 50 years between 2 and 3 thousand have approached the Sun though the
larger part have been entirely unnoticed by us.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">It may
probably be asked what becomes of those comets which retire from the power of
our Sun and never return. Are these bodies lost in the immense deserts which
separate our primary from the nearest of the fixed stars. It rather appears
from mechanical principles that when a comet has by the action of any
extraneous force acquired such a velocity as to cause it to quit the sphere of
our Sun that it would pursue its unimpeded course until some other sun should
exercise its influence on it and attract it towards a new centre. It might then
descend with immense velocity towards this new primary and thus continue to
visit system after system making as it were the tour of the universe. If this
be really the arrangement of Nature there is doubtless some final cause from
which these wanderings are directed. On this we can only form conjecture. It
has been supposed that in the transit of these bodies through the immense
regions which separate our system from the nearest fixed stars the comets pass
through regions of nebulous matter which they convey to distant systems to
supply the waste occasioned by the emission of light.</span></div>
Unknownnoreply@blogger.comtag:blogger.com,1999:blog-1290567400778113129.post-36198393729641785422013-11-11T10:19:00.003+00:002013-11-11T18:29:26.337+00:00Lecture 10: On the Theory of Universal Gravitation<h1>
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Lecture
10</span></h1>
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="http://upload.wikimedia.org/wikipedia/commons/9/91/Pierre-Simon-Laplace_(1749-1827).jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" src="http://upload.wikimedia.org/wikipedia/commons/9/91/Pierre-Simon-Laplace_(1749-1827).jpg" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">Laplace</td></tr>
</tbody></table>
<h1>
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"> On the Theory of Universal
Gravitation</span></h1>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>By observation and experiment we gain
the first materials of our knowledge. These are the foundations of all
philosophy. But facts however certain or numerous would by themselves be of
comparatively small importance if the inquiring spirit of Man did not seize him
to generalise his observations, and to form some theory to connect and account
for the various appearances presented to him by Nature. The very number of
these facts would soon overpower his memory unless he made use of method and
arrangement. Thus then the accumulation of his observations would necessarily
produce speculative knowledge and give rise to an attempt at theory.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>It is to a legitimate use of theory
that the science of Astronomy is particularly indebted. She presents us with
some of its most happy illustrations. The indiscriminant zeal against
hypothesis so generally avowed by the followers of Bacon has been much
encouraged by the strong and decided forms in which they have been reprobated
by Newton. But the language of this great man must be qualified and limited by
the exemplification he has availed himself given of his general rules. And it
should be remembered that they were particularly directed against the vortices
of Descartes which were purely fictitious and were the prevailing doctrine of
the time. A very learned and acute writer has observed that "the votaries
of hypothesis have been challenged to shew one useful discovery in the work of
Nature that was ever made in that way." In reply to this challenge it will
be sufficient on the present occasion to maintain the Theory of Gravitation and
the Copernican System. Of the former I shall presently endeavour to prove from
a sketch of its history that it took its rise entirely from a conjecture, or
hypothesis suggested by analogy. Nor indeed could it be considered in any other
light until that period of Newton's life when, by a calculation founded in an
accurate measurement of the Earth by Picard, he evinced the evidence to be
true, even the law which regulates the fall of heavy bodies, that power which
retains the Moon in her orbit.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">The
Copernican System offers however a still stronger case, inasmuch as the only
evidence which the author was able to offer was the advantage it possessed over
every other hypothesis in explaining with beauty and simplicity all the phenomena
of the heavens. In the mind of Copernicus therefore this system was nothing
more than a hypothesis, but it was an hypothesis conformable to the universal
law of nature, always accomplishing her ends by the simplest means.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>Nor is the use of hypothesis
confined to these cases in which they have subsequently received confirmation.
It may be equally great where they have completely disappointed the
explanations of their authors. Indeed any hypothesis which possesses a
sufficient degree of plausibility to account for a number of facts will help us
to arrange those facts in proper order and will suggest to us proper
experiments either to confirm, or refute it. Nor is it solely by the erroneous
results of his own hypothesis that the philosopher is assisted in his enquiry
after truth.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Similar
lengths may often be collected by the errors of his predecessors: it was from a
review of the endless and hapless wanderings of preceding enquirers that Bacon
inferred the necessity of avoiding every beaten track, and it was this which
encouraged him with a confidence in his own powers, amply justified by the
event, to explore and to open a new path to the mysteries of Nature.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>In this respect the maturity of
reason in the species is analogous to that in the individual. It is not the
consequence of any sudden or accidental cause but the fruit of re-iterated
disappointment connecting the mistakes of youth and inexperience. "There
is no subject," says [Bernard le Bouvier de] Fontenelle, "on which
men ever come to a reasonable opinion till they have once exhausted all absurd
views which it is possible to take of it. What follies" he adds,
"should we not be repeating this day if we had not been anticipated in so
many of them by the ancient philosophers."</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Those
systems which are false are therefore by no means to be regarded as altogether
useless. That of Ptolemy, for example, as has well been observed by the elegant
historian of Astronomy, is founded on a prejudice so natural and so unavoidable
that it may be considered as a necessary step in the progress of astronomical
science, and, if it had not been proposed in ancient times, it would infallibly
have preceded among the moderns the system of Copernicus and have retarded the
period of its discovery.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>Among the numerous discoveries which
have rendered illustrious the name of Kepler there are none more important than
those with which he enriched Astronomy at the commencement of the 17th century.
Galileo had begun the investigation and cleared away some of the difficulties
but it was Kepler who transported geometry to the heavens, who discovered the
laws of their movement. Let us for a moment follow Kepler in the ideas and
conjectures which led him to these memorable results.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">By
comparing the different velocities of the Sun at different seasons of the year
with variation of its apparent diameter, he found that it was impossible to
account for the phenomenon by supposing the Earth to revolve [around the Sun]
in a circle. He therefore supposed that it might move in an oval, but of this
figure there are various kinds, and the first one he hit upon did not answer
his purpose. He was however more successful in the next trial and found that
the common ellipse would satisfy all the conditions. From this period we may
date the knowledge of the elliptic motion of the planets and the destruction of
the ancient prejudice which attributed to the heavenly bodies a uniform
circular motion which by its simplicity had seduced the ancient philosophers of
Greece.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>After discovering that the planets made
an ellipse Kepler wished to find some law which might regulate their motions.
He knew that when they were nearest the Sun they moved fastest, but was not
acquainted with the law of the change of velocity. After numerous trials he
found out the following analogy. If we conceive a line drawn from the Sun to
the centre of any planet this line will always pass over equal areas in equal
time.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Kepler
observed that the more distant a planet was from the Sun the longer time it
required to perform its revolution round that body, and this led him to the
discovery of another law which prevails throughout the planetary system. He
found that the square of the time of any planet's revolution always bore a
certain proportion to the cube of its distance and that this ratio is constant.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span></span><span lang="ES-MX" style="mso-ansi-language: ES-MX;">Mercury<span style="mso-tab-count: 1;"> </span>[=
0.13050]<span style="mso-spacerun: yes;"> </span>39 x 39 x 39</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="ES-MX" style="mso-ansi-language: ES-MX;"><span style="mso-tab-count: 1;"> </span></span><span lang="EN-GB" style="mso-ansi-language: EN-GB;">Venus<span style="mso-tab-count: 2;"> </span>[=
0.1356]<span style="mso-spacerun: yes;"> </span>72 x 72 x 72</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>Earth<span style="mso-tab-count: 2;"> </span>[= 0.1332]<span style="mso-spacerun: yes;">
</span>100 x 100 x 100</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>Mars <span style="mso-tab-count: 1;"> </span><span style="mso-spacerun: yes;"> </span><span style="mso-tab-count: 1;"> </span>[= 0.1344]<span style="mso-spacerun: yes;">
</span>152 x 152 x 152</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>Jupiter<span style="mso-tab-count: 1;"> </span><span style="mso-spacerun: yes;"> </span><span style="mso-tab-count: 1;"> </span>[= 0.1335]<span style="mso-spacerun: yes;">
</span>520 x 520 x 520</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">[Table of
the square of the number of days required by a planet to revolve round the Sun
divided by the cube of its mean distance from that body in millions of miles]</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">The laws
which I have just mentioned were discovered by Kepler after many trials and numerous
failures. They rested on no other foundations than experiment and were in
precisely a similar situation to the laws relating the planetary distances
which I endeavoured to explain in a former lecture. That is to say Mankind were
astonished at their coincidence with Nature, but were unable to divine the
cause which produced it. </span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>Kepler is particularly distinguished
from the philosophers of his time by the great boldness and frequently by the
great correctness of his views in enquiring into the cause which produces the
phenomena of Nature. He considered the Sun as the supreme moderator of the
celestial bodies. "This star" says he,<span style="mso-spacerun: yes;"> </span>"is possessed with a power of moving bodies, which it
spreads with immense rapidity throughout all space and hence arise the motions
of the planets."</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>At one time he compares the weight
of heavy bodies on Earth to the gravity of the planets towards the Sun. In
another place he suspects that the combined action of the Sun and the Earth
produces the irregularities of the [motion of] the Moon. And he imagines that
the tides may possibly arise from the attractions of this body. One of his
fundamental doctrines is the motion of the Sun on its axis, an hypothesis which
was completely justified a few years after by the discovery of the spots which
cover its surface. These ideas and conjectures bear the evident stamp of
genius, the daring flight of a powerful and comprehensive mind, and they opened
to Newton that glorious path which led him to the most sublime discoveries.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Before
however we take the steps which led this great philosopher to his theory of
gravitation, it will be proper to pass in review the theory of Descartes, which
at the time universally prevailed. Philosophers of the highest antiquity had
recognised the existence of the heavenly bodies. They had each calculated their
distances and appreciated with some accuracy the motions by which they are
animated. But no one before the 17th century, had endeavoured to snatch from
Nature the secret mechanism by which they hold together the planetary system.
The honour of this bold enterprise was reserved for Descartes. Determined to
produce a system entirely novel in which everything should be reconciled with
his ideas of the harmony of Nature, he conceived himself at the formation of
the Universe, and thus presented to himself the spectacle of Creation -an
infinity of molecules of matter repose in the immensity of space. All possess
an extreme hardness, and their infinitely varied form victoriously opposes the
existence of a vacuum. "Creative power" says Descartes, "imposes
on them a force, which at the same time carries them forward in space, and
causes them to revolve on their axis. And certain laws are prescribed to them
by which their actions are to be regulated." Such is the chaos from which
Descartes conceives a universe like ours might arise, whose spectacle, although
habitual, daily excites fresh reason for surprise and admiration. It is
needless to pursue the speculations of this philosopher through their varied
course to the existence of vortices of extremely subtle matter, through whose
assistance the planets and satellites were conceived to revolve.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">We have
already seen that they are totally devoid of proof, and are, in fact,
physically impossible. Yet were not the speculations of Descartes without their
use in the progress of philosophy? They were errors, but it must be confessed
that they were splendid ones, and that the mind which could frame such a theory
might under better guidance have arrived at more accurate results.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>Kepler, Galileo and Descartes
contributed to dissipate the darkness which had for a long time enveloped
mankind. They became the benefactors of the human race, but for the shame of
the age which produced them, they received as the reward of their labours
nothing but injustice, persecution and disgrace. Galileo, whose brilliant
discoveries had merited a better fate, was dragged to an unworthy prison.
Kepler surrounded by the glory he had acquired by the his sublime views of
Nature experienced in his old age all the misery of want and indignance. And it
was not until 100 years after the death of Descartes that his grateful country
raised even a monument to his memory.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Doubtless
nothing would be more satisfactory to the mind than the physical system of Descartes
if it could sustain the process of examination and observation. Those vortices,
that is to say, those torrents of ethereal matter, which according to this
philosopher carry with them the planets round the Sun, present to the mind an
intelligible mechanism which enchants by its simplicity. But this theory, which
at first glance is so seducing, is subject to many difficulties. It is
unfortunately found to agree so little either with the phenomena or with the
laws of mechanics that notwithstanding the efforts of many ingenious writers,
it is universally allowed that the system of Descartes is not that of Nature.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>Newton pursued a different course,
and on the ruins of this system he has erected a new, more solid one, which
presents every appearance of the greatest durability. In fact his system
exhibits a perpetual coincidence between theory and observation. Whether we
regard the grander laws which regulate the Universe, or whether we examine
these minute and almost insensible ramifications of observation and geometry
[it] has nothing to fear from the vicissitudes of time, or from the more
changeable opinions of men. The system of Newton is founded on the principles
of Universal Gravitation.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Every
particle of matter, whatever may be the mechanism or cause which produces this
effect, tends, according to this philosopher, to every other particle with a
force which decreases inversely as the square of the distance. This gravity
causes on the surface of the Earth the weight of a body, and among the celestial
bodies it is the source of the most complicated motions. I shall endeavour to
explain the proofs of this principle and the reasoning which lead Newton to it
after shortly stating what was known on the subject before the time of his
writings.</span></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span></span></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>We find among the writings of the
ancients a glimpse of several of the most brilliant truths. This is
particularly the case with the principles of Universal Gravitation, of which we
discover some decided marks. Anaxagoras as we have already seen attributed to
all the heavenly bodies a tendency towards the Earth, and other traces of the
same opinion may be found among the writings of Democritus and Epicurus. It was
from this principle that Lucretius drew the bold conclusion that the world is
without bounds.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">As soon
the true system of the world revived by Copernicus arose from its ashes that of
Universal Gravitation threw some rays of light. This celebrated astronomer
attributed the round figure of bodies to the attraction of their parts. He did
not extend this attraction from planet to planet. But Kepler more bold and more
systematic made this step. He attributed to the Moon a tendency towards the
Earth and said that they could meet in their common centre of gravity if they
were not prevented by their rotation. Nobody however before the time of Newton
so clearly perceived the principle of that attraction, or more nearly
approached in making a proper application of the system of the Universe than
Dr. Hooke. The philosophers whom we have mentioned had some of them seized one
branch, some another, but Hooke embraced it in all its generality.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>His anticipation of that theory of
planetary motions which was soon after to present itself with increased and at
length demonstrative evidence to a still more powerful mind furnishes a
remarkable instance of this philosophical sagacity. This conjecture I shall
state in his own words, and it affords a decisive reply to the undistinguishing
censures which have so often been bestowed on the presumptuous vanity of
attempting by means of hypothesis to penetrate into the secrets of Nature.
"I will explain" says Dr. Hooke, in a communication made to the
P[resident of the Royal Society?] in 1666, "a system of the world very
different from any yet received. It is founded on the three following
positions."</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">"1st
that all the heavenly bodies have not only a gravitation of their parts to
their own centre, but that they mutually attract each other within their
spheres of action."</span></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>"2ndly that all bodies having a
simple motion will continue to move in a straight line unless continually moved
out of it by some extraneous force."</span></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>"3rdly that as this attraction
is so much greater as the bodies are nearer, as to the proportion in which
those forces diminish by an increase of distance."</span></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Dr. Hooke
adds "I [on my] own have not discovered it although I have made some
experiments for this purpose. I leave this for others who have time and
knowledge sufficient for the purpose."</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>It should be observed that there is
a wide difference between the conjecture of Hooke and the proofs and sublime
demonstrations by which Newton supported this law of the Universe. Such however
was the state of the question when this profound philosopher appeared. It was
in 1666 that he first [began] to suspect the existence of this principle and to
endeavour to apply it to the motions of the heavenly bodies. He had retired
into the country to avoid the plague, which at that time prevailed in London.
His meditations were one day accidentally directed toward the weight of bodies.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">His first
reflection was that the cause which produced the fall of heavy bodies always
acts upon them to whatever height we convey them. It may then be extended much
further than we think, possibly as far as the Moon or even beyond. From this he
conjectured that it might possibly be this same force which retains the Moon in
her orbit. At the same time he considered that though Gravity does not sensibly
alter at different heights to which we can attain, yet at greater distances it
may vary and these altitudes are too small to conclude that it is the same at
all distances. It now remained to discover the law by which it varied,. For
this purpose he argued that if gravity retained our Moon in her orbit round the
Earth, it must be a similar cause which retains the sattelites of Jupiter in
their orbits round that body, and by comparing the periods of these bodies with
their distances, he found that gravity must decrease inversely as the square of
the distance.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Newton
did not however rest here. He continued to examine an account of the Moon's
distance from the Earth: the force by which she is attracted will be 3,600
times less than that by which a body falls on the surface of the Earth. If we
can compare the space through which the Moon falls towards the Earth in a given
time with that through which a body on the Earth's surface falls, we shall have
a criterion by which to judge of the truth of our theory. But here arose a
difficulty, how shall we find how much the Moon falls towards the Earth in a
given time? This difficulty Newton overcame, and these are the means he made
use of. This had nearly overthrown his whole edifice. He supposed the
terrestrial degrees to contain 60 miles and in consequence of this the two
quantities (on whose relation the truth of his theory was to be tried) did not
afford a result favourable to it.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>Many philosophers would have been
but little troubled by this disagreement and would have continued to construct
their theoretical edifice. But this incomparable man, whose object was the discovery
of truth and not the formation<span style="mso-spacerun: yes;"> </span>of a
system, when he found that a single fact overthrew all his conjectures which
had hitherto been so well founded, immediately relinquished them.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">It was
not till 10 years after that he resumed the train of his ideas. In this
interval the opinion of Dr. Hooke had been published and a very important step
had been made by Ricard in ascertaining the magnitude of the Earth. From this
Newton learned that the terrestrial degree was nearly 70 miles in length, and
not 60 as he had considered it in his calculations. He now therefore again
returned to his theory and having calculated the magnitude of the lunar orbit,
he found to his great satisfaction that the space fallen through by the Moon
precisely agreed with what it ought from his theory. After this demonstration
Newton no longer hesitated to consider the force by which heavy bodies fall at
the surface of the Earth and that by which the Moon is retained in her orbit as
one and the same.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>He assumed gravity as a well ascertained
fact and proceeded to reason upon it. He showed that it followed necessarily
from his theory that the planets move in ellipses and he demonstrated the laws
which Kepler had only found by induction. By a skillful application of
mathematical calculation to the phenomena of Nature aided by the theory of
gravity, he unravelled numberless irregularities to which the heavenly bodies
are subject. Yet such was the excessive modesty of this great man that it was
with the greatest difficulty that he was persuaded to publish his profound
discoveries.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">At the
urgent request of his friend, Dr. Halley, and at the entreaty of the Royal
Society, he was persuaded to collect together his discoveries, which he did in
his Principia, a work which also is alone sufficient to immortalise its author.
This work however in which<span style="mso-spacerun: yes;"> </span>this great
man has built a new system of natural philosophy upon the most sublime geometry
did not at first meet with all that applause it deserved and was one day to
receive.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>Two reasons concurred to produce
this effect. Descartes' system had at that time got full possession of the
world. His philosophy was indeed the creature of a fine imagination; he had
given her some of Nature's features and had painted the rest to a seeming
resemblance to her. Newton, on the otherhand, had with unparalleled penetration
and force of genius pursued Nature up to her most secret abode and was intent
to demonstrate her residence to others rather than anxious to describe
particularly the way by which he arrived at it himself. But at last that
approbation which had been so slowly gained became universal, and nothing was
heard from all quarters but one universal burst of applause [and] admiration:
"Does Mr. Newton eat, drink or sleep like other men?" said the
Marquis de l'Hospital, one of the most enlightened foreigners of the age, to
his English visitors, "I represent him to myself as a celestial genius
entirely disengaged from matter."</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Yet in
the midst of these profound enquiries Newton had leisure for other pursuits. When
the privileges of the University [of Cambridge] were attacked by James II he
appeared as one of the most strenuous defenders. And he made a very successful
defence before the high commission court. He was also a member of the
Convocation Parliament in which he sat till it was dissolved.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>In his private life Newton was
modest and unassuming in the highest degree. His temper was so mild and equal
that no accident could disturb it. He would have rather chosen to remain in obscurity
than to have the calm of life ruffled by the storms and disputes which genius
and learning so frequently draw on those who are eminent for them. From this
love of peace arose that unusual horror which he felt for all disputes, and
that steady unbroken attention which was his peculiar felicity; he knew and
well esteemed its value.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">When some
objections hostily made to his discoveries concerning light and colours induced
him to lay aside his design of publishing his optical lectures, we find him reflecting
on that dispute into which he was unavoidably drawn in these terms: "I
blamed my own imprudence" said Newton, "for parting with so real a
blessing as quiet, to run after a shadow." Yet this shadow was one of the
most splendid and most original of these discoveries which have contributed to
make his name so illustrious.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>In contemplating the genius of
Newton the penetration, the strength and the originality of his mind, in his
moral capacity the pre-eminent trait is his modesty and love of quiet. In his
intellectual character the most predominant feature is the astonishing power
which he possessed of concentrating his attention to the object on which he was
employed. It was to this almost supernatural power that he himself attributed
his profound discoveries. When he declared that if he had done the world any
service it was due to nothing but industry and patient thought; that he kept
the subject of consideration constantly before him and waited till the first
dawning opened gradually, by little and little into a clear and full light.
Such was Newton as a man, and as a philosopher both characters were tinged with
a similar colouring. As a man he did not possess that warmth and enthusiasm
which we should admire in a friend, but he displayed that calm unruffled
serenity, which we should reverence in superior beings. As a philosopher he was
not led away by the fire of genius whose too daring grasp by sometimes
fostering error, reminding us of his mortal origin. But he was the patient, the
accurate investigator of Nature, the deep, the profound philosopher.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">To trace
the consequences of the law of Universal Gravitation through its numerous and
almost endless ramifications would lead us in succession through every branch
of astronomical science. Each seeming objection which has successively been
brought against its truths has furnished new arguments in its favour and
afforded new ground of triumph to the followers of Newton. It has frequently
anticipated observation and has predicted to future astronomers irregularities
that are yet to be recognised. To such a point of perfection has this science
been carried, that there does not now remain one single irregularity of the
heavenly bodies of any magnitude which does not follow as a consequence of this
law, and whose quantity cannot be calculated by it. It is obvious that an
enumeration of these varied irregularities would be of little improvement, but
there are some important questions which present themselves and which relate to
the law of gravity.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>There are many of the planetary
irregularities, which increase and decrease alternately in longer and shorter
periods. Some however, since we have observations of them recorded, have been
found uniformly to decrease. Such for instance is the obliquity of the ecliptic.
If this obliquity were to decrease continually, the ecliptic would at last
coincide with the equator. This would not take place until after the lapse of
millions of years, but when it did the days and nights all over the world would
become equal; the Earth would possess a perpetual spring. Whether this change
would be for the advantage of the human race if it then existed, or whether it
would not render nearly half the globe uninhabitable, are doubtful questions;
yet remote as this change may be it is interesting to enquire whether it is
possible, because there are other irregularities which increase much more
rapidly and which might if they continued to increase totally derange the
system. The question then in the most general sense is to investigate whether
the irregularities of the planetary system will continually increase or whether
after attaining a certain point they will not decrease and return again in the
same order.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">This
question which is a very important one has occupied some of the greatest philosophers
of the age. The progress was very gradual but the solution is complete. It is
to the united labours of Lagrange and Laplace that we are indebted for the
solution of this interesting question. It has been demonstrated that every
irregularity to which the planetary [system] is subject must from its nature be
periodical: that is, it will increase to a certain point and then decrease to
another point, between which two it will constantly oscillate, never exceeding
either of them, just in the same manner as a pendulum which constantly moves to
and fro, but never exceeds a certain fixed distance from its point of rest.
Almost everything in the system will therefore be in motion, but admidst this
universal change some few elements will remain constant: thus the mean distance
of each planet from the Sun will be unaltered. Thus then it appears that unless
some foreign force disturb the harmony of our System, it will forever continue
in its present arrangement, that it does not contain within itself the seeds of
destruction, but on the contrary that it is destined to an eternal duration,
unless the mechanical laws which govern matter be subverted or some influence
foreign to the System be exerted on it.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">There
were, however, in its original constitution some conditions necessary, that the
inclination of the orbit of the planets to the Sun's equator be small, that the
ellipses which they describe should be nearly circular and that they should all
move round the Sun in the same direction. And if these conditions had not been
fulfilled it is possible that the beautiful system from its own action have
produced its destruction. If we enquire from the doctrine of chances whether it
is probable that the conditions should have been accidentally accomplished we
find that the contrary is indicated with the highest possible degree of
probability. In fact Laplace, when speaking on this subject, says that we have
stronger grounds for believing that the planetary motions and inclinations were
all influenced by the same primitive cause, than we have for giving credit to
any of the most authentic accounts in history. This wonderful contrivance of an
intelligent mind by which the permanence of our System is secured is highly
calculated to excite our admiration, yet it has been perverted to the worst,
the most unphilosophical purposes. It has been urged as the supporter of fate
and of necessity, and has been inconsiderately advanced as an argument against
the superintendence and existence of a first cause. It is a singular circumstance
that a fact which had a tendency so directly contrary should have been so
misunderstood, and it would perhaps be needless to refute it, but that it has
been said, though I believe falsely, to have received the sanction of one of
the most eminent of the continental philosophers. The difficulty is easily
removed. We have only to ask ourselves this question: which is the most skilful
artist? he who makes a clock which requires winding up every day and cleaning
every year, or he who contrives one which winds itself up and never requires
cleaning, to which </span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">it may be
further added that among the infinite number of laws by which gravity might
act, all equally possible, this of Nature alone aided by the conditions already
specified will ensure the stability and permanence of the System. Had there
been any other originally established this Universe the beneficent result of
creative power would have long since have returned to its primitive chaos.</span></div>
<span lang="EN-GB" style="font-family: "Arial Black"; font-size: 14.0pt; mso-ansi-language: EN-GB; mso-bidi-font-family: "Times New Roman"; mso-bidi-font-size: 10.0pt; mso-bidi-language: AR-SA; mso-fareast-font-family: "Times New Roman"; mso-fareast-language: EN-US;"><br clear="all" style="page-break-before: always;" />
</span>Unknownnoreply@blogger.comtag:blogger.com,1999:blog-1290567400778113129.post-55610293916641550402013-11-11T10:18:00.003+00:002013-11-11T23:44:20.921+00:00Lecture 9: The Outer Planets<h1>
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Lecture
9</span></h1>
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="http://upload.wikimedia.org/wikipedia/commons/thumb/3/36/William_Herschel01.jpg/490px-William_Herschel01.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="200" src="http://upload.wikimedia.org/wikipedia/commons/thumb/3/36/William_Herschel01.jpg/490px-William_Herschel01.jpg" width="163" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">William Herschel - Discover of Neptune</td></tr>
</tbody></table>
<div>
<br /></div>
<h1>
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">The Outer Planets*</span></h1>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">[script
of lecture missing]</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">* Note:
probable title of lecture.</span></div>
<span lang="EN-GB" style="font-family: "Arial Black"; font-size: 14.0pt; mso-ansi-language: EN-GB; mso-bidi-font-family: "Times New Roman"; mso-bidi-font-size: 10.0pt; mso-bidi-language: AR-SA; mso-fareast-font-family: "Times New Roman"; mso-fareast-language: EN-US;"><br clear="all" style="page-break-before: always;" />
</span>Unknownnoreply@blogger.comtag:blogger.com,1999:blog-1290567400778113129.post-86615451658389775232013-11-11T10:17:00.003+00:002013-11-12T00:07:55.380+00:00Lecture 8: On the Minor Planets (Asteroids) and also the History and Development of the Reflecting Telescope<h1>
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Lecture
8</span></h1>
<h1>
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">On the Minor Planets
(Asteroids) and also the History and Development of the Reflecting Telescope</span></h1>
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh0sqZgAep5mZI6dIW8c9ojScjBlGDnw_iJ3npzcEqh-Cv9W6Yom4vS-eMQW3tpKbAVO4KJ_9Iw6CUKTLLBED_sRMlFeKQdA3NM993OUgdXpyB4l2epu3i7ZbvgLow3KutvpgflAvFymUo/s1600/511px-40_foot_telescope_120_cm_48_inch_reflecting_telescope_William_Herschel.png" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="320" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh0sqZgAep5mZI6dIW8c9ojScjBlGDnw_iJ3npzcEqh-Cv9W6Yom4vS-eMQW3tpKbAVO4KJ_9Iw6CUKTLLBED_sRMlFeKQdA3NM993OUgdXpyB4l2epu3i7ZbvgLow3KutvpgflAvFymUo/s320/511px-40_foot_telescope_120_cm_48_inch_reflecting_telescope_William_Herschel.png" width="272" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">William Herschel's 40 foot Reflecting Telescope at Slough</td></tr>
</tbody></table>
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<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>We have already considered the
motions, phases and appearances presented by the planets which are included by
the orbit of the Earth. We have also extended our view to the planet Mars,
which is next in order to our globe. Beyond this body are situated four small
bodies whose diminutive size would have ever hid themselves from our sight
without the assistance of the telescope. These are on many accounts remarkably
worthy of our attention. The recency of their discovery, the smallness of their
magnitudes and the nearly equal periods of their revolution round the Sun,
these and the numerous other points in which they differ from the rest of the
planetary bodies with which we acquainted combine to give them a singular
interest.</span></div>
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<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>There is, however, another point of
view in which these bodies appear in no less striking light. A law has been
discovered to which all the planetary bodies are submitted. This law was
incomplete until the discovery of these bodies, but it is now found to prevail
throughout the system. Some astronomers have contended that it is a law of
Nature, whilst others have attributed the coincidence entirely to chance. I now
propose to trace the history of this singular question and the consequences to
which it leads. In explaining the law itself I fear I shall necessarily appear
abstruse, for this the nature of the subject will, I hope, be a sufficient
apology.</span></div>
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<span lang="EN-GB" style="mso-ansi-language: EN-GB;">It is
needless to collect the vague notions of a few of the ancients respecting the
number of the planets. They were for the most part conjectures without the
slightest foundation. Kepler was the first who had some notions real or
imaginary respecting the number and distances of the planets. He even pointed
out two vacancies in the system in which he supposed new ones ought to be
discovered.</span></div>
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<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>Kepler imagined every thing in
Nature must be harmonious. He conceived certain mystical properties to be
attached to numbers, and imagined that there must exist some law which should
connect together these wandering stars. These were the objects of his constant
enquiries of his ardent pursuit and the result was the discovery of those laws
which have received his name. These have subsequently been confirmed by the
investigations of the mechanical philosophy but at the time of their discovery
they were merely the results of trials and were only judged to be true from
their coincidence with fact. This is the only kind of evidence which can be
offered for the law we are about to consider. Kepler spent a considerable time
in endeavouring to find by trial whether there did not exist some relation
among the distances of the planets from the Sun, but after a long and
unsuccessful labour he gave up the task in despair. All his calculations were
overturned from the want of a planet situated between Mars and Jupiter, and
that<span style="mso-spacerun: yes;"> </span>such an one did exist he strongly
suspected but could not discover any law nor assign the distance at which it
should be placed from these two bodies.</span></div>
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<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Titius, a
professor of Astronomy at Wittenburg, was the next who applied himself to these
researches. After much labour he found out a law to which all the planets then
known accorded , and from this he concluded that there must be a planet
situated between Mars and Jupiter, and he even determined the distance at which
it ought to be placed from the Sun. the law which Titius discovered was this:
the distance of the planets Mercury, Venus,<span style="mso-spacerun: yes;">
</span>the Earth, Mars, Jupiter and Saturn may be represented by the numbers:
4, 7, 10, 16, 28, 52, 100 [and] 196. If now from each of these quantities we
subtract the number 4 and if we divide the remainder by 3 the result will
always be a power of two. This is very nearly true for all the planets that
were known at the time. Titius lived, but there was a vacancy between Mars and
Jupiter at the distance of 28. According therefore to his theory he concluded
that some one would be found to be situated between them.</span></div>
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<span lang="EN-GB" style="mso-ansi-language: EN-GB;">These
observations were made before the discovery of Uranus by Dr. Herschel in the
year 1781. The proportional distance of that planet is 196, and it is somewhat
remarkable that this number corresponds with the law of Titius, for if we
subtract four from it there remains 192. This divided by 3 gives 64 which is
the 6th power of two. Dr. Herschel's planet then is in a certain sense a proof
of the law discovered by Titius. It was found out after the law was known and
is situated at the precise distance which that law indicated. We shall find
however that there is another perhaps a stronger proof.</span></div>
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<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>The law of Titius does not seem to
have excited that astonishment which so singular such a subject might have been
expected to create. It was, however, much considered in Germany, and met with
many warm advocates. It excited a strong belief in the assertion of Kepler that
another planet must exist between Mars and Jupiter. This was so much increased
during the latter years of the last century, that Bode who was quite a convert
to the opinion wrote to appoint a meeting to consider of the best means of
discovering the supposed planet. Those who found the journey inconvenient sent
word that they would undertake a share in any of the labour which might be
resolved on for this purpose. Gothe in Saxony was the place appointed for the meeting
and here were assembled Bode, Lalande, Schroeter, Harding, Olbers and many
others of the most respectable observers in Europe. The result of their
consultation was that they would divide the heavens in zones of a few degrees
each and that each astronomer should take one of these zones and examine
scrupulously every star it contained above a certain magnitude. This was the
plan adopted. So each observer was appropriated a zone and to those who were
absent an account of their task was sent.</span></div>
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<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Piazzi,
an astronomer at Palermo in Sicily, had one of these zones assigned to him. He
was at that time occupied in a description of the starry heavens and
consequently had occasion to examine other parts besides that which was
appointed to him. In this pursuit he was occupied when he observed one evening
the 87th star in the Zodiacal Catalogue of La Caille situated between the Ram
and the Bull. Near this he perceived a small star of the 8th magnitude which he
thought an unknown one, and it appeared to possess a proper motion of its own.
This happened on 1st January 1801 and according to his usual custom he wished
to observe it on several of the following days for the purpose of determining
its position with better success.</span></div>
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<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>He made several other observations and
perceived a motion in the star and suspected that it might possibly be a new
planet. To verify this conjecture he resolved on following the motions of this
body very assiduously, but a dangerous illness he was attacked occasioned by
excessive fatigue had nearly at once deprived the world of the astronomer and
his discovery. When he was sufficiently recovered to pursue his observations
the star was no longer visible to the Earth; it had disappeared in the rays of
the Sun. Piazzi now reconsidered his former observations. These were the only
guides he had to conduct him in his search after this new body. He found that
they accorded very well with the supposition of its moving in an ellipse. These
conclusions were similar to those of Burckhardt, an astronomer of acknowledged
skill, and confirmed him in the idea of its being a planet. He therefore gave
it the name Ceres to inform posterity that Sicily which was formerly
consecrated to this goddess was the place from which she was first discovered.</span></div>
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<span lang="EN-GB" style="mso-ansi-language: EN-GB;">The new
planet, however, was not easily rediscovered from her extreme minuteness. She
escaped all observation. The greater part of the year 1801 was employed in
searching for her. After many fruitless attempts she was again found by Zach on
the 31st December and by Dr. Olbers on the 1st January 1802, that is just
twelve months after her first discovery. These gentlemen, however, were not the
first to whom the planet became visible. On the 7th of December it had been
observed by Gauss, and as it affords a remarkable instance of the powers of
[Mathematical] Analysis I shall mention the circumstances attending it.
"All hope" observed this excellent mathematician, "of again
gaining a sight of this planetary atom depended entirely on our being able to
find its orbit with a sufficient degree of approximation from the few
observations that were made on it when visible. I could not" continued he,
"desire a better opportunity of trying whether my ideas on this subject
were of any practical utility than by using these observations for the
determination of the orbit of Ceres." This planet had during 41 days only
described an arc of 3 degrees and now after the lapse of a year it was to be
sought for in a far, distant part of the heavens. The first applications of
Gauss' method was made in the month of October of 1801. And on the first fine
night which occurred he directed his telescope to that precise spot in the
heavens which it ought to occupy from his calculations and the planet was
immediately visible. This is a striking instance of the perfection which theory
has attained. With only 3 observations on a new planet we may find its distance
from the Sun in a rough manner, and with a very few more we may approach
tolerably near the truth.</span></div>
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<span lang="EN-GB" style="mso-ansi-language: EN-GB;">From the
new observations which were now made on this planet it was found that she
completes her circuit round the Sun in about 1,618.5 days or in 3 years 7
months 10<span style="mso-spacerun: yes;"> </span>degrees. Having thus
discovered the nature of her orbit and the principal irregularities to which
she is subject there is no longer any danger of her eluding the enquiry of
astronomers.</span></div>
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<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>The discovery of this minute planet
has suddenly changed many of the received opinions concerning the Solar System.
The extent of the Zodiac in which the motion of the planets was confined was 16
degrees. This was the Zodiac of the ancients, but Ceres has extended these
limits and requires a zodiac of 37 degrees, which is more than double the
extent of the former. The apparent inclination of her orbit varies from 11
degrees to 18<span style="mso-spacerun: yes;"> </span>degrees. She has also
disarranged our ideas respecting the rank established among the bodies which
constitute the planetary system. Nature appeared to have placed the largest
under the immediate dominion of the Sun and around these smaller bodies or
satellites revolved, but this arrangement is destroyed. Ceres is one of the
smallest bodies of the planetary system. Her apparent diameter does not amount
to 1 minute of arc according to Dr. Herschel and from this it would follow that
her real diameter is 17 times less than that of the Earth or that our Moon is
five times as large as the planet Ceres, and yet this diminutive body does not
describe a narrow circle round some primary planet but pursues her lengthened
course through the heavens beyond the orbits of the Earth and Mars.</span></div>
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<span lang="EN-GB" style="mso-ansi-language: EN-GB;">The
discovery of Ceres has by some been regarded as the effect of accident, but it
should not be considered in such a point of view. It is the honourable fruit of
an immense labour. It is the well deserved reward of the care and attention
bestowed by its author on the formation of his catalogue of fixed stars. This
skilful observer never placed any star in his catalogue until he had viewed it
on several successive nights, and it is owing to these repeated observations
that the discovery of Ceres must be attributed. It was difficult from its
extreme smallness and it has become more glorious to its author from the
important consequences which have followed.</span></div>
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<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>It was in examining the path which
this body describes in the heavens that the other planets have been discovered
which were before equally unknown. The most singular circumstance attending
this new planet is that it occupied the interval between Mars and Jupiter which
was predicted by Kepler, and that its distance corresponds very nearly with the
law discovered by Titius. In fact 28 = 4 + 3.2. This remarkable law occupied
much of the attention of the German philosophers. It must however be confessed
that it is not completely accurate, yet it so far agrees with the truth as to
excite considerable surprise and to make us almost doubt whether it could be
the effect of accident. Whatever it may be planets to whose discovery it
contributed will always remain to us and the law itself if found to be
fallacious will furnish an example of the happy effect which have sometimes
casually resulted from systems entirely erroneous.</span></div>
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<span lang="EN-GB" style="mso-ansi-language: EN-GB;">On the
subject of this law I cannot lay before you the calculations of Professor
Schweigger of Nurnberg which would I have no doubt throw some light on this
curious question. They are contained in a paper read before the Philosophical
Society of Munich on the 6th August 1813. It is entitled a dissertation on a
general law which subsists between the distance of the planets and their
satellites. From this title I should imagine that its author had discovered
some law between the distances of the secondary as well as the primary planets.
The volume, however, which contains this paper if it is printed has not yet
arrived in this country.</span></div>
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<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>The third year of the 19th century
produced another new planet for whose discovery we are indebted to Dr. Olbers,
a physician of Bremen, who was known to the astronomical world as the author of
a treatise on comets. On the 28th of March 1802 he was observing with the
design of determining the position of Ceres all the stars which form the
constellation of the Virgin. At a short distance from that marked 20 near which
he had observed the planet about two months before he saw a star of the 7th
magnitude which he had not perceived in his former observations. He had some
suspicions about this star and examined her more attentively. In the interval
of two hours he found that she had altered her situation and on the following
two nights he ascertained that she was in motion at the rate of 10 minutes of
arc in 24 hours.</span></div>
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<span lang="EN-GB" style="mso-ansi-language: EN-GB;">If the
astronomer considers the accidental discovery of a comet as a piece of the
greatest good fortune how much more highly must he estimate the advantage of
enriching the system with another planet. Dr. Olbers enjoyed the satisfaction<span style="mso-spacerun: yes;"> </span>almost at the moment of his discovery. He
had no doubt respecting the nature of the body he was viewing. Its disc was
better defined than that of Ceres and it had not the least resemblance to a
comet. He had besides learnt from the discovery of Dr. Herschel and Piazzi that
the ancient planets were not the only ones belonging to our System. His
satisfaction was not therefore interrupted by any of those doubts which had
alarmed the former observers. Thus after the first few days of the discovery
Dr. Olbers announced the new planet to the astronomical world. The astronomer
Burckhardt and Gauss as soon as they were informed of its existence commenced
their observations on it. They soon found that it revolved in an ellipse but
were much astonished to discover that the inclination of its orbit was greater
even than that of Ceres.</span></div>
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<span lang="EN-GB" style="mso-ansi-language: EN-GB;">A star
which embraces in its course from north to south a zone of about 70 degrees
wanders too far from the ordinary course of the planets not to leave at first
some hesitation as to the rank which ought to be assigned to it. But since this
body as well as Ceres is placed between Mars and Jupiter, and since it is not
like comets subject to disappear by its recess from the Sun it has been placed
among the number of the planets and it received from its discoverer the name of
Pallas.</span></div>
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<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>The effect of this great inclination
of its orbit combined with its eccentricity which is larger even than that of
Mercury causes the greatest inequalities and perturbations in its motions and
at the same time renders their disturbances more difficult to calculate.
Burckhardt undertook some of the most laborious calculations with a view to
ascertain its elements. Pallas performs it revolution round the Sun in 1,681.7
days. This is about 2<span style="mso-spacerun: yes;"> </span>hours longer than
Ceres occupies for the same course, so that the two planets are situated almost
precisely at the same distance from the Sun. The eccentricity of Pallas is very
considerable. She is in one part of her course almost twice as far distant as
she is at the opposite part. From the united effect of these considerations it
may happen that Pallas whose mean distance from the Sun is greatest may pass
between Ceres and the Sun and thus to an observer situated on the planet Ceres
there would be a transit or rather from the nearness of the two planets it
should be called an eclipse since the Sun would probably be completely hid. If
the inclination of their orbits were equal this eclipse might last months or
upwards and it would perhaps not again recur for about 33,000 years, but from
the difference in the inclination of their orbits it will last but a short time
and will occur still less frequently. After the lapse of ages it may happen
from the differences of their eccentricities that that which was the inferior
shall become the superior planet and that the inhabitants of Pallas shall
observe Ceres pass between themselves and the Sun.</span></div>
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<span lang="EN-GB" style="mso-ansi-language: EN-GB;">To the
discovery of the planets of Piazzi and Olbers shortly succeeded that of another
new planet by Professor Harding. This astronomer, the worthy colleague of Schroeter,
undertook the task of forming a map of that zone of the heavens which contains
the paths of Ceres and Pallas. He executed this zodiac of Ceres on twelve large
sheets and not only marked down all the stars contained in the different
catalogues all of which he found in Lalande's list of the 50,000 he observed,
but added a great many from his own observations which had hitherto escaped the
attention of astronomers. On the 1st of September 1804 in comparing these maps
with the heavens he discovered between two stars whose places were known a new
star which had not before been seen in that place.</span></div>
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<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>On the 4th September he no longer
perceived it but at a short distance he saw another which he had not seen 3
days before. He immediately suspected that this might be the same as the first
but that its motion had made it appear in two different places. This suspicion
was soon changed into certainty: on the next day he plainly observed its
movement and as the body presented [had] neither nebulosity nor the appearance
of a tail he immediately concluded that it was a planet. This was soon
confirmed by the other observations of other astronomers and by the
calculations which resulted from them. The new planet received from Professor
Harding its discoverer the name of Juno. It performs its revolution round the
Sun in 1,590.998 days so that the length of its year is about 90 days shorter
than those of Ceres and Pallas. Its distance from the Sun is nearly 26.5 if we
consider that of the Earth as represented by the number 10. The eccentricity of
Juno is very considerable; it is rather greater even than that of Pallas, but
the inclination of its orbit is not so considerable being only 13 degrees.</span></div>
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<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Since the
commencement of this century a fourth planet has been discovered in almost
every respect similar to those already described. Although a plan was as we
have seen formed for the discovery of a planet between Jupiter and Mars, yet it
was not strictly owing to this laborious undertaking that the three first were
found. For the discovery of Ceres we are indebted to the formation of a
catalogue of stars by Piazzi; that of Pallas arose from the examination of the
heavens which [were] undertaken to re-discover Ceres and Juno was found from
the investigations undertaken by Harding to form a chart of all the small stars
in the path of the two former bodies. The fourth is the only one discovered
from pursuing a plan with the express view of finding it.</span></div>
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<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>The hypothesis on which it was founded
is certainly very extraordinary and may perhaps be controverted, but it has
been too fortunate in its result to incur the disapprobation of astronomers.
The idea itself and the consequences which resulted from it are equally the
property of Dr. Olbers. This skilful observer to explain the phenomena
presented by the smallness of the new planets and their nearly equal distance
from the Sun framed this hypothesis.</span></div>
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<span lang="EN-GB" style="mso-ansi-language: EN-GB;">That
possibly these small bodies might be the fragments of a much more considerable
planet which some extraordinary cause had burst in pieces and that these parts
continued to circulate round the Sun at the same distance and with equal
velocities. This theory does credit to the ingenuity of its author and is not
opposed by an argument which has frequently overturned such speculations. It is
not repugnant to the principles of mechanics. It is not impossible that such an
occurrence should have taken place and if such had been the case it might have
happened that several fragments would revolve in nearly an equal time and the
orbits of all would cut each other in two points. If however any of these parts
should pass within the sphere of attraction of any large body its orbit might
be considerably altered. This has perhaps happened in the present case. It is
not probable to suppose that the convulsion which thus destroyed a planet
should have divided it into precisely the parts which have been discovered. It
is more likely that an immense number of pieces of different magnitude should
have been formed, the larger parts would revolve regularly in certain orbits
but possessing a considerable mass they would only be disturbed by the action
of the other planets and would perform their course subject to these
irregularities. The smaller fragments would be much more considerably affected
by the attractions of the larger, and as they passed within the reach of each
new body their orbit would be altered. Thus it might happen that some of these
small fragments coming within the sphere of attraction of the Earth may be
precipitated on its and thus produce those meteoric stones which are frequently
discovered. It is not impossible that at the original disruption one part of
the planet might be projected nearly in a right line towards the Sun. This
would revolve in a very eccentric ellipse and would consequently become a
comet.</span></div>
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<span lang="EN-GB" style="mso-ansi-language: EN-GB;">This
hypothesis of Dr. Olbers will answer another purpose. It was observed that the
law proposed by Titius was deficient before the discovery of a planet between
Mars and Jupiter. The knowledge that four planets exist there would be equally
fatal to this law but, according to Dr. Olbers, they are the remains of an
original one and, if we suppose as is most probable that this was situated at
the mean distance of all its parts, it will coincide very well with the law
alluded to. From the knowledge we possess of the elements of the three planets
already noticed it appears that they may at some future period come into
contact with each other and that in preceding ages they might have done so before.
If we were more completely acquainted with their motions it might be possible
to assign with some considerable degree of probability the epoch of the
original catastrophe.</span></div>
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<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>In pursuing his hypothesis Dr.
Olbers considered that the orbits of these fragments possessed of different
inclinations ought to cut each other in two opposite points of the heavens
which would be the common intersection of them all. He thought that if we
wished to discover the other scattered fragments of the planet that we should direct
our attention to these two points. According to observations on the course of
Ceres and Pallas and from calculations on the inclinations of their orbits it
was found that one of these points is situated in the constellation of the
Virgin and the other towards that of the Ram.</span></div>
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<span lang="EN-GB" style="mso-ansi-language: EN-GB;">The
discovery of Pallas in the first of these points and that of Juno in the second
seemed to confirm this ingenious hypothesis and determined Dr. Olbers in his
resolution of seeking for some new planet. He resolved therefore three times
annually to pass in review all the small stars which compose the opposite
constellations of the Virgin and the Ram. Fortune favoured this project and on
the 29th March 1807 he discovered in the northern wing of the Virgin a small
unknown star whose motion from day to day was very perceptible and it was
immediately placed in the rank of planets. She appeared to shine with a pure
white light and to be surrounded by a thinner atmosphere than those of her
colder sisters Ceres, Pallas and Juno. Vesta is the name assigned to this new
planet which soon occupied the attention of the principal observers in Europe.
It completes its revolution round the Sun in 1,335.2 days and its mean distance
from the Sun is nearly 24 if that of the Earth be considered as represented by
ten. The eccentricity of its orbit is considerably less than that of the other
recently discovered planets and the plane of its orbit is inclined to the
ecliptic only at an angle of 7 degrees. It results from the values which have
been assigned to its elements that Vesta is about 36,000,000 miles nearer the
Sun than Ceres, Pallas and Juno; that the inclination of her orbit is not much
greater than that of Mercury and that its eccentricity is nearly equal to that
of Mars.</span></div>
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<span lang="EN-GB" style="mso-ansi-language: EN-GB;">From
these causes it appears that she must be much less exposed to perturbations
from the action of Jupiter and that they will be more easy to calculate. In
fact Gauss, having compared his calculations with 22 observations of the
astronomer Bouvard, found that they differed only 17 minutes of arc. This is
certainly a wonderful degree of precision considering the shortness of the time
the planet had been discovered.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>The examination by Dr. Olbers of the
stars in the constellation of the Virgin and the Ram were crowned by a fortunate
result. It must, however, be observed that the intersections of the orbit of
Ceres and Pallas and the new planet have not that precise coincidence which was
expected. They are separated by an angular distance of about 20 degrees. This
however is too small a quantity to afford an argument against the hypothesis of
Dr. Olbers.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>The four planets we have just
considered present a singular spectacle in the system of the world, differing
from all the other. They have among themselves many points of resemblance and
appear associated by nature to the destinies. Collectively they fill up the
vacancy which was thought to exist between Mars and Jupiter. Placed at a mean
distance between these two planets they describe orbits of nearly equal
magnitude and move with nearly an equal pace.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Several
philosophers at first refused to bestow the name of planets on these stars
whose discovery signalised the commencement of the 19th century. The principal
reason was their extreme smallness which might make these bodies be regarded as
of an inferior order. Dr. Herschel proposed to distinguish them by the name
asteroids. But these bodies revolve round [the] Sun as well as the others; like
them their elliptical orbit is but little elongated. They are scarcely smaller
when compared to Mercury than that body is in respect to Jupiter. The
magnitudes of the planets are subject to no law: they have no relation to their
distance from the Sun. Mars is further distant than the Earth yet it is
smaller. Jupiter is much greater than Saturn though this latter body is most
remote.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>It has been objected that these
planets are without the limits of the ancient Zodiac, but the bounds of this
Zodiac were fixed principally on account of Venus. They would have been much
less if this planet had been unknown. It may therefore be extended at will from
one pole to the other, that is to say they are artificial limits of no real
utility and may be abandoned altogether. In fact there is no reason why there
may not exist in the heavens planets whose orbits cut the ecliptic at right
angles as the equator of Venus nearly does and as the satellites of Uranus
actually do. We should beware of establishing from partial observations
arbitrary laws which future discoveries may oblige us to abrogate.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">The
discussion is however merely verbal and could not long engage the attention of
astronomers. The denomination of planet is now universally applied to designate
these newly discovered bodies and also all of a similar nature which may be
hereafter found.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>The hypothesis of Dr. Olbers
respecting the formation of these planets by the explosion of a former one has
been considered with some attention by Lagrange whose numerous investigations
have contributed much to the splendid progress of Physical Astronomy. he calculated
the force necessary to project a body from our Earth so that it should revolve
in a very eccentric orbit and become a comet, and found that if a body could be
projected with a velocity about a 120 times as great as that of a cannon ball
it would quit this globe and revolve in an elongated ellipse round the Sun.
Applying similar principles to the case of other planets he found that if a
large planet had existed between Jupiter and Mars and if by some internal cause
it should be torn asunder, its parts might form small planets and circulate
round the Sun in nearly circular orbits provided that at their first projection
they moved with a velocity only about 20 times greater than that of a cannon
ball. From these calculations it results not only that the hypothesis is a
possible one but also since the power required to produce the effect is not
exhorbitant it receives from them a certain degree of probability. It appears
that the only method of increasing the evidence on which it rests would be by
discovering other similar bodies whose orbits intersect those of the small
planets already known nearly in the two points before alluded to. Should this
ever be the case it will indeed afford us satisfactory evidence and in fact the
only kind which the subject admits.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">It has
already been observed that these new planets are by far the smallest of any we
are acquainted with and which revolve as primaries round the Sun. Of their
magnitude different opinions have been entertained. Schroeter of Liebenthal
whose observations have acquired deserved reputation estimated their apparent
diameter at from 2 to 5 seconds of arc.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>This however differs widely from the
opinion of Dr. Herschel who undertook a series of experiments of a curious
nature purposely with a view to ascertain the diameter of these objects. He
found that their extreme smallness rendered the common methods inapplicable and
therefore resorted to others of his own invention. Having heated some sealing
wax and drawn it out into small threads he passed the ends of them through the
flame of a candle. They consequently had at the end of each thread a small
round globule of wax. It was now necessary to measure the diameter of these
balls and this was accomplished by means of a solar microscope which projected
their images on a sheet of paper and their size was thus ascertained with great
accuracy. A row of these waxen balls thus arranged was placed on a card at the
distance of 7 or 8 hundred feet and viewed with a telescope. By knowing the
distance at which they were placed and their real diameters it was easy to
calculate the angles under which they would be seen.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Dr.
Herschel examined them attentively with different magnifying powers. For
instance with a telescope magnifying 150 [times] he could perceive a globule
subtending only an angle of<span style="mso-spacerun: yes;"> </span>part of a
second in diameter. (It results from this that Ceres is about 161 miles in
diameter and Pallas 147 according to greatest extent or 40 times smaller than
the Moon.)</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>For the discovery of the planetary atoms
and of numerous other bodies with which we are acquainted the unassisted eye
would be utterly incompetent. It is only by the aid of instruments of a most
powerful kind that the observations which I have had occasion to notice can be
repeated. They are generally carried on by means of the reflecting telescope.
Some account therefore of an instrument which has contributed so much to extend
our acquaintance with the magnitude of the Creation may not be uninteresting.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>The first idea of the reflecting telescope
was undoubtedly entertained by Mersennus and communicated by him to Descartes.
But this philosopher rejected the idea and endeavoured to convince his friend
of the impossibility of effecting it. Some years after Gregory a young man of
uncommon genius was led to the invention by seeking to correct two
imperfections of the common telescope. The first was its too great length which
made it unmanageable; the other was the incorrectness of the image it produced.
These inconveniences he imagined might be obviated by substituting for the
object glass a metallic speculum of a parabolic figure to receive the image and
afterwards reflect it to a small speculum of the same metal. This was again to
return the image to an eye glass placed behind the great mirror which for this
purpose [was] to be perforated in the centre.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">This
construction was published in a work entitled Optica Promota in 1660, a work
which in every respect does credit to the talents of its author. But Gregory
was he himself declares possessed of no mechanical dexterity, nor could he find
any workman capable of realising his invention. And after some fruitless
attempts he was obliged to give up the pursuit and probably had not some new
discoveries in light and colours been made a reflecting telescope would never
more have been thought of, particularly if we consider the difficulty of
execution and the little advantage that would accrue from it according to the
principles of optics at that time known.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>But Newton whose happy genius for
experimental knowledge was equal to that for geometry and to these talents, in
a supreme degree joined patience and mechanical abilities, fortunately
interposed and saved this noble invention from perishing in its infant state.
He also had employed himself at an early period in his life in endeavouring to
improve the telescope but imagining that neither Gregory's specula were neither
very necessary nor yet easily to be executed he turned his attention towards
improving the common telescope. While he was thus employed about three years
after the publication of Gregory's book he made that celebrated discovery of
the refrangibility of different rays of light. This convinced him of the great
errors of refracting telescopes and forced him as it were to turn his thoughts
towards reflectors. In a letter to Mr. Oldenburg he observes, "I
understood that by their mediation optical instruments might be brought to any
degree of perfection imaginable, provided a reflecting substance could be found
which would polish as finely as glass transmits and provided also the art of
communicating to it a parabolic figure could be obtained. "Amidst these
thoughts" he adds, "I was forced from Cambridge by the intervening
plague and it was more than two years before I proceeded further." It was
not until the end of 1668 or the beginning of the next year that Newton
returned to his studies and not relying on any artificer for making his specula
he began the work himself. Early in 1672 he completed two small telescopes. One
of these he sent to the Royal Society and in the letter which accompanied it he
writes that though he then despaired at attaining the parabolic figure by
geometrical rules he doubted not that it might in some measure be accomplished
by mechanical devices.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">But,
though the invention was admirable and the theory perfect, the discoveries,
even of Newton, were not exempted from the general fatality which so frequently
attends great and useful inventions, that of making a slow and vexatious
progress to their authors. The fact is that, excepting an unsuccessful attempt
made by the Royal Society by employing an artificer to imitate the Newtonian
construction, no reflector was heard of for nearly half a century. But when
that period had elapsed, a reflecting telescope was at last produced to the
world of the Newtonian construction, which the venerable author, ere yet he had
finished his much distinguished course,<span style="mso-spacerun: yes;">
</span>had the satisfaction to find executed in such a manner as left no room
to fear that the invention would continue longer in obscurity. This memorable
event was owing to the genius, dexterity and application of Mr. Hadley, the
inventor of the reflecting quadrant.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">The two
telescopes which Newton had made were 6 inches long and were held in the hand
for viewing objects. hey were equal in power to about a 6 foot refractor,
whereas Hadley's was above 5 foot long, was provided with a well-contrived
apparatus for managing it and was equal in power to the celebrated aerial
telescope of Huygens of 123 feet in length. Mr. Hadley very liberally
communicated to others the results of his experience in the construction of
these instruments. About 1734 Mr. James Short of Edinburgh signalised himself
by the construction of some excellent reflecting telescopes. Since that time
there have been several improvements in the modes of giving them the parabolic
figure and also in the composition of the metal made use of for the speculum,
but until Dr. Herschel turned his attention to this subject the highest
magnifying power which was usually given to telescopes did not exceed a few
hundred times. The reason of this deficiency arose from the difficulty of
giving to the mirror the requisite parabolic figure.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">In
viewing heavenly bodies mere magnifying power is not always most favourable for
observation. Dr. Herschel found that with different telescopes of the same
magnifying power am object did not appear equally distinct. On enquiring into
the cause he found that there was another property possessed by telescopes
which is totally different from and sometimes interferes with the magnifying
power. To this it is necessary to attend in the construction of telescopes and
he designates it the power of penetrating into space. It depends on the
magnitude of the polished surface.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>A striking instance of the
difference of these two powers occurred on the erection of a 20 foot telescope
image. One of its effects was that when towards evening on account of the
darkness the natural eye could not penetrate far into space, the telescope
possessed that power sufficiently to show, by the dial of a distant church
steeple what o'clock it was, notwithstanding the naked eye could no longer see
the steeple itself. Here it is clear there was a greater penetrating power; for
though it might require magnifying power to see the figure of the dial it could
require none to see the steeple.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">It is
this power which is so necessary in resolving nebula into stars and it is on
this account principally that Dr. Herschel's large telescope of 40 ft. in
length is so powerful an instrument. The smaller telescopes will bear a
magnifying power of several thousand. Indeed some of his 7 ft. reflectors will
magnify 5 or 6,000 times, which is nearly a big a power as has been used in the
larger ones. Their powers of penetrating into space are however very different.
If that of a 7 ft. telescope be described by 20 the power of the 40 ft. will be
represented by 190. These powers are both susceptible of increase but not to an
unlimited extent. With respect to magnifying power Dr. Herschel thinks a
telescope of 25 ft. in length may possibly admit of high a power as the nature
of our atmosphere will admit. Many of his observations particularly on double
and triple stars were made with powers of 4, 5 or even 6,000. The highest
amplificative power which he has yet made use of is 7,200, but it is only at
very favourable opportunities that such powers can be used.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">The
penetrating power of telescopes the same skilful observer thinks can be
increased. Yet this has its limits which are perhaps more easily found. The
greatest penetrating power which it would perhaps be possible to give it would
be about 500. This would be about 3 times that of the 40 foot telescope and the
diameter of the polished surface required for this purpose would be 10 ft 6
ins..</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>Besides the two already mentioned
there are several extraneous causes which modify the action of telescopes.
These arise chiefly from the various states of the atmosphere and present some
unexpected phenomena. A moist state of the atmosphere is generally favourable
to observers even if it is so excessive that the vapour condenses and runs from
the tube of the telescope. Wind produces a curious effect. It increases the
diameter of stars. They all appear like planets. It is otherwise unfavourable.
Clouds produce a contrary effect. If they gradually intervene the stars
diminish in size and at last become invisible. It is rather singular that a fog
which prevents objects being visible at the distance of 40 feet should yet
permit excellent observations to the telescope. Frost is not a hindrance. In
some cases Dr. Herschel found his feet frozen to the ground while he was making
some very favourable observations. And in January 1783 we find in an extract in
his journal, "I made a number of very delicate observations yet at four in
the morning my ink was frozen and at 5 the frost was so intense that a speculum
in the tube of my 20 ft. telescope we off with a crack and broke into two
pieces".</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Generally
speaking a calm state of the atmosphere undisturbed by any motion in the air is
most favourable for telescopes of large power and aperture. These occur but
rarely and it is calculated that there are not perhaps more than 100 hours
occur in the course of a twelve month which are favourable for observations
with Dr. Herschel's 40 ft. reflector.</span></div>
<span lang="EN-GB" style="font-family: "Arial Black"; font-size: 14.0pt; mso-ansi-language: EN-GB; mso-bidi-font-family: "Times New Roman"; mso-bidi-font-size: 10.0pt; mso-bidi-language: AR-SA; mso-fareast-font-family: "Times New Roman"; mso-fareast-language: EN-US;"><br clear="all" style="page-break-before: always;" />
</span>Unknownnoreply@blogger.comtag:blogger.com,1999:blog-1290567400778113129.post-45409777898887166722013-11-11T10:16:00.001+00:002013-11-12T00:07:22.107+00:00Lecture 7: The Inner Planets<h1>
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Lecture
7</span></h1>
<div class="separator" style="clear: both; text-align: center;">
<a href="http://images.nationalgeographic.com/wpf/media-live/photos/000/543/cache/venus-transit-preview-historical_54375_600x450.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="263" src="http://images.nationalgeographic.com/wpf/media-live/photos/000/543/cache/venus-transit-preview-historical_54375_600x450.jpg" width="320" /></a></div>
<h1>
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"> </span></h1>
<h1>
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">The Inner Planets</span></h1>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Synopsis
of lecture:-</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Observations
on the planets: Mercury, Venus, Transits of Venus and Mars.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">[script
of lecture missing]</span></div>
Unknownnoreply@blogger.comtag:blogger.com,1999:blog-1290567400778113129.post-84841144703009925702013-11-11T10:07:00.004+00:002013-11-11T18:03:02.894+00:00Lecture 6: On the Sun<h1>
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Lecture
6</span></h1>
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://upload.wikimedia.org/wikipedia/commons/thumb/e/eb/J%C3%A9r%C3%B4me_Lalande.jpg/196px-J%C3%A9r%C3%B4me_Lalande.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" src="https://upload.wikimedia.org/wikipedia/commons/thumb/e/eb/J%C3%A9r%C3%B4me_Lalande.jpg/196px-J%C3%A9r%C3%B4me_Lalande.jpg" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">Lalande</td></tr>
</tbody></table>
<h1>
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"> </span></h1>
<h1>
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"> </span></h1>
<h1>
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">On the Sun</span></h1>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>All the heavenly bodies participate
in the apparent motion which arises from the diurnal rotation of the Earth, but
several of them possess a proper motion of their own. The quickness of this
change of place is most evident in the Moon, a body which we have already
considered. It is these proper motions which it is most interesting to follow,
because it is from them alone we can derive a complete knowledge of the systems
of the world. In discovering the distance of a terrestrial object we observe it
in two different positions. The same principle must be applied to the heavens.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>In our attempts at discovering the
laws of Nature we must observe her in various points of view and detect these
laws from the change of appearance she presents to us. There is, however, this
striking difference between the philosophical enquiries of Astronomy and those
of any other science. In the various branches of natural philosophy we may vary
the phenomena presented to us by means of experiments and thus put our theories
to the test; but the case is widely different in Astronomy: Nature presents the
phenomena and the skill and industry of Man must be directed to search for
those situations from which he may most easily discover the chain of cause by
which she operates.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Of all
the heavenly bodies which appear to have a proper motion of their own the most
brilliant and remarkable is the Sun. Its proper motion is in a direction
contrary to the daily motion of the Earth. This will be evident from the
appearance of the heavens during the night which changes and is renewed with
the seasons.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>The stars which are situated in the
path of the Sun and which set a little time after him soon are lost in his
light, and at length reappear before his rising. The Sun therefore advances
towards them in a direction contrary to his diurnal motion. It was by this
means that his proper motion was examined by the ancients, but at present it is
determined with much more accuracy by observing every day his altitude on the
meridian and the time which elapses between his passage and that of some known
stars across the meridian.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>By these means it has been found
that the Sun appears to move in an orbit round the Earth, that this orbit is
not in the plane of the equator but inclined to it at an angle of about 23
degrees 28 minutes. The path in which the Sun moves is called the ecliptic.
This path intersects the equator in two points; these are named the equinoxes.
The reason is that, when the Sun is situated in either of them, it appears by
the daily motion of the Earth to move in the equator. And, as the horizon of
every place divides the equator into two equal parts, the day will then be equal
to the night in every part of the Earth. The Sun now advances from the equinox
of Spring and its altitude on the meridian increases daily. The arc it
describes continually augments and increases the length of day, until it
arrives at its greatest altitude. At this period of year the days are longest,
and the situation at which the Sun has arrived is named the summer solstice.
The reason of this name is that during several days the Sun preserves nearly
the same meridianal altitude and seems therefore in a manner stationary. The
circle which the Sun appears to describe is called the tropic of summer. There
is a similar one represented on the globe.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">It now
redescends to the equator and continuing its course it arrives at the winter
solstice, where its altitude is least. This is the shortest day of the year.
When the Sun has arrived at this term, it again ascends and returns to the
vernal equinox. Such is his constant progress and that of the seasons. The
spring is comprised between the vernal equinox and that point of the ecliptic
where the sun appears stationary. The interval between this point and the
autumnal equinox forms the summer. Autumn is the period which elapses between
this equinox and the winter solstice, and the remaining part of the year,
between the winter solstice and the vernal equinox, constitutes winter.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>As the presence of the Sun above the
horizon causes heat, it might perhaps be imagined that the temperature would be
the same during the spring as in the summer, and in autumn the same as in winter.
But temperature is not the instantaneous effect of the presence of the Sun, it
is the consequence of its continued action. During the course of a day it does
not produce the greatest effect until a considerable time after it has arrived
at it highest altitude. The different degrees of heat in summer and in winter
do not then arise entirely from the different times the Sun is above the
horizon. The direction and force of the solar rays have a considerable
influence.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">During
the short days the effect of the Sun is less both with respect to the intensity
of the Sun's rays, their direction and the time of their continuance; and
during the long days it is greater in all these respects. It may also be
remarked that the severest frosts usually take place after the days have begun
to lengthen, and the most oppressive heat prevails when the days are
decreasing: the reason of which is, that during the summer months the Earth,
having imbibed more heat than it gave out, is not exhausted of its
superabundant warmth until towards the close of the year. In a similar manner
the waste of the Earth's heat being greater in winter than its supply, it
continues to get colder and colder until a month or longer after the winter
solstice. From a similar cause arises the difference between spring and autumn
though the position of the Earth is the same in both.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">That
remarkable variety of seasons which takes place in the various parts of the
world is owing to the different elevation of the pole in the different
climates. To a spectator situated on the equator the poles are on the horizon.
The day is consequently always equal to the night and at the two equinoxes the
Sun passes through the zenith of the place, and the least meridian altitude of
the Sun happens in the solstices. In this situation a curious circumstance
takes place. On the 21 June and 21 December the shadow of an upright rod will
fall in two opposite directions. On one of those days it will be directed
toward the north on the other to the south point.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>This is a circumstance which can
never take place in our climate. At noon the shadow will always be directed to
the north. The inhabitants of the equator enjoy two summers and two winters
every year. And the same happens to all those countries where the height of the
pole is less than the obliquity of the ecliptic. Beyond this limit there is
only one summer and one winter in every year. As we approach the pole the
longest day in summer augments and the shortest in winter decreases. To the
inhabitants at the polar circle on the 21 June, which is the summer solstice,
the Sun never sets and similarly, on the day of the summer solstice, it never
rises. As we approach still nearer the poles the continuance of the Sun above
the horizon becomes longer it exceeds several days and even weeks. At the pole
itself it rises and continues to illuminate it for six months. It then sets and
is invisible for an equal length of time.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">By
observations on the Sun, that is by comparing its place with that of known
stars, it is found that it moves faster in winter than in the summer months.
This variation in its motion is unequal. As in the case of the Moon we found
that the apparent diameter is variable. It is natural to examine whether the
same does not take place with respect to the Sun. On applying a micrometer to
this luminary we find that it appears larger in winter than in summer, that is,
when the velocity of the Sun is greatest, its diameter is largest and
conversely, when the Sun moves slowest, its diameter is least. From this we immediately
infer that the Sun is at a greater distance from our Earth in the summer when
she moves slowest than she is during the winter months when her apparent
diameter is greatest.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>The apparent diameter of the Sun
only informs us by its changes of the relative variation of our distance from
that body. What may be our real distance becomes an object of considerable
interest. The methods of ascertaining this have been successively improved, but
the extreme accuracy with which the observation ought to be made and the
minuteness of the quantities to be determined will probably, for a long time,
render the result in a measure uncertain. The easiest method of finding our
distance from the Sun is by employing a method similar to that by which we
measure distances on the Earth.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">It has
been shown that, if at the two ends of a line of known length we measure the
angle formed by a distant object, we may, by calculation, determine the
distance of this latter object. To apply this to our case let us suppose two
observers, one situated in the northern hemisphere the other on the same
meridian on the southern. If, at the same instant, they observe the altitude of
the Sun's centre, it is obvious we shall have the two angles required. From
these observations we may determine the angle under which the Earth's diameter
would appear to a spectator at the Sun. It is this angle which is called
parallax, but it is too small a quantity to be determined by this method. All
that we can learn from it is a negative result. We may infer that the Sun
cannot be nearer to the Earth than 6,000 of its diameters, but how much farther
off that luminary [we] may be situated we cannot from this method determine.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>Astronomy, however, affords other
means of ascertaining the Sun's parallax which will be hereafter noticed. The
most successful is the transit of an inferior planet over the disc of the Sun.
From this it would result that the Earth is situated at rather less than
95,000,000 miles distant from the Sun. This is determined from the Sun's horizontal
parallax, which is very nearly 8 seconds [of arc], and we may form some idea of
the investigation when it is considered that an error of 1 second in this small
quantity would produce an error of many millions of miles in our estimation of
the Sun's distance.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">On the
surface of the Sun black spots are frequently observed. Their number, position
and magnitude are variable. They are sometimes very numerous and of
considerable extent. Some have been observed which have been four or five times
large as the Earth's diameter. Sometimes, though rarely, they have disappeared
altogether for a few years. The solar spots are almost always surrounded by a
penumbra, which is enclosed in a luminous cloud, which is almost always more
brilliant than the rest of the Sun's body. Whatever may be the nature of these
spots it is sufficient for our present purpose that they preserve their
relative situation on the Sun's surface, and though variable they continue
fixed for a few months or perhaps years.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>It is by means of these spots that
the astronomer ascertains that the Sun, like the Earth and the Moon, revolves
on its axis. This immense body, which is at least 1,300,000 times larger [in
volume] than the Earth, turns on its axis. From accurate observations on the
returns of the spots the time which this revolution occupies is about 25 days
and a half. This revolution might appear slow, but when we take into
consideration the immense magnitude of the Sun it will be found that its
equatorial parts are moving at the rate of 5,000 miles in an hour. The axis
round which the Sun turns is not exactly perpendicular to the plane in which
the Earth moves. It is inclined at a small angle, about 7<span style="mso-spacerun: yes;"> </span>degrees.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">There is
a curious remark relative to the solar spots already alluded to. They are
almost all contained in a zone which extends about 30 degrees on each side of
the equator. It is but very rarely that they have reached as far as 40 degrees.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>The intensity of the Sun's light is
not the same on every part of his disc. It ought according to theory to be much
brighter towards the border than in the centre of the Sun, but the reverse is
the fact. This has been shown by the experiments of Bouguer. He admitted rays
of light from the Sun into a darkened room, but, as their light would be still
too intense he diminished it, by causing it to pass through a concave lens. It
now became weakened and could readily be compared to the light of a wax taper.
He repeated these experiments with rays from various points of the Sun's disc
and compared them with the light of the Moon. He came to the conclusions that
the light of the Sun is 300,000 times as great as that of the Moon, that some
parts of the Moon's disc are 3 times as bright as others and that the centre of
the Sun is considerably more brilliant than the edge. The light of the Sun's
border must therefore be obscured or extinguished in a certain degree and it
becomes interesting to enquire into the cause. The only one which affords a
complete explanation is that the Sun is surrounded by a very dense atmosphere;
the rays which issue from his limb are obliged to traverse a more considerable
portion of this atmosphere than those which emanate from his centre.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">It
appears from this that there is a very great probability of the Sun's
possessing an atmosphere. There is indeed another phenomenon which has been
explained<span style="mso-spacerun: yes;"> </span>on the hypothesis of its
existence and which would assign to it very considerable limits. I refer to
that faint light which is sometimes visible a little before the rising and
after the setting of the Sun, particularly in the spring and which is called
the zodiacal light. It appears generally at the end of winter and in the
beginning of spring after sunset and it appears before sunrise in autumn and at
the commencement of winter. It resembles in form a pyramid lying lengthways
along the zodiac, its base being placed obliquely in respect to the horizon.
This phenomenon was first noticed by Cassini the Elder, in 1683. The zodiacal
light is, according to Mairan, the solar atmosphere. It is a rare and subtile
fluid, either luminous itself or made so by the rays of the Sun. He supposes it
to surround the Sun but in greater quantity and more extensively about his
equator than at any other part.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">The
length of the zodiacal light varies, sometimes in reality sometimes only in
appearance. Its length is seldom less than 60 degrees nor its breadth more than
20, but it has been observed to extend to 100 or 103 degrees, and then its
breadth was only 8 or 9 [degrees]. It has, however, been suggested that this
phenomenon is not owing to the atmosphere of the Sun, but that this body is the
fountain of electricity, which<span style="mso-spacerun: yes;"> </span>is
thrown off from the equatorial parts from the rapidity of its rotation on its
axis, and, it is added, as a probable conjecture, that the zodiacal light, the
tails of comets, as well as the aurora borealis and lightning<span style="mso-spacerun: yes;"> </span>are only its various and not dissimilar
modifications.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>The succession of our own ideas or
of external objects present to us the notion of time. We conceive time, or absolute
time, to flow uniformly in an unchangeable course. This alone measures the
changing of all other things and unless we apply to the common measures of
time, which are gross and inaccurate, some proper corrections, the conclusions
we arrive at are found to be erroneous. It becomes then necessary to fix on
some natural object whose motion is conspicuous and nearly uniform, or some
artificial contrivance to determine this measure. Of the first kind the Sun has
been chosen and of the latter, a pendulum clock, but as the motion of the Sun
is not uniform and as a clock is liable to variations from heat and cold and
other accidental causes, it becomes desirable to establish some mode of
comparison by which we may gain a correct measure of time. This is best
effected by tracing out the irregularities of the Sun's motion.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">The
astronomical day at any place begins when the Sun's centre is on the meridian
of that place. It is divided into 24 hours, which are reckoned from one to 24.
The interval of time between two successive transits of the Sun over the same
meridian is called a solar or astronomical day.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>At the end of a diurnal revolution
of the Earth which is known to be a uniform motion, the same star will come to
the meridian that transited it at the preceding noon, but the Sun during this
period has moved from that star to another which has greater Right Ascension.
Therefore, before the Sun can be again on the meridian, the Earth must have
described this additional arc. This may, perhaps, be illustrated by considering
the hands of a clock. At midday the two hands are both together and point to
the hour of twelve. After one revolution of the minute hand they are not again
together for the hour hand will have advanced about 5 minutes and will point to
one. The minute hand must therefore move over more than a whole circle before
the two are in the same relative position. The apparent motion of the Sun is
unequal in different parts of the year. A dial therefore which measures time by
the Sun's motion will measure it unequally. Hence there ought to be a
difference between the hour indicated by a sun dial and that by a good clock.
This difference is called the equation of time. The two principal causes of
this equation of time are the inclination of the Earth's orbit to the equator.
This is generally called the inclination of the ecliptic. And the other cause
is the unequal motion of the Sun in its orbit.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">We have
hitherto only considered this question on the hypothesis of the Sun revolving
round the Earth, but all the appearances we have described are equally capable
of explanation by supposing the Sun at rest and the Earth revolving round it.
It remains then to enquire which is the fact in Nature. A similar reason has
already occurred with respect to the question whether the heavens revolve
around the Earth at rest or whether this latter body moves on its axis in the
contrary direction. The appearances are the same in both cases, but we have
decided both from reason and experiment that the latter is the true theory.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>In the case of the annual motion of
the Earth we must depend on arriving at a solution rather from reason and
analogy than from any direct experiment. The following are the considerations
which lead us to adopt this conclusion. The mass of the Sun is immensely
greater than that of the Earth. Is it not therefore more simple to make the
latter body revolve round the Sun than to put the whole solar system in motion
round the Earth?</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Besides
this latter supposition involves a physical impossibility. It is known from the
laws to which matter is subject that, when two bodies acting on each other
revolve round their common centre of gravity, it is physically impossible that
the larger can revolve round the smaller one at rest. This is precisely the
case with the Sun and in physical point of view decides the question, but it
may nevertheless be useful to state a few of the other arguments by which this
important point is supported. The analogy of the Earth compared with other
planets strongly confirms it. They all revolve on their axis and several have
satellites or moons. An observer situated on any one of them would have just as
much and, in some cases, a greater right to consider himself the centre of the
system, and to view the Sun as a dependent body revolving round his world for
the purpose of affording him light. Shall we then presume that what would be an
illusion in each of these worlds would be a fact in ours? There is no apparent
difference to justify such an inference. These bodies, like our own globe, shine
with a borrowed light. The Sun alone is the source of light, the largest body
in the system. Shall we then refuse to consider him in its centre? If we
conceive ourselves for a moment transported to the Sun, this arrangement will
restore order and harmony to the system. The annual motion of the Earth will
form no exception to the general laws of Nature.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">I shall
only state one other fact which clearly proves the point in question. This
arises from the progressive motion of light. About the close of the 17th
century Roemer discovered that the propagation of light is not instantaneous,
but that it takes about 8 minutes in passing from the Earth to the Sun. In
consequence of this when we observe an eclipse of Jupiter's satellites, if this
planet is in opposition, it will appear to begin sooner, and, if he is in
conjunction, it will seem to commence later than it ought by calculation. This
and many other phenomena are all explained by supposing the Earth in motion
round the Sun. It is therefore strong presumptive evidence of this fact. It
appears then that the Earth revolves around the Sun once in about 365 days and
on its own axis once in 24 hours. The knowledge of these important truths were
not the result of the labours of one individual. Their perfect demonstration
was the united work of ages.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Man
ignorant of the true constitution of the Universe and seduced by self-love and
by the illusion of his senses for a long period considered himself as placed in
the centre of the movements of the heavenly bodies, and his pride was
sufficiently punished by the vain terrors with which they inspired him. In
throwing aside the veil, which concealed from him the true system of the world,
he perceived himself far from the centre of the Universe, placed on a planet
almost imperceptible in the vast extent of the Solar System, which itself is
but a point in the immensity of space. But the sublime knowledge at which he
has arrived concerning these grand and interesting objects may well console him
for the scanty limits he occupies in the scale of Nature.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>We have already observed that that
elevation of water, called a tide, is caused by the attraction of the Moon
acting on the water of the ocean, and that this is augmented and diminished at
different times by the action of the Sun. The atmosphere which surrounds our
globe being a fluid body might be supposed to be acted upon by the same
influence, and the consequence would be winds and variations in the height of
the barometer, which would recur at the same intervals as the tides do in the
sea. This we should expect from theory, which at the same time informs us that
the variations of the barometer, even in the most favourable situations, such
as at the equator, must be very small. The greatest is calculated at ?th part
of an inch [of Hg]. It is evident that this very minute quantity cannot always
be observed when the variations from other causes may produce a difference of
several inches. The Sun then, on account of its attraction, has but a small
effect in producing winds. On another account, namely by the heat which it
affords, its effect is much more considerable.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Several
philosophers, amongst whom we meet with the name of Descartes, have ascribed
the general winds to the diurnal motion of the Earth. They contend that, as the
Earth turns eastward, the particles of air being very light are left behind, so
that, in respect of the Earth's surface, they move in a contrary direction and
thus become an easterly wind, which is generally the case near the equator. It
is sufficient for the refutation of this hypothesis to observe that it
contradicts the laws of mechanics, and that the air being attracted to the
Earth by gravity would, in a short time, acquire the same velocity.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>Dr. Halley, dissatisfied with this
explanation, substituted another cause capable of producing the same effect and
more consistent with the known laws of the motion of fluid bodies. He
attributes the trade winds to the action of the Sun's rays heating the air.
Thus the air being rarefied by heat must become lighter and ascend, and fresh
air must rush in to supply its place and bring the whole to an equilibrium. But
from the motion of the Sun an equilibrium cannot take place. There will
therefore be a constant current produced. By combining this cause with the variations
which must arise from the situation of continents and chains of mountains Dr.
Halley explained many of the regular winds which blow periodically.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">From the
action of the Sun the air will be heated and consequently ascend. There will
therefore ensue two currents in the upper regions of the atmosphere. There will
be a stream from the equator to the poles, and in the lower strata near the
Earth's surface there will be a contrary current from the pole to the equator.
This combined with the motion of the Earth on its axis will produce a wind from
west to east, which is in fact the direction of the trade winds.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>The density of the Sun, which is
about<span style="mso-spacerun: yes;"> </span>part of that of the Earth,
indicates that this body consists of something more solid than a simple flame.
If we admit the ideas of Buffon concerning the formation of the planets, we
should consider it a body exactly similar to our Earth, only in a state of
fusion. This celebrated naturalist has explained at considerable length an
hypothesis, according to which the planets primitively constituted a part of
the Sun. According to this theory a comet moving with immense velocity grazed
along the surface of the Sun and tore from it and projected to a distance some
portions of its body. These being in a state of fusion by the mutual attraction
of their particles formed themselves into globes which continued to circulate
round the Sun, from the combined effect of their centripetal and centrifugal
forces. Some detached parts of the largest masses formed the satellites, which
circulate round them. These bodies gradually parted with the heat that kept
them melted. The smallest bodies cooled soonest and the larger masses more
slowly. Buffon even attempted to calculate the time they would require to cool,
by comparing it with that in which known weights of red-hot iron required for
the same purpose. He found that the Moon and other satellites were already
cooled below the temperature of ice.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">That the
Earth must have required 75,000 years to arrive at her present temperature, and
that she would continue insensibly to lose her heat, and that, at the end of
about 93,000 years more, the zones of ice at the poles would increase to the
equator and thus put an end to the human species. Such is the ingenious fiction
of this celebrated naturalist. It is more to the credit of his imagination than
to his acquaintance with the mechanical philosophy. It will be sufficient to
mention one insurmountable objection to this theory. It is known from
mechanical principles that, if this had been the origin of the planets, they
must all have returned nearly to the Sun's surface once in each revolution. But
this is far from the case: they move in ellipses of very small eccentricity.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>The phenomenon of the solar spots
are a subject on which astronomers have entertained various opinions. In
considering the Sun as an immense ocean of melted fluid it was natural to
regard the spots as the scoria floating on the matter in fusion. But it may be
observed that we have no proof of the Sun's being a body in such a state.
Granting however that it were what causes the formation of new scoria. For this
purpose we must suppose new bodies to have fallen onto the Sun, for we well
know that, when a pot of melted metal has been sufficiently purified by fusion,
no new scoria arises unless from the accession of fresh matter which finds a
difficulty in uniting with the former. Perhaps, however, it may be said that
some comet has fallen into the Sun and caused these spots, but this is a mere
conjecture and can but rarely happen, besides the cause, if it ever happens,
would not answer the effect.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Another
theory which has met with several supporters considers the Sun as a solid
nucleus covered with a melted fluid, that from certain causes this fluid is
subject to several motions of flux and reflux, and that in consequence of these
motions the mountains and other asperities with which this solid nucleus is
covered are sometimes left exposed by the recess of the fluid, and at other
times covered by its return. This will account for the successive appearance
and disappearance of spots in precisely the same place, but this system, though
more probable than the former, is founded on arbitrary suppositions and is
contrary to observation.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>In fact, if a solar spot were a mountain
of the solid nucleus elevated above the melted matter, it would, when it
arrives near the edge of the Sun, form a new kind of dark projection. This,
however, very rarely happens. On the contrary a spot, as it approaches the
Sun's border, becomes narrower and at length disappears. This clearly indicates
that it is not an elevation above the Sun's surface.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Lalande
made a great many observations on the solar spots and concludes that they
result from the scoria attaching themselves to the summits of the mountains.
This was published in the Memoirs of the French Academy in the years 1776 and
1778. Nearly at the same time appeared in the Philosophical Transactions a very
different hypothesis. In this the Sun was regarded as a solid nucleus covered
with a semi-fluid ignited substance. The cause of the appearance of the spots
we supposed to be owing to the effects of volcanoes, which by their action
drove aside the semi-fluid matter and exposed to view the dark nucleus. After
the eruption the half-melted matter gradually subsided into the vacant space
and thus the spot disappeared until a new eruption should again produce the
same effect.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>There is, in this theory of the
cause of the spots on the Sun's disc, an appearance of the truth, which renders
it inviting. It would be still more so if the solar spots always presented the
same appearances. Its ingenious author was, however, aware of many of the
objections that might be urged against it and endeavoured in a subsequent paper
to explain them.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Such is
an account of the chief of those theories of the Sun which consider this body
in a state of violent ignition. It may be remarked that as yet we are scarcely
acquainted with the nature either of light, of fire or of heat, nor even with
all the means by which they are produced. They are affections of our senses,
and it may not be necessary that the Sun or other object which produces them
should itself be either light or fire.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>As a system which supposes light to
be emitted from the Sun, notwithstanding the difficulties which it presents,
appears to meet with the largest number of advocates, there results from it a
curious question relative to this body which we shall now examine. As the Sun
is the source of a continual torrent of luminous particles, how does it happen
that during so many ages it has not been entirely exhausted? During 2,000 years
we possess astronomical observations, and these indicate no diminution either
in mass or volume. Comets which have frequently destroyed the finely woven web
of the theorist have, on this occasion, been invoked to supply the imaginary
waste of matter in the Sun, but a more probable explanation may be given
without their aid. It may be observed that many millions of years would have
but a small effect in diminishing the solar bulk on account of the immensity of
his volume and the great levity of light. The philosopher, Niewentiit, has
calculated that the<span style="mso-spacerun: yes;"> </span>of a grain of wax,
which is consumed by a taper in one second of time, produces a greater number
of particles of light than 1,000 million of Earth's equal to our own could
contain grains of sand. This will afford an idea of the extreme tenuity of the
molecules of light and convince us that the Sun would require no fresh supply
from comets.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">It has
already been observed that the Sun is surrounded by a light which extends to
some distance and is termed the zodiacal light. It has a lenticular shape and
forms a kind of atmosphere. If, therefore, our Sun were viewed from any star
situated in direction perpendicular to the solar equator, it would appear to an
observer placed there like a small luminous point plunged in a round
nebulosity, similar to what we observe in several of the stars, and which Dr.
Herschel has ascertained not to consist of a multitude of small stars, but to be
a space filled with luminous matter. If the same object were viewed from a star
situated in the plane of his equator, the Sun would appear like a star plunged
in a nebulous atmosphere of a lenticular shape.</span></div>
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<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>The spots on the Sun's surface have
by their periodic return enabled us to ascertain his rotation on this axis.
They have also given rise to several theories respecting their nature. We have
already seen that some observers have considered them as owing to the eruption
of volcanoes, whilst others have regarded them as the scoria of an immense
furnace. Neither of these systems accords with the latest observations of Dr.
Herschel. In the year 1776 this astronomer discovered a spot on the Sun
sufficiently large to be visible to the naked eye. Having observed it with a 7
foot telescope and a very high magnifying power, it appears to him divided into
two parts, the largest of which had an extent of 30,000 miles; the total length
of the whole spot was about 40,000. This spot evidently occupied too large an extent
of surface for a volcanic eruption. During the years 1783, 1791 and 1792 Dr.
Herschel saw a great variety of spots on the Sun's disc. Many were much lower
than his apparent surface. These spots did not appear like floating masses, but
resembled portions of his solid nucleus seen through an atmosphere driven aside
by some great agitation.</span></div>
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<br /></div>
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<span lang="EN-GB" style="mso-ansi-language: EN-GB;">According
to the theory of this admirable observer the surface of the Sun is very
unequal: there are numerous deep cavities and lofty elevations. Above the solid
body of the Sun he places a very extensive atmosphere composed of elastic
fluids, of which some are luminous and others transparent. He compares the
formation of the luminous fluids in the solar atmosphere to the formation of
the clouds in that of the Earth. He perceives in both bodies two immense
laboratories where the different combinations and decompositions are carried on
in a manner analogous to the chemical action of the bodies they contain. He
estimates the height at which these luminous clouds are formed in the solar
atmosphere at not much less than 1,800 miles, nor much more than 2,700.</span></div>
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<br /></div>
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<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>In considering the atmosphere of the
Sun and the numerous points in which this body resembles the planets in its
solidity, in its lofty mountains, its deepest cavities, in its rotation on its
axis and in the gravity of the bodies at its surface Dr. Herschel was led to
view this star as an immense and brilliant planet, which alone deserves the
name of a primary one, and to consider it a habitable globe. This ponderous body
ought not to be viewed, according to this able astronomer, merely as a centre
of attraction appointed to return the planets in their orbits, but for the
nobler purpose of affording an abode adapted to the reception of innumerable
generations of living things. The same idea has been extended to the stars,
which in many respects resemble our Sun.</span></div>
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<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">It must
not be dissembled that this beautiful and interesting theory is subject to a
great objection which may be urged on account of the heat, which, according to
the impressions we receive from it on the Earth, ought at the Sun's surface to
exceed any thing of which we can have a conception. To this objection founded
on the nature of our sensations Dr. Herschel replies, that the solar rays may
possibly not carry heat along with them, and that that which they excite may
depend entirely on the nature of the bodies on which they fall. But can these
rays, which at the distance of 95,000,000 miles melt the metals, be entirely
without energy at the brilliant body from which they emanate, or shall we
suppose that the substances which exist at the Sun's surface are so constructed
by Nature as not to receive the strong impression which they produce? These are
questions we can hardly hope to solve. Of the theory itself it may be observed
that it is more consistent with facts than any which has been proposed and that
it accords with the universal provision of Nature by which the material
universe every where beams with animated life.</span></div>
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<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">It is
from observation alone that we derive materials for the formation of any
legitimate theory, but in the case of the object we are now considering some
difficulties arise from the powerful light and dazzling brilliancy with which
it shines. The human eye is too delicate an instrument to endure for a moment
the solar rays: some shade must be made use of to diminish their power and
protect it from their influence.</span></div>
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<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>For common purposes, such as viewing
the commencement of an eclipse, or in observing the Sun with a telescope of
very small magnifying power, it is sufficient to smoke one of the glasses made
use of in the flame of a candle: but when it is necessary to examine the disk
of the Sun with telescopes of high magnifying power this contrivance fails.</span></div>
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<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>With a view to determine the most
advantageous method of making observations on the Sun Dr. Herschel instituted a
series of experiments which are detailed in the Philosophical Transactions of
the year 1800. He found that the smoked glasses were, by the heat of the Sun,
soon covered with blisters and endeavoured to substitute glasses differently
coloured. The first he made use of was two red glasses. These intercepted
sufficient light, but the heat penetrated through them and became insufferable
to the eye. Two pale green glasses were used, that next the eye being smoked.
This acted incomparably well, but the heat soon passed the first and raised
blisters on the smoked side of the second. Many other combinations were tried
in which the coloured glasses cracked from heat. The most successful was the
application of a very dark green glass with the side nearest the eye a little
smoked. This was the arrangement with which Dr. Herschel made a very extensive
series of observations on the solar spots, and it always protected the eye very
effectively, both from too great light and also from the too powerful effect of
heat.</span></div>
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<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">There is,
however, another means by which the same purpose may be effected. It is
well-known that some substances reflect more heat than others, and that they also
reflect different quantities of light. This is remarkable in the instance of
glass and the metals. A speculum of a telescope of large aperture will reflect
a very considerable portion of heat to its focus. Dr. Herschel therefore,
instead of a metal mirror, ground one out of a solid piece of glass, and having
made the back of it rough, that it might reflect as little light as possible,
he used it in one of his 7 foot telescopes. The effect is that no inconvenience
is found from the reflection of heat, and a single coloured glass is sufficient
to protect the eye from the Sun's light.</span></div>
<span lang="EN-GB" style="font-family: "Arial Black"; font-size: 14.0pt; mso-ansi-language: EN-GB; mso-bidi-font-family: "Times New Roman"; mso-bidi-font-size: 10.0pt; mso-bidi-language: AR-SA; mso-fareast-font-family: "Times New Roman"; mso-fareast-language: EN-US;"><br clear="all" style="page-break-before: always;" />
</span>Unknownnoreply@blogger.comtag:blogger.com,1999:blog-1290567400778113129.post-20418110817767493692013-11-11T10:06:00.003+00:002013-11-11T18:17:07.870+00:00Lecture 5: On the Moon<h1>
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Lecture
5</span></h1>
<h1>
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">On the Moon</span></h1>
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="http://www.jpmaps.co.uk/mapimages/originals/38779.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="264" src="http://www.jpmaps.co.uk/mapimages/originals/38779.jpg" width="320" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">Eclipseos Solis Totalis 12 Maji 1706 </td></tr>
</tbody></table>
<h1>
<span lang="EN-GB" style="font-size: small;">Advertisement
for lecture taken from the Royal Institution's
archives:-</span></h1>
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</div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Guard
Book Volume 1 f139</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Thursday
April 6, 2 o'clock, Mr Babbage Astronomy Lecture V. On the appearance and
phases of the Moon; its orbit. Cause of Eclipses. Theory of Tides. The Moon's
Libration. On the constitution of its surface, and the appearance which would
be exhibited to a spectator situated on it.</span></div>
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<br /></div>
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<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>Next in point of importance to the
theory of the apparent motion of the stars and to the determination of the
magnitude of the globe we inhabit are the motions of the Moon and Sun. The
natural arrangement might at first seem to require our attention to be directed
to this latter body, but there are other considerations which indicate the
convenience of a different arrangement. The Moon is, as we shall presently
discover, nearer to the Earth than any other body with which we are acquainted,
and owing to its appearance above the horizon during the absence of the Sun, we
can readily, by comparing it to some of the stars in its course, determine its
motions.</span></div>
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<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">We will
suppose that our astronomer, who has constantly occupied himself in observing
the positions of the stars, has not omitted to bestow his attention on this
interesting luminary, and that he has remarked the various particular
circumstances and appearances which attend its motion and which change daily.
These different appearances which are generally termed the phases of the Moon
constitute in fact its most striking characteristic and were calculated to
excite the earliest attention among astronomers.</span></div>
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<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>The Sun always<span style="mso-spacerun: yes;"> </span>presents to us a disk perfectly circular.
The Moon, on the contrary, only exhibits a round appearance during a few hours.
Her figure changes rapidly and in the space of 29 or 30 days, which she occupies
in making the circuit of the heavens, she presents all possible variations
between a circular disk perfectly illuminated and an obscure crescent almost
invisible.</span></div>
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<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>This revolution, between 13 and 14
times more rapid than that of the Sun, probably in ancient times, furnished
mankind with the idea of dividing time into months; and possibly into weeks, as
7 days is nearly the interval of time which elapse between each of her
quarters. That this division of time is more ancient than any other is, I
think, evident from this circumstance, that it is interwoven with the structure
of language. It is a remarkable fact that every country where that period which
we term month is made use of, the name which they give to it is always derived
from that which they assign to the Moon. This clearly indicates its origin and
likewise shows that the formation of the period must have been nearly that of
language.</span></div>
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<br /></div>
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<span lang="EN-GB" style="mso-ansi-language: EN-GB;">The
ancients made use of these phases for the purpose of regulating time before
they enquired into their causes. This cause, however, could not long escape the
notice of a reflective mind. In order to follow the phenomena in the most
methodical manner, although not perhaps in the most natural one, let us
consider the Moon in the evening just after sunset, when she herself is near
the horizon and about to disappear. The Moon then appears to us a narrow
crescent [bounded] on one side by a circle and on the other by an ellipse. She
sets a few minutes after the Sun and we may observe that the luminous segment
is turned towards that body, that the line which joins the two points of the
crescent is inclined to the horizon, and that these two points, which are
called cusps, are equally distant from the Sun.</span></div>
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<br /></div>
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<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>From day to day the crescent increases,
but the point or cusps still continue to terminate its diameter. The interior
curve becomes [a] narrower ellipse and the Moon sets later every evening and
illuminates us during a greater part of the night. The line which joins the
cusps is always inclined to the horizon when the Moon sets, and its smaller
diameter is always directed to the Sun. On the seventh day the Moon appears
like a semicircle and is visible nearly half the night. On the following days
the luminous portion continues to increase: the interior ellipse takes a
contrary position.</span></div>
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<br /></div>
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<span lang="EN-GB" style="mso-ansi-language: EN-GB;">On the
14th or 15th day the whole disk of the Moon appears illuminated and its figure
is round, but the light has not a uniform tint. We may observe some points
which are more luminous, others that are darker than the rest of the surface.
With a telescope we may perceive large round holes, which are deep like wells,
and which are luminated even to the bottom. On every part there appear places
which are elevated above or depressed below the rest of the surface, and since
at this time no shadow is projected by the most elevated point on the lower
grounds, our observer will do well to profit by this circumstance and also of
its being visible during the whole night for the purpose of making a drawing of
it, and determining the relative position of the most remarkable asperities.</span></div>
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<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>The next day the western side of the
Moon begins to become less distinctly terminated. On each succeeding day it
becomes more and more obscure. The illuminated part is terminated by an ellipse
which gradually flattens. On the 22nd day the Moon again appears an illuminated
semicircle. All the phenomena which we have observed reappear in an<span style="mso-spacerun: yes;"> </span>order. The mountains again project shadows
and the enlightened part diminishes. About the 28th day the Moon rises but a
short time before the Sun, and is scarcely visible for a few days, after which
it reappears on the west in the form of a fine narrow crescent.</span></div>
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<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">During
the whole course of a revolution the enlightened part of the Moon is always
nearest to the Sun, and the dark part is turned from that body. Another
important remark will result from these observations. In all the phases of the
Moon, the spots, and other remarkable points, always occupy the same parts of
the lunar disc. The obscure part is not so completely dark, but that with a
little attention the whole disc may be perceived, on which may sometimes be
distinguished the most remarkable spots, which are thus found always to
preserve their places. The excavations resembling wells are now not illuminated
to the bottom, as they were when we observed them on the day of the full Moon,
but it may be distinctly seen that the shadow of one part of their edge is
projected on the opposite side. From these remarks it evidently follows that
the Moon is not luminous of itself, that she only shines by a borrowed and
reflected light, and it also appears that she always turns towards us the same
face.</span></div>
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<br /></div>
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<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>We shall likewise easily conclude,
although not quite so quickly, that the Moon is not simply a flat surface, but
that it is a globe whose enlightened side is but seldom directed entirely
towards us.</span></div>
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<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">It was
imagined by some of the ancients that the Moon was a flat surface resembling a
mirror and that the appearances on its disc were owing to the reflection of the
Earth's surface. This opinion however scarcely needs refutation. It is
obviously incorrect. The apparently elliptic curve which terminates the
enlightened surface is in fact a great circle of the lunar globe which is
presented to us in [an] oblique direction. It is well known that in painting,
when we wish to depict a circular object which is not quite in front of us, its
perspective representation is an ellipse.</span></div>
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<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>But, if the Moon is an opaque body,
it is natural to enquire whence she derives the light by which she shines.
There is not much difficulty in the decision on this point. We only possess the
knowledge of one body in the universe that shines with sufficient lustre to
lend its splendour to the Moon. The following considerations will confirm this
conjecture. At the time of the full Moon, when this luminary possesses the
greatest brightness, she passes the meridian nearly at twelve o'clock at night.
The Sun is then distant from her almost or about half a circle. It is
consequently in the opposite side of the heavens. I say "almost" for
it is evident that, if the Sun were exactly in opposition to the Moon, the
interposition of the Earth would prevent the transmission of light to this
latter body.</span></div>
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<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">The Earth
as well as the Moon receives light from the Sun, and it must also reflect part
of the rays which it receives. From this circumstance it happens that in lunar
eclipses the Moon is still visible, though very obscurely, for from the
relative magnitudes of the two bodies the Earth will, to an inhabitant of the
Moon, appear thirteen times as large and afford thirteen times as much light as
we receive from that body.</span></div>
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<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>It is well known that when the solar
rays are concentrated by means of a mirror or by a large convex lens the
effects are very powerful. The heat produced is intense and metals and other
substances presented to it are immediately melted. It is natural to suppose
that if the Sun's rays produce an effect those of the Moon which are borrowed
from the same source would also produce some small degree of heat: it was not
expected that the heat thus generated would be very considerable but merely
that it might be sufficient to be made sensible. An experiment was therefore
made with a view to determine this point: a large convex glass was made use of
which by the Sun's rays had melted a small piece of platina in 3 seconds. With
this powerful instrument the light of the Moon was collected and condensed upon
the ball of a delicate thermometer: but not the least elevation of temperature
was observed. This certainly appears rather unfavourable to the opinion of the
Moon receiving illumination from the Sun, but it fortunately admits of an
explanation. It is found that those bodies which reflect light do not always
reflect a proportionable quantity of heat. There is a striking difference in
this point between a silvered looking glass and a polished plate of metal. Both
reflect nearly the same quantity of light but the quantity of heat reflected is
very different. The glass reflects very few of the rays which produce heat,
while the metal speculum reflects almost all. The glass, on the contrary,
absorbs into its substance the rays of heat, and consequently becomes warm,
while the mirror of metal propelling them from its surface receives<span style="mso-spacerun: yes;"> </span>accession of temperature.</span></div>
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<span lang="EN-GB" style="mso-ansi-language: EN-GB;">The Moon
appears to be constituted of materials which possess in an eminent degree the
property we have just described as belonging to the glass. She reflects many of
the rays of light but absorbs the whole of those which produce heat. This
appears to be the case with our Earth and with the other planets, and we may
admire the wise provisions of Nature, by which each body retains those parts
which contribute to the warmth and comfort of its inhabitants, while it
reflects from its surface the remainder for the illumination of distant worlds.
Without this admirable contrivance we should, during the presence of the Sun,
be burnt with heat and in his absence suffer the extreme of cold.</span></div>
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<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>It will now become necessary for our
astronomer to determine the path of the Moon. In this enquiry he will not find
much difficulty. It is easy to measure its distance from any star to which it
approaches. By means of the sextant, and by repeating this a few nights he will
find the number of degrees it passes over in 24 hours. And at the end of about
27 days and<span style="mso-spacerun: yes;"> </span>he will perceive it to have
arrived near the same point at which he first began to observe it.</span></div>
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<span lang="EN-GB" style="mso-ansi-language: EN-GB;">From
these observations he will conclude that the Moon revolves round the Earth,
that it is nearer than any of the other celestial bodies, for in its progress
it eclipsed all the stars in its path and, when we are deprived of the light of
the Sun in an eclipse, the Moon is situated between us and that luminary. It
will be further observed that the interval between two full Moons is about 29
days 12 hours. It seems natural to suppose that the Moon would suffer an
eclipse every time she is in opposition to the Earth, for this body, being
perfectly opaque, will necessarily cast a long shadow in a direction opposite
to that from which [s]he is illuminated, and in consequence of the revolution
of the Moon round the Earth, it might be expected that the Moon would enter
this shadow once every month and suffer an eclipse. This would be the case if
the three bodies were always in the same plane, but the orbit of the Moon is
inclined at a small angle to the line which joins the Sun and the Earth, and in
consequence of this inclination it does not at every revolution enter the
Earth's shadow.</span></div>
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<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">From the
same cause it results that the Moon does not always pass exactly between the
Earth and the Sun so as to cause an eclipse of the latter body. It should be
observed that in an eclipse of the Moon the commencement is visible precisely
at the same moment over all parts of the globe, provided she is above the
horizon. The reason is that the Moon herself suffers a deprivation of light,
but this is not the case with the Sun: an eclipse of this body being caused by
the passage of the Moon between the Earth and the Sun. It will begin at
different times in different parts of the globe. In some parts it will be total
the whole disc being hid, or perhaps it may be annular, that is the Moon will
only cover the central part, and leave a ring of light all round. The same
eclipse may in other countries be partial [for example: a similar one was
observed at London in 1764].</span></div>
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<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>An eclipse of the Sun always begins
on the western side and ends on the Eastern one, and in this latitude it seldom
happens that total darkness continues for more than four if 5 minutes.
Generally in a solar eclipse the Moon's disc is covered with a faint light,
which we have before observed is owing to the reflexion from the illuminated
part of the Earth. Though the darkness in solar eclipses is not of long
duration, it is, while it continues, very intense. The stars and planets become
visible and instance[s] are related of birds going to roost. A singular
appearance takes place in these eclipses: the Moon's limb is surrounded by a
pale circle of light, which some astronomers have considered as an indication
of a lunar atmosphere, but others as the atmosphere of the Sun, because it has
been observed to move equally with the Sun and not with the Moon. And it is
generally believed that the Moon has no atmosphere, or at least a very small and
rare one.</span></div>
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<span lang="EN-GB" style="mso-ansi-language: EN-GB;">With
respect to the number of eclipses of both luminaries it may be observed that
there cannot be fewer than two nor more than seven in one year. The most usual
number is four and it is rare that more than six occur. Generally speaking there
are more solar than lunar eclipses, nearly in the proportion of 4 to 3, but if
we consider any particular spot its inhabitants will see more eclipses of the
Moon than of the Sun, because a lunar eclipse is visible to a whole hemisphere
at once, whereas a solar eclipse is confined to a small tract of country.</span></div>
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<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>Since eclipses depend on the
relative situations of the Earth, the Moon and the Sun, it is evident that when
these bodies arrive at the same position the eclipses will begin again to recur
in the same order. It was from observing this that the ancients discovered a
period of nearly 18 years at the end of which the same eclipses happened in the
same order. It was called the Chaldean Saros and contains 18 Julian years 11
days 7 hours 42 minutes. This was probably the method made use of by Thales to
predict the eclipses for which he was celebrated.</span></div>
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<span lang="EN-GB" style="mso-ansi-language: EN-GB;">The
doctrine of eclipses is of considerable advantage and utility. From it we may
deduce a strong confirmation of the conclusions we arrived at respecting the
figure of the Earth, for in all lunar eclipses the shadow of the Earth on the
Moon's disc is always bounded by an arc very nearly circular, and this could
not happen unless the shadow of the Earth were in all situations nearly
conical, and consequently the Earth itself nearly spherical. Lunar eclipses
also prove that the Sun is larger than the Earth, and the Earth larger than the
Moon; that the Sun is larger than the Earth appears because the shadow of the
Earth ends in a point, which it could not do if the Sun were smallest; and that
the Earth is larger than the Moon because the whole of the Moon's body is
sometimes involved in the Earth's shadow, and a section of this shadow at the
Moon's orbit is much less than the Earth itself.</span></div>
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<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>Eclipses also that occur in similar
circumstances and at distant intervals are of use in ascertaining the periods
of the Moon's motions. In Geography the advantages resulting from them are
great. By their means the longitudes of different places may be discovered.
Those of the Moon are most serviceable for this purpose.<span style="mso-tab-count: 1;"> </span>We have already seen what great assistance they afford to
history and to the examples already adduced. I may be permitted to add another.
The time of a very celebrated event in the history of our own country has been
ascertained with great accuracy in considerable degree though not entirely by
means of an eclipse. The invasion of Julius Caesar is stated by some historians
to have [taken place] in the year 52 before Christ, by others in the year 54,
and some place it as early as 56. But Dr. Halley has determined the time of
this invasion to a day and says that the landing was effected on the 26th
August, in the year of the world 3950, or the 55th before the usual Christian
era. He was lead to this decision partly by calculating the time of the lunar
eclipse, of which Drusus made such successful use to quiet the Pannonian army
upon the death of Augustus, and partly by computing the time of full Moon
previous to which Caesar landed his legions, and also by considering the
phenomena of the tides on the Kentish coast at that particular period.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Of all
the phenomena presented to us by Astronomy eclipses are certainly the most
remarkable. In every age they have excited the highest interest; in early
periods they were attended with the fears and terrors of the ignorant: in more
modern times the accuracy with which every circumstance attending them is
predicted has produced equal admiration. The theory is now so well understood
that they are always mentioned in the Almanac. I shall just notice some of the
most remarkable which have occurred during the last century.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>On the 12th May 1706 there was a
total eclipse in the southern parts of France and Spain; at Paris about eleven
digits were eclipsed. (It should be observed that the disc of the Sun or Moon
is imagined to be divided into twelve parts called digits and the magnitude of
an eclipse is measured by the number of these digits which are covered.) This
eclipse was observed by Cassini and he made the following remarks on it.
"The duration of total darkness was about five minutes. In this interval
of time the stars and planets became visible. The cattle appeared terrified and
returned towards their stables with evident marks of consternation. The bats
and owls quitted their retreats while the birds of the day betook themselves to
their places of rest. The inhabitants of the country ignorant of the cause of
this wonderful phenomenon participated in the general terror. The obscurity
which prevailed," observed Cassini, "could not be compared to the
darkness of the night nor yet with the twilight; it had a gloom peculiar to
itself. Round the obscure disc of the Moon there appeared a luminous ring which
extended on all sides to about the distance of 4 degrees. The moment the least
portion of the Sun's disc re-appeared the effect was instantaneous like
lightning, and the superior splendour of his rays dissipated the faint gleam of
this luminous ring."</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">In the
year 1776 there was a great eclipse of the Sun; it is particularly remarkable
from the observations of some Spanish astronomers. They observed a luminous
point near the border of the Moon, which they explained by supposing it to be
an opening in the body of the Moon through which the Sun's rays penetrated.
Subsequent observations have rendered it very probable that this luminous spot
was a volcano which has been discovered by Dr. Herschel and since that time
been frequently observed. It must have been in a state of active eruption. This
eclipse was total and annular at about the middle of the Atlantic Ocean.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>Among the remarkable eclipses of the
last century may be mentioned that of 28th July 1748. It was total annular for
some parts of Scotland. The French astronomer Lemonnier travelled from Paris to
Edinburgh on purpose to observe this singular phenomenon and profited by making
observations, which we have rarely an opportunity of repeating.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">In the
year 1764 there occurred another total eclipse of the Sun. It excited much
attention among astronomers, who usually, on the expectation of these
phenomena, calculate the time of the various appearances and draw a plan of the
eclipse. One of the best of these published at this time was entitled- A Map of
the Passage of the Moon's Shadow across Europe during the Total and Annular
Eclipse of 1st April 1764. Its singularity, however, consisted in this, that it
was the work of three ladies. The calculations were from the pen of Madame
Lepante a pupil of the astronomer Lalande, the engraving was executed by
another lady and the ornamental designs and embellishments were the work of a
third.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>It was soon found from observation
that the Moon moved round the Earth and it would perhaps be natural to conclude
that the curve it describes is a circle, for this is one of the most simple in
Nature. To verify this hypothesis will not be a matter of much difficulty. It
is well known that, when objects of equal magnitude are viewed at the same
distance, they appear of equal size: and that, if we increase our distance from
an object, it appears smaller; thus a person on the top of a high tower sees
the people walking below him much diminished in size, and they always appear
smaller the higher he ascends. Let us apply these principles to the case of the
Moon. If that body moved in a circle round the Earth she is always at the same
distance and must consequently always appear of the same size. A difficulty may
now arise: how shall we ascertain the apparent magnitude of a heavenly object?
The eye is not a sufficiently accurate judge. For this purpose several devices
have been made use of. They all go by the name of micrometers.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">The
description of one of them will be sufficient for my present purpose. When
describing the transit instrument I mentioned that it was usual to place small
wires in the field of view of the telescope. In that case the usual number is
six, but for the purpose of a micrometer two are sufficient. A screw must be
adapted to the instrument, by means of which these two wires may be made to
approach or recede from each other at pleasure. If now any object, such as the
Moon, be viewed through a telescope to which this micrometer is attached, the
screw must be turned until the disc of the object be contained between the two
wires; if at any subsequent period the telescope be again pointed to the same
object (supposing the wires still at the same distance), it will readily be
observed whether the object exceeds in breadth the distance of the wires or
falls short of it. With an instrument thus prepared let us suppose our
astronomer to commence his observation, and, having made the two wires coincide
accurately with the edges of the Moon, let him turn the screw until the two
wires touch each other. Suppose that it takes 20 revolutions of the screw to
open the wires to the distance of the Moon. Then, about 14 days afterwards, let
him again examine this body and he will find that its diameter is increased,
for it will now require 21 turns of the screw to open the wires to such a
distance as to include the disc of the Moon.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">It need
scarcely be observed that it is not the variable illuminated part which should
be measured but the distance between its two horns or cusps. From these
observations our astronomer finds that in one part of its revolution the
apparent diameter of the Moon is ?th part greater than in the opposite quarter,
and he will consequently conclude from the principle we have just stated that
the Moon is one twentieth part nearer in one part of its course than it is in
another. This immediately solves the question he had proposed. The Moon does
not revolve in a circle of which the Earth is the centre, for it is evident
that its distance from the Earth is variable. From observations on the Moon's
apparent diameter it may be presumed that it revolves in an ellipse, and our
subsequent enquiries will confirm this supposition.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>While our astronomer is measuring
the diameter of the Moon his attention will naturally be attracted to the
appearance of the various spots and figures on its surface. These, as they
appear immovable, will furnish a ready method of ascertaining whether it has a
revolution on its axis. After viewing it during several revolutions he will
observe that the same spots always appear in the same place, and that the same
side is constantly turned to the Earth. From this circumstance it has sometimes
been erroneously inferred that the Moon does not revolve on its own axis. This,
however, is not a legitimate deduction from our observations. The Moon, by
keeping the same face to the Earth during a whole revolution in its orbit, must
have turned once round its axis.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Thus then
the general law is established that the Moon revolves round its axis in exactly
the same time that it moves round the Earth. From this it would seem to follow
that not more than one half the Moon's surface could ever be visible to an
inhabitant of the earth, but there are some modifications by which we are
enabled to see rather more than one half. This is owing to what is called the
Moon's libration.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>One of the causes of this libration
is the unequal motion of the Moon in her orbit, for though both the revolution
round the Earth and that round her axis are completed in the same time, yet
they are not both of them uniform motions. In consequence of this there is an
apparent oscillation of the Moon by which we see a few degrees on each side
alternately more than a hemisphere. This is called the Moon's libration in
longitude.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>The Moon is also subject to a
libration in latitude. It arises from the inclination of her axis to the plane of
her orbit, and its effect is to render visible sometimes the parts of her
surface adjacent to the north, and sometimes those situated near the south
pole.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">The third
species of libration to which the Moon is subject was discovered by Galileo. If
the two former librations did not exist, the same face of the Moon would be
turned not to a spectator on the surface but to an imaginary spectator in the
centre of the Earth. Now two lines drawn respectively from the centre and
surface of the Earth to the centre of the Moon form an angle of some magnitude.
Hence when the Moon rises parts of her surface situated towards the boundary of
her upper limb are seen by a spectator which would not be seen from the Earth's
centre. As the Moon rises these parts disappear, but, when the Moon having
passed the meridian declines, other parts situated near that boundary, which,
whilst the Moon was rising, was the lower [limb], are brought into view and
which would not be seen by a spectator at the centre of the Earth. The greatest
effect of this diurnal libration will be perceived by observing the Moon first
at her rising and then at her setting. It should be observed that this species
of libration, as well as the two preceding ones, are merely optical; they arise
from the circumstances in which we are placed not from any oscillating motion
in the Moon.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">In
considering the singular coincidence (which takes place in the equal duration
of the Moon in her orbit and her rotation round her axis) we are led to enquire
whether there is any physical cause to which it can be attributed, whether it
is a necessary consequence of some more general law or whether it is the effect
of chance. The reasoning by which these questions are answered is among the
most difficult and successful efforts of human enquiry. It will be sufficient
at present merely to state the results. In the first place it soon appeared
that the improbability of this coincidence, being the effect of chance, is
excessively great. It then remained to enquire what might be the cause, and
this required a knowledge of the shape of the Moon.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>It is found from theory, as well as
actual mensuration, that the Earth is a flattened spheroid. It is evident that
it is on theory alone we attain a precise knowledge of the Moon's figure. From this
it is found that the Moon will not be quite globular but must have a small
compression at the poles. This, however, is too slight to be sensible even to
our best instruments.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">There is,
however, another and greater deviation from a perfect sphere. Owing to the
great attraction of the Earth that diameter of the Moon which passes through
the Earth's centre will be larger than the other diameters. In consequence of
this superior thickness of the Moon in the direction of the Earth it would
happen that, if from any circumstance that diameter of the Moon were turned a
little out of its direction, it would return to its original position. This
effect is owing to the attraction of the Earth and is similar to a pendulum
drawn a little from the perpendicular; it returns and oscillates to and fro.
Another consequence of this attraction is that, if the time of the Moon's
revolution on her axis had at first been nearly but not quite equal to that in
her orbit, this attraction of the Earth would have rendered them quite equal.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>It appears then that the Earth
causes by its attraction the Moon to assume a certain figure. In consequence of
this figure, and also of the Moon having a slow rotation on its axis, it is, by
the Earth's attraction, always retained with the same face directed towards us.
If the Moon is thus affected by the attraction of the Earth it might probably
be enquired, since the action of gravity is reciprocal, why the same part of
the Earth is not always directed towards the Moon. To this it is a sufficient
answer to state that the Earth possesses a quick rotatory motion, but it will
be both curious and interesting to trace the different effects of the same
cause modified by various circumstances.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">The Earth
by its attraction preserves the same face of the Moon constantly directed
towards us. The effect of the Moon's attraction on our planet is to create
tides in the ocean. It is well known that the more distant a body is the less
will be the force of attraction exerted on it. The surface of the Earth is nearer
the Moon than its centre, and this again is nearer than the opposite side. In
consequence of this the waters will rise at the part immediately directed to
the Moon, and also in the part diametrically opposite.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>If the Earth and the Moon were
without motion and the Earth covered all over with water, the attraction of the
Moon would raise it up in a heap in that part of the ocean to which the Moon is
vertical, and there it would always continue, but by the rotation of the Earth
on its axis each part of its surface is presented to the action of the Moon,
which causing an elevation in two opposite hemispheres thus produces two floods
and two ebbs every day. The effect of the Moon is not confined to the parts to
which she is vertical but acts, though in a less degree, on others adjacent. If
the Moon were stationery there would be exactly two tides in twenty four hours,
but, as this body moves in her orbit, it takes rather more than 26 hours for
the same spot on the earth to return to the same situation with respect to the
Moon. From this cause it arises that two tides take place in about 24h 50m.
this is the reason why the high tide/water is usually<span style="mso-spacerun: yes;"> </span>hour later each day than it was the preceding.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">That
there is a great variation in the height of the tides is a fact well known to
everyone. There are two causes which produce this effect. The waters of the
ocean are affected by the attraction of the Sun as well as by that of the Moon,
but owing to the immense distance of the former body it produces but a small effect.
Sir I. Newton computed that the force of the Moon raised the waters in the
great ocean 10 feet whereas that of the Sun only produced an elevation of only
two feet. When the attraction of both Sun and Moon act in the same direction,
that is at new and full Moon, the combined forces of both raise the tide 12
feet. But when the Moon is in her quarters the attraction of one of these
bodies raises the water while the other depresses it. In this case the smaller
force of the Sun must be subtracted from that of the Moon. Consequently the
tides will be no more than 8 foot high. When the tides are highest they are
called spring tides and are caused by the united actions of the Sun and Moon.
When the tides rise least they are called neap tides and are owing to the
difference of the action of these two bodies. This is the general principle on
which the tides depend, but particular circumstances will considerably modify
the effects. In lakes and seas of small magnitude there will be scarcely any
perceptible tide for the action of the Moon will be nearly equal in every part.
This is the case in the Black Sea and even in the Mediterranean. If the ocean
communicates with any narrow inlet the tides will rise to a great height. At
the mouth of the River Indus the elevation is above 30 ft. This is the reason
of the great rise at Bristol and other places similarly situated. A question
intimately connected with the theory of the tides has lately occupied the
attention of one of the greatest philosophers from whose labours the science of
Astronomy has derived immense improvements. I allude to the question relative
to the stability of the </span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">equilibrium
of the ocean.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>There are several irregular causes
such as hurricanes and earthquakes which agitate the sea, elevate it to a great
height and sometimes oblige it to forsake its limits. We find from observation
that it has always a tendency to return to its former state, and that the
resistance it experiences would soon bring it to this state even without the
action of the Sun and Moon. This tendency constitutes what is called a stable
equilibrium. Amongst the infinite variety of disturbances to which it is liable
from irregular causes, it might be supposed impossible that some extraordinary
cause may communicate to it a shock which, though inconsiderable at its origin,
may augment continually and elevate it above the highest mountains. If this
were the case we should have an explanation of several phenomena in natural
history and at least the possibility of such an occurrence is a subject worthy
of our enquiry.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Laplace
has directed his attention to this interesting question with great success. He
has investigated the conditions which are necessary for the absolutely
stability of the ocean and has examined whether these conditions exist in
Nature. The result of this investigation is, that the equilibrium of the ocean
is stable if the mean density of the Earth is greater than that of the water.
And this is found to be the fact from experiments on pendulums and the
attractions of mountains.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>It appears then that the equilibrium
of the ocean is stable, and if as appears certain the waters have formerly
covered continents which are now at present much above its level, we must
search for other causes than a want of stability in the ocean. Another singular
result of Laplace's analysis is that this stability would cease to exist if the
mean density of the sea exceeded that of the Earth. Thus if the ocean instead
of water consisted of quicksilver extraneous causes might impress on it oscillations
which might continually increase in magnitude and which subverting the loftiest
mountains would involve in one universal ruin this beautiful abode of Man. Such
destructive revolutions are not the appointed doom of the planet we inhabit.
The general deluge, which is alike certain from history and from an examination
of the structure of the Earth, appears to have been arisen from the immediate
will of that power which created and not from any secondary cause.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">To
determine the distance of the Moon from the globe round which it revolves has
long been a problem with astronomers. Its solution has progressively increased
in precision. We have seen that by means of micrometer the apparent diameter of
the Moon may be ascertained. In order to determine her distance we must find
the angle under which the Earth would appear if we were placed on the surface
of the Moon. If we suppose an inhabitant of the Moon measuring the apparent
diameter of the Earth, it is this angle we are in search of. By an easy
calculation we may from this angle and the Earth's diameter determine the
distance of the Moon. It may be stated in round numbers at 240,000 miles. This
is in fact about 1200 miles too much. Astronomers, however, have not confined
themselves to determining the Moon's distance they have ascertained her
magnitude and weight; they have measured the height of the mountains which
cover her surface and have determined her attractive force. The diameter of the
Moon is about 2180 miles; this is a little greater than<span style="mso-spacerun: yes;"> </span>of that of the Earth. If, for instance, we
suppose a spring is bent 5 inches by a weight of 10 lbs and if the same spring
is transported to the Moon's surface, the same weight of 10 lbs will only bend
it one inch.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">On directing
a telescope of moderate power towards the Moon we perceive that it is a body
much resembling our Earth, covered with appearances of the form of mountains,
the greater part resembling those which form the craters of our volcanoes. We
perceive a number of mountains in the form of<span style="mso-spacerun: yes;">
</span>cones on whose summits are cavities resembling craters. Volcanic
mountains on our globe present the same appearance. These mountains cast a
shadow on the face of the Moon in a direction opposite to that of the Sun. This
shadow diminishes as the mountain becomes less obliquely exposed to the Sun's
rays, and when the Sun has passed its meridian the shadow falls on the opposite
side.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>The mountains are, comparatively
speaking, much more elevated than those of the Earth. Galileo estimated some of
the at 3 miles. This, however, is considerably too great. Schroeter, who
occupied himself with great success in examining the phenomena of the Moon, has
found that the depths of the largest cavities are about 18,000 ft. below the
Moon's surface and the elevation of the loftiest mountains are about 25,000
[ft.], that is about 3,000 ft. higher than Chimboraco [in Ecuador], which is
the highest<span style="mso-spacerun: yes;"> </span>mountain on our Earth.
Schroeter has made a very curious remark on this subject. He observes that the
highest mountains on the Earth, on the Moon, on Mercury and on Venus are all
situated in the southern hemisphere, and that the deepest cavities in the Moon
are in the same hemisphere. From this he concludes that the southern parts of
these globes have undergone the greatest revolutions and been subjected to the
most powerful actions.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">I have
mentioned that most of these mountains are volcanic. This fact is proved by an
observation of Dr. Herschel. In the beginning of May 1783 he saw two mountains
gradually forming on the Moon's surface. On the 4th he perceived a luminous
point near the spot called Aristarchus. This light appeared still more vivid on
the 19th and 20th of April following and left no doubt that it was a volcano on
this part of the Moon. This new discovery was soon spread over Europe and
excited the attention of astronomers. The volcano was not however recognised by
foreigners -perhaps the eruption had ceased. It reappeared in March 1794 and
was distinctly perceived by two people at the same instant, who were nearly 100
miles distant from each other. It appeared to the naked eye like a star on the
obscure part of the Moon. This must undoubtedly have been a large eruption of
lava, but we have on our own globe the remains of one issuing from Hecla in
Iceland which extends 300 miles in length. This to an inhabitant of the Moon
would have been very distinctly visible.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">With
respect to the dark spots on the Moon we are not so well informed. All that can
be said of them with certainty is that they reflect less light than the rest of
its surface. But whether they are seas or whether they are forests is quite
undecided.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>Whether the globe we are now
considering is a place of abode for rational beings or whether it is a desolate
waste is a question which has been frequently disputed. I shall at present
state the probability of our determining this question and the means we possess
of effecting it. If an object occupying as much space as London were placed in
the middle of the lunar disc, it is easy to prove that it would be visible by
means of a telescope magnifying 100 times. A slight extension of this
calculation will inform us that with such a telescope we might distinguish a
spot of about 4,000 feet in diameter. With a telescope which magnified 50 times
as much or 5,000 times we might distinguish an object of about 80 feet in
diameter. Dr. Herschel's 7 foot telescopes admit of such a power. But, if we
enquire what would be the magnifying power necessary to render visible an
object of about 6 feet, it will be found that it must be at least 60,000 times.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">From this
we may judge that there is little probability of our ever seeing, on the
surface of the Moon, beings of the same species with ourselves. Dr Herschel
imagines he has observed changes which could only be produced by the labour of
the inhabitants of the Moon. The only method of deciding the question is by
examining particular districts of the Moon with telescopes of large power and
making very accurate maps of them. This labour has been undertaken by Schroeter
the celebrated astronomer of Liebenthal.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>The phenomena which would be
observed by a spectator at the Moon are of a singular nature. Since the Moon
revolves round the Earth in the space of a month and always presents the same
face to it it follows that the whole surface is successively turned towards the
Sun. Thus a lunar day will be nearly equal to 15 of ours and their nights will
be of equal duration. Such would be the appearance in respect to the Sun. But
with regard to the Earth it would be much more varied and singular. An
inhabitant of the Moon situated in the centre of that hemisphere which is
always directed towards [the Earth] will constantly see [it] in his zenith. It
will appear to oscillate a few degrees on each side owing to the libration. But
an inhabitant of the hemisphere on the opposite side to the Earth will never
see it.<span style="mso-spacerun: yes;"> </span>But the most singular spectacle
will be to those inhabitants, if there are any who live on the borders of the
lunar disc. They will see the Earth sometimes rise above the horizon a few
degrees, and then by a retrograde motion replunge itself below and disappear
during an equal time. It appears then that the two hemispheres of the Moon have
a very different lot. One of them during a night equal to 15 of our days never
sees the Earth. The hemisphere directed towards us enjoys more advantages:
although she has a night of 15 days in length her inhabitants have always the
Earth above their horizon, which affords, when full, nearly 14 times as much
light as we receive from the Moon. Just such an illumination we should ...
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</span>Unknownnoreply@blogger.comtag:blogger.com,1999:blog-1290567400778113129.post-41340656737028128502013-11-11T10:04:00.003+00:002013-11-11T17:27:47.075+00:00Lecture 4: Ascertaining the Figure of the Earth <h1>
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Lecture
4</span></h1>
<h1>
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"> <div class="separator" style="clear: both; text-align: center;">
<a href="http://images.stampwants.com/igpiw.img" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="232" src="http://images.stampwants.com/igpiw.img" width="320" /></a></div>
</span></h1>
<h1>
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"> Ascertaining the Figure of
the Earth</span></h1>
<h1>
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"> </span></h1>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>We have now arrived by a gradual progress
at the discovery of the relative immobility of the stars. We have seen that
they all apparently revolve together round the Earth with a uniform velocity.
This was merely an hypothesis assumed for the purpose of connecting together
the various facts. It will now become necessary for our astronomer to examine
its truth and to endeavour to discover whether it accords with other facts
which gradually present themselves.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>It has already been remarked that
the rotation of the stars round the Earth afford proof that this body is
isolated in space. The investigation of its figure and magnitude will now
become not merely an object of curiosity but one of the greatest importance. It
will soon be perceived that the appearance of the heavens may be accounted for
on two hypotheses. One has already been mentioned, the other is that the Earth
itself revolves on its axis. It is evident that either of these will account
equally for the phenomena; only if the Earth moves it must turn in a contrary
direction to the apparent motion of the stars. Before we survey the proofs by
which this latter hypothesis is supported it will be necessary to ascertain in
a rough manner the figure of the Earth we inhabit. For this purpose we may have
recourse to some very familiar appearances which afford a ready solution of the
question.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">A person
standing on the sea shore and viewing a ship approaching towards him will
observe that the masts and rigging become visible a considerable time before
the hull of the vessel. If the Earth were a plane surface this could not take
place or rather the contrary would be [the] case, for the body of the vessel
being so much larger than the slender mast would, as is well-known from optical
principles, be visible much sooner. If the vessel be viewed through a telescope
it will be very evident that the hull is prevented from being seen by the
elevation of the sea between the observer and the object. This observation
indicates a roundness in the figure of the Earth, and if it be repeated in
different parts of the globe the result will always be the same. Another proof
of the rotundity of the Earth is afforded by the practice of engineers in the
construction of canals: they are obliged to make an allowance for the Earth's
convexity since the true level is not a straight line but a curve which falls
below it about 8 inches in every mile. This deviation is nearly the same in
every part of the globe.</span></div>
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<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">From
these two arguments it clearly follows that the Earth is, if not perfectly
spherical, very nearly so. This is still further confirmed by observations on
the height of the pole at different places in the same meridian. The farther
northward we travel the higher the pole appears to be elevated. For every 70
miles its altitude is increased one degree. Calculation will inform us that
this could not take place if the Earth were a flat surface. But its spherical
shape accounts for these appearances.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>If Man had confined himself solely
to the collection of facts science would have presented a barren detail of
arbitrary names; and he would never have attained the knowledge of the great
laws of Nature. It is by comparing phenomena together and by endeavouring to
trace their mutual connection, by gradually correcting his theory that he has
succeeded in discovering these laws, the existence of which may be perceived in
their most complicated effects.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>When we reflect on the diurnal
motion to which all the heavenly bodies are subject, we are naturally led to
infer the existence of some one general cause which moves or which appears to
move them round the axis of the Earth. When we consider that these bodies are
insulated with respect to each other and are apparently placed at various
distances form the Earth the simplicity of the two theories stands strikingly
contrasted, and it seems much more natural to admit this latter motion and to
regard that of the heavens as only apparent.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Many
objections have been raised against the motion of the Earth but these may be by
attentive consideration of its attendant circumstances be easily removed. It
has been urged that this motion would, if it existed, be perceptible to the
senses. But this is not the fact. In a similar case the passengers in the cabin
of a vessel sailing on a smooth sea are not aware of the motion of the ship.
Every thing near them with which they can compare it moves likewise. If however
they look at objects situated of the land from which they are departing these
will appear to recede and it is only by a process of reasoning that they
discover that it the vessel which moves. That this is the case is obvious from
the remarks of children in these circumstances who always believe that the land
and the objects with connected with it are in motion. The more evenly the ship
moves the stronger is this deception. Is it then suprising that the Earth which
meets with no obstacles and no resistance, whose motion is perfectly equal
should seem to its inhabitants at rest?</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">It has
been objected that this theory would assign to the inhabitants of the equator
an immensely rapid motion but this objection is of vastly greater weight
against the other hypothesis. If the Earth revolves on its axis the equatorial
parts will move some few hundred miles in an hour. But if the starry heavens
move round their axis in the space of 24 hours these luminaries must fly with a
velocity of which it is impossible for the human mind to form any adequate
conception. It were easy to multiply millions by millions. I might assign to
figures a name which should be said to measure this inconceivable velocity. The
powers of numbers are unlimited but the finite capacity of Man must forbear to
apply his diminutive scale estimate the magnitude of the Earth.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>These, it is true, are but probable
reasons for the rotation of the Earth. There are, however others which rest on
stronger evidence. Newton has established three laws to which all matter is
subject and all succeeding philosophers have acknowledged their truth. From
these it follows by a strict train of demonstrative reasoning that two bodies
acting on each other will not revolve round the centre of one of them but will
move round the common centre of gravity of both. If this be applied to the
stars, which are immensely larger than the Earth, it will follow that it is
impossible for them to perform their revolutions round this globe. This is a
proof derived entirely from theory. It is not on that account the less
conclusive. I shall only at present mention one other of a mixed nature. It is
found from theoretical calculations that if a sphere covered with any fluid
revolve round its axis it will change its spherical shape and become spheroidal
or flattened at the poles, and this compression will be greater in proportion
as the swiftness of its rotation increases. The globe we inhabit is precisely
in this situation. It is nearly a sphere, is partly covered with a fluid and
revolves on its axis. It ought therefore from theory to be flattened at the
poles, and we shall presently find on investigation that this is the fact. It
must be observed in justice to the illustrious Newton that his hypothesis of
gravity, from which this follows as a necessary consequence, was published long
before those measurements were undertaken which subsequently confirmed the
fact.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">The first
among the moderns who openly maintained the rotation of the Earth, an opinion
alike repugnant to the vanity and prejudices of Mankind, was undoubtedly
Copernicus. Galileo quickly embraced it and contributed greatly by his writings
and discoveries to spread its belief over Europe, but an opposition arose from
a quarter from which it was little to be expected. Religion was alarmed and the
power of the Inquisition was called in to arrest the progress of reason and of
truth. The history of this persecution is too remarkable to be omitted and
forms a singular era in the progress of philosophy. It arose from this
circumstance. A carmelite monk, Father Toscarini, who was converted by the
writings of Galileo to the opinion of Copernicus was the innocent cause of it.
In 1615 he published a letter addressed to the general of his order where he
examined the sense in which appeared the sense in which several passages of
scripture which appeared contrary to Copernicus ought to be understood. And
without deviating in the least from that respect which was due to the sacred
writings he proposed a method of reconciling them equally wise and ingenious.
Some time antecedent to this a Spanish theologian in a commentary on the book
of Job, had embraced the system of Copernicus, and had remarked that in matters
of philosophical discussion the holy writings always expressed themselves
conformably to the language and opinions of the common herd of Mankind. This
same doctrine had been taught before him by many learned doctors and
commentators much respected in the church. But these authorities were
insufficient to preserve Galileo and Toscarini from censure. Their works were
presented to the tribunal of cardinals, whose office it was to inspect the
different books which were published. They were condemned and the work of
Copernicus, which had occasioned them, was also involved in </span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">the
condemnation. It was also ordered that in all new editions certain parts should
be omitted, particularly those which treated this system as a reality, and also
two chapters in which the opinions of those who maintained that the holy
writings were to be understood literally was treated with a sort of contempt.
Finally this tribunal came to the following resolution: the opinion of those
who place the Sun immovable in the centre of the Universe was declared to be
heretical, false and absurd, and that which placed the Earth in this centre but
gave it rotation on its axis was qualified as being erroneous in point of faith
and a dangerous tendency. Galileo's reputation was too great and his
discoveries had contributed too much credit the system of Copernicus for him to
escape the censure of the Inquisition. No sooner was this new heresy of the
Earth's motion brought before the consideration of the tribunal that this great
man was cited to appear before it as one of the greatest promoters. He did not
think fit to expose himself to a long imprisonment or perhaps to some worse
punishment by too obstinate an attachment to his opinions. He therefore
disadvowed them and they suffered him to depart about the beginning of 1616.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Galileo,
however, meditated a piece of revenge which he put in execution a few years
after. He laboured in retirement at his Dialogues on the three celebrated
systems of the world which is a complete apology for that of Copernicus
considered in a physical point of view. His next difficulty was to get it
printed a licence was necessary but how was it to be procured. For this purpose
he wrote a preface in which he explained that several foreigners had thought
and even published their opinions that the condemnation of the Copernican
system was the work of a tribunal which was ignorant of the reasons that might
be alleged in its favour and that he wished to show that the Italian doctors
were not less conversant with the reasons for and against it than the most
learned strangers. By this artful representation he was permitted to print his
book: it appeared in 1632 and consists of a dialogue between three
interlocutors. The first is Sagredo a Venetian senator, another is himself
under the name of Salviati, and the third is Simplicius a peripatetic. Of
course the unfortunate Simplicius is only introduced to be beaten and confuted
in the clearest manner, although he is furnished by Galileo with the strongest
arguments ever adduced by the Aristotelian school.</span></div>
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<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">It was
impossible that the object of these dialogues could for a long time be
concealed. The success which they had and the ridicule they cast on the
adversaries of Copernicus re-awoke the Inquisition. Galileo had had several
disputes on hydrostatical questions about comets with a certain Father Grassi,
a Jesuit, and it is asserted that the good father contributed not a little to
animate the rage of the inquisitors. Doubtless Galileo considered himself
secure from the resentment of this tribunal under the protection of the Grand
Duke of Tuscany, but this prince either through weakness or from political
motives dared not support him: and Galileo cited a second time before the holy
office was obliged to make his appearance at Rome. On his arrival he was thrown
into prison and detained until his sentence was pronounced. They threatened him
with the severest punishments if he did not a second time recant his opinions
and if he should ever again presume to teach by word or writing the heretical
doctrine of the Earth's motion.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">By these
means they obtained from him that humiliating recantation which was published
all over Europe and which furnished matter of<span style="mso-spacerun: yes;">
</span>triumph to the enemies of Copernicus and himself. Part of the abjuration
he was compelled to sign runs thus "I Galileo, in the seventieth year of
my age brought personally to justice and being on my knees with a sincere heart
and faith I abjure and detest the absurdity error and heresy of the motion of
the Earth". What a spectacle must this have been. What a contrast between
the venerable sage and his intolerable persecutors. A philosopher respectable
for his age, illustrious from his attachment to science, whose discoveries had
exalted our idea of the Creator from a wider acquaintance with His works,
compelled by an ignorant tribunal of bigots to assert his belief in doctrines
contradicted by the evidence of his senses. </span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Posterity
has in some measure compensated the misfortunes of Galileo, if posthumous fame can
be considered in this light. His name is ever joined with his brilliant
discoveries while those of his uncharitable judges loaded with the contempt of
the wise and good have faded from the pages of history. This, however, was not
sufficient to satisfy the tyranny of the Inquisition; the rest of his sentence
was that he should repeat a certain number of prayers daily and be imprisoned
for life. This latter part of it was mitigated at the intercession of the Grand
Duke of Tuscany and, after the expiration of a twelvemonth, Galileo was
liberated, but forbidden to leave the territory of Florence lest he should
withdraw himself from the power of the Inquisition. He devoted the remainder of
his life to those pursuits which had occupied his youth and terminated his
illustrious career in 1642 regretted by all Europe.</span></div>
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<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">If
curiosity alone were interested in the measure of the Earth it would be a
sufficient reason for undertaking the enquiry. What can be more natural to Man
than the desire of becoming acquainted with the magnitude of the globe which
has been assigned him for an habitation? But there are other motives which
amply justify the anxiety that has been displayed particularly during the two
last centuries to arrive at the knowledge of this measure. A slight acquaintance
with geography will convince us that it is of the greatest importance and that
it would be impossible to estimate the errors which would be committed in the
distances of a vast number of places are only determined by astronomical
observations, if we did not know what length on the Earth corresponded to a
certain number of degrees. The art of navigation relies constantly on this
measure. We have already seen that several ineffectual attempts were made for
completing this object by the Greeks and Arabs, and that from the numerous
errors to which their methods were subject that little reliance can be placed
on their results.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">The
celebrated Fernel, a physician and mathematician of the 16th century, is the
first among the moderns who undertook to determine the magnitude of the Earth.
He went from Paris to Amiens which is nearly in the same meridian and measured
the distance by the number of revolutions made by the wheel of his carriage. He
travelled in this manner until the height of the pole was altered one degree
and determined the length of a degree on the Earth's surface at 56,746 toises,
which does not differ from its real length by one part in eighty. This accuracy
would have done much credit to Fernel if it could be ascribed to the method he made
use of but it is obvious that a philosopher travelling in a carriage is
ill-prepared for operations in which the greatest precision is required and
that it was only by a fortunate chance that he approach so near the truth.</span></div>
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<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Snellius
[Snell] is the next who attempted a more accurate measure. He is the author of
an excellent method of measuring a large arc of the meridian. As it is the
foundation of this operation and is the only method that has been used by
astronomers during the last half century some account of it will be necessary.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>If at the two ends of a base line of
known length the angles, which some distant object forms with the other end be
measured, the distance of the observed object from either end may be found by
calculation. If two lines of known length form an angle, and if this angle be
observed, the distance between the two ends may be found without the trouble of
measuring it. It is on these two principles that the determination of a
terrestrial arc depends. Some convenient level spot must be chosen on which a
base of considerable length must be measured; from the two ends of this the
angles with some distant object must be found. From the principles already
mentioned the distances of these objects from each end of the base may be
ascertained and consequently their mutual distance. This line whose length is
thus determined may be used as a base, and other objects must be chosen whose
distances are to be determined in the same manner. These operations are to be
continued until we arrive at the end of the arc proposed to be measured. We
have thus a series of triangles from which by trigonometrical methods it is
easy to determine the distance in a straight line of the first from the last
point.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">This is
but an outline of the plan usually pursued in such undertakings. There are
numerous little niceties made use of for avoiding even the minutest cause of
error. These however will be better understood in the account of each
particular operation. Snellius was the first who made use of this method; he
measured a degree in Holland between the towns of Alcmaer and Bergen-op-Zoom.
Finding his measurement incorrect he revised it and observed all the angles a
second time. From the calculations resulting from this second attempt he
deduced the length of a degree which corresponds much more accurately with
[the] modern result.</span></div>
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<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>Shortly after this Richard Norwood,
an Englishman, undertook the task of measuring the distance between London and
York. The method he pursued was different from Snellius. He measured with a
chain the length of the road and by means of the compass he determined how much
it deviated from the meridian. When he came to a hill he observed the angle
which it formed with the horizon and made allowances for it in his
calculations. At each of the two stations which determined the length of his
arc he observed the altitude of the pole, and thus determined the number of
degrees he had measured.</span></div>
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<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">The next
astronomer who undertook the difficult task of assigning the magnitude of the
Earth was Riccioli. It would be uninteresting to detail the means he made use
of and the numerous errors to which they were subject. It is sufficient to
remark that he commenced his undertaking with a conviction that the ancient
measures were perfectly accurate and consequently used every means to make his
own coincide with them, and to this may be probably be attributed the enormous
error which occurs in the value he assigned to the length of a degree.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>The diameter of the Earth is, as it
were, the scale by which we measure the distance of the heavenly bodies; it is
in some respects the first element of astronomy and its importance naturally
led Men to<span style="mso-spacerun: yes;"> </span>undertake operations on a
large scale to determine it. It must however be confessed that notwithstanding
the labours already mentioned it still remained a matter of considerable
uncertainty.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">The two
most accurate measures yet recorded differed from each other more than 7,000
fathoms in the length of a degree. The Academy of Sciences [of France]
considering that the art of observing had lately made considerable advances and
wishing to determine so important a point selected Abbe Picard for the
purpose<span style="mso-spacerun: yes;"> </span>of measuring a new degree in
the neighbourhood of Paris. In this new attempt the greatest pains were taken
to ensure accuracy, but in these delicate operations there are so many
precautions to be taken the greater number of which are only suggested by time
and by the errors of others that it is no reflection on the accuracy of Picard
that the most scrupulous examination of his observations has only caused an
alteration of about 30 toise in the result of his measure.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>About this time a voyage was
undertaken by several French astronomers for the purpose of observing some
important points. It is only mentioned at present from the discovery to which
it gave rise. It was observed by Richer that a pendulum clock, which he had
brought from Paris where it had kept regular time, on being set in motion at
Cayenne [French Guiana], which is situated near the equator, lost nearly two
minutes and a half during 24 hours. This obliged him to shorten the pendulum
[by] about ?th [of an] inch, the quantity by which it had been shortened while
he was at Cayenne.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">This
singular fact excited great astonishment. It was impossible to explain it by
the effect of heat on the pendulum for it [also] took place on the summits of
the Andes, where the temperature of the air is the same as in Europe, or even
colder. The first who gave a satisfactory explanation of it was Newton. He
attributed it to the spheroid figure of the Earth and proved that its two
diameters were not equal. If the Earth is flattened at the poles the length of
a degree at the equator will be shorter than that of one at the pole. The
results of the degrees already measured could not be reconciled to the
hypothesis of the Earth being flattened at the poles, but it must be confessed
that they were very unfavourably situated for determining the question. The
learned world was at this time much divided. Newton and all the philosophers
who relied on physical enquiries maintained that the Earth was flattened at the
poles, while all the followers of Descartes and a multitude of foreigners
assigned to it an elongated figure.</span></div>
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<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">It is a
very singular circumstance that the first operation undertaken with a view to
determine this point should have been decidedly unfavourable to the Newtonians.
From the results of 8 or 9 degrees measured by Cassini in France it appeared
that they increased as they approached the equator. The only plan which now
remained was to decide the question by the measurement of two degrees, one near
the pole and the other at the equator. For this purpose two expeditions were
undertaken which do infinite credit to the country which gave birth to them.
Maupertuis, the President of the French Academy, accompanied by Clairaut ,
Camms and Lemonnier undertook a voyage to the polar circle. The spot they
selected for their operations was situated near the town of Torneo in Lapland.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>It is impossible sufficiently to
admire the zeal and ardour with which these philosophers surmounted the
greatest difficulties. They traversed the thick forests of Lapland which had
never yet been penetrated. They climbed the steepest mountains and remained
whole weeks on their summits exposed to all the inclemencies of the weather. By
such means, after nearly 3 months labour, they were in possession of an
excellent series of triangles, which extended from one end of the arc to the
other. This however was not the only labour. A base must be measured, but the
circumstances of the country which were adverse to every other part of the
operations were favourable to this. A line of 7,500 fathoms in length was
measured on the ice of the river which flows near Torneo and by a second
measurement there only appeared the slight difference of four inches.</span></div>
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<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">The
expedition to the equator was undertaken by Godin, Bouguer and La Condamine.
They assembled at Quito in South America and were joined by two Spanish
astronomers appointed to assist them. They commenced their undertaking by
measuring a base, but in a country covered with mountains it was difficult to
discover a piece of ground sufficiently flat for their purpose. They were
fortunate in finding a spot of ground of sufficient length near Quito. This
they measured twice and the difference of the two measures only amounted to 2
inches. But the difficulties they met with in the determination of the base
were but a slight prelude to those which they afterwards experienced. If we
imagine a lofty chain of mountains whose summits though situated in<span style="mso-spacerun: yes;"> </span>the torrid zone are covered with perpetual
snow and whose less elevated parts are continually involved in rain and fogs we
shall have a faint idea of the places in which these philosophers were obliged
to fix their stations. Here they were compelled to stay sometimes for whole
months waiting for some fortunate interval of fine weather in order to observe
their signals placed on other distant mountains.</span></div>
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<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">These
local inconveniences prolonged their stay during two years. The work was at
last completed in 1739 and fixed the length of 3 degrees. Such was the
termination of one of the grandest expeditions ever undertaken for the purposes
of science. Our philosophers now separated each to return to Europe by a
different route. The adventures they met with are rather curious. Bouguer was
the first who arrived in France. He pursued the shortest course and crossed the
Atlantic. La Condamine possessed of the greatest intrepidity and an insatiable
curiosity boldly traversed the vast continent of South America and descended
the River Amazon. Of this interesting voyage he published a separate account
which by the elegance of its style and the animated picture which it presents
of the dangers he escaped and of the manners and customs of the countries he
traversed, excites all the interest of fiction while it possesses the higher
recommendation of truth and information.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">The two
Spanish officers returned round Cape Horn, the southernmost point of America.
They arrived in Spain in 1746, the first without any accident, the second was
captured by an English cruiser. He however pays a grateful tribute to the
hospitality he experienced in this country. His books and papers were
immediately restored [to him] and he was elected a member of the Royal Society.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>Godin another of the party was
detained in Peru by various circumstances until 1748 and witnessed the horrible
catastrophe which destroyed the towns of Gallao and Lima. A brother of Godin,
who had accompanied him as far as Paraguay, remained there to be joined by his
wife and family whom he had desired to meet him by descending the River Amazon.
They had the misfortune to be lost in the vast woods of the interior of
America. This circumstance gave rise to a pathetic letter of La Condamine in
which he relates the fate of the various astronomers who had participated in
the measure of the Earth. He there details the sufferings she experienced in
wandering those vast deserts after seeing seven of her companions perish until
by accident she regained the bank of the river.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">The
surgeon who accompanied the expedition was assassinated at a bull feast at
Cuenca. La Condamine travelled 500 leagues to obtain the punishment of the
assassin, who was transported to the island of Chiloe, but soon after obtained
his liberty.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>The result of the operations we have
detailed left no doubt respecting the figure of the Earth. It was decided that
the degrees diminish as we approach the equator and consequently the Earth is a
flattened spheroid.</span><br />
<b><span lang="EN-GB" style="mso-ansi-language: EN-GB;"><br clear="all" style="mso-special-character: line-break; page-break-before: always;" />
</span></b><br />
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><b>References</b></span><br />
<br />
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><b><span style="-webkit-text-stroke-width: 0px; color: black; display: inline !important; float: none; font-family: sans-serif; font-size: 11px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: auto; text-align: left; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px;"><span class="Apple-converted-space"> </span></span><span class="reference-text" style="-webkit-text-stroke-width: 0px; color: black; font-family: sans-serif; font-size: 11px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: auto; text-align: left; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px;"><span class="citation book" style="word-wrap: break-word;">Bouguer (1749).<span class="Apple-converted-space"> </span><a class="external text" href="http://books.google.com/books?id=LiHvRISNxFsC" rel="nofollow"><i>La Figure de la Terre: déterminée par les observations de messieurs Bouguer, & de La Condamine .</i></a> chez Charles-Antoine Jombert.</span></span></b></span><br />
<br />
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><b><span class="reference-text" style="-webkit-text-stroke-width: 0px; color: black; font-family: sans-serif; font-size: 11px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: auto; text-align: left; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px;"><span class="citation book" style="word-wrap: break-word;"> </span></span> </b>
</span></div>
<span lang="EN-GB" style="font-family: "Arial Black"; font-size: 14.0pt; mso-ansi-language: EN-GB; mso-bidi-font-family: "Times New Roman"; mso-bidi-font-size: 10.0pt; mso-bidi-language: AR-SA; mso-fareast-font-family: "Times New Roman"; mso-fareast-language: EN-US;"><br clear="all" style="page-break-before: always;" />
</span>Unknownnoreply@blogger.comtag:blogger.com,1999:blog-1290567400778113129.post-26821957381112226692013-11-11T10:03:00.002+00:002013-11-11T23:49:45.111+00:00Lecture 3: Observing and Cataloguing the Heavens<h1>
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Lecture
3</span></h1>
<div>
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><br /></span></div>
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<div>
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><br /></span></div>
<h1>
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Observing and Cataloguing
the Heavens</span></h1>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>In my last lecture I described some
of the instruments which are made use of by the practical astronomer. There are
others which are sometimes employed, but as these are not absolutely necessary
to our fictitious observer I shall defer their description untill we arrive at
those parts in which they are more particularly concerned.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>Let us now pursue the path of our
imaginary Astronomer. Let us suppose him placed in an elevated station having
an uniterrupted prospect on every side to the horizon. He will naturally
distinguish in the heavens three kinds of bodies marked by entirely different
characters. The Sun, whose preserve is always indicated by the copious emission
of light and heat, whose duration above the horizon constitutes that period
called day. The Moon, whose rays never produce the least sensible heat, but
illuminate in different degrees at various times. And the third kind of objects
are the Stars, which yield no heat and but just sufficient light to render them
visible. These are the less conspicuous and least splendid objects which
attract his notice, but accordingly the laws of investigation to which we prescribed
to him, he must for the present defer the consideration of the lunar and solar
orbs which, though the most brilliant, are but solitary individuals and confine
his attention to the examination of that numerous class called Stars.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">The first
question respecting them which will present itself to his mind will probably be
this. What becomes of these objects during the day? is their light
extinguished? have they themselves been removed? or what is the cause of their
disappearance? By looking at their gradual appearance at sunset he will soon be
convinced that they have not been removed from their places during the presence
of this luminary, and it will soon occur to him that the probable cause of
their being invisible is owing to the superior splendour of the Sun preventing
their feeble rays from making any impression on our organs of sight. To put
this explanation to the test he will make the following experiment.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>Having provided a long tube or a
long telescope would be still better he makes the apertures at the ends
extremely small, so that only a few rays shall enter at once. When this is
directed to any part of the heavens at a distance from that in which the Sun is
situated, the Stars will be visible by means of it. The reason is obvious. The
rays from all other objects but that to which it is directed are excluded from
the eye, and thus the faint rays proceeding from a star are rendered visible,
even during the day.</span></div>
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<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">After
looking at them for a short time, he perceives that they appear all in motion,
some moving but slowly, others with considerable velocity. He observes, in one
quarter of the horizon, new stars appear, and at the opposite side those which
were visible seem to sink below. The first conjecture he makes on these varied
appearances is, that all the stars move in the same direction. To determine
whether this is just he turns his eyes to that part of the heavens where the
greater number appear to rise, and he examines whether there may not be some
few which set in this part of the heavens. He will find that there are none.
And on turning to that quarter where the stars appear to set he will likewise
find that there is no exception. He is thus led to conclude that all the stars
move in the same direction.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>This first result to which our observer
arrives he is still anxious to confirm and continues to examine if there be no
exception to it. Upon viewing some few, however, he finds to his disappointment
that their apparent motion is in a contrary direction. On these stars he
particularly fixes his attention, and after looking at them for some time, he
finds that they never sink below the horizon, but appear to describe some
curve, and when they get to the lowest point they reascend. This curve, it soon
appears, is a circle, and the apparent exception is now explained. These stars
move in their circles in the same direction as the others which he had
observed, but<span style="mso-spacerun: yes;"> </span>from the nature of this
kind of motion, when they are in the lowest part of the circle, their motion
appears contrary to the direction in which they move when they are in the upper
half.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Thus then
it is determined that this first law, namely that the stars all move in the
same direction is confirmed without a single exception. Our fictitious
astronomer has, however, made an important step by the doubt which at first
assailed it: he has found that some of the stars continue above the horizon
during the whole night, and that they describe circles round some imaginary
point. For the sake of convenience he gives a name to this point which will
frequently be referred to and calls it the Pole. No star is situated precisely
in this point, but numbers move round it. One, however, is placed sufficiently
near to mark its position, and is from this circumstance called the Pole Star.
This will indicate in a rough manner the height of the Pole above the horizon.
But our astronomer will devise a more accurate method. His attention, however,
must for the present be turned to other objects.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>In order to discover the various
motions of the stars, he considers a few of the principal and most luminous and
measures with his sextant their relative distances from each other. For the
purpose of distinguishing those which he observes he may perhaps represent
their positions on a globe or a sheet of paper marking on it carefully their
measured distances. This he puts aside in order to compare it with future
observations. After making several of these drawings or maps of different parts
of the heavens and comparing them on many successive evenings with the sky he will
most probably find that they have always retained their respective distances,
or that the figures which he had drawn of them will still represent their
positions. It was doubtless by observing this constant regularity in the
heavens that the idea of dividing them into Constellations first occurred,
obviously for the purpose of distinguishing the various stars from each other
and likewise for the convenience of describing their situation when they are
not visible.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">It may
however possibly happen that a few of these observations indicate a relative
motion in some of the stars. If this should be the case our astronomer will,
for a short time, be puzzled: but on comparing a great variety of similar
observations he will find that it is only two or three stars which have this
motion, and he will therefore leave them out of his consideration at present.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>This discovery brings a new class of
bodies to his notice. And the result of the observations he has now made is
this: that the stars all move in one direction with considerable speed, and
that the greater part of them never change their relative position, but that
some few are exceptions to this rule and have a peculiar motion of their own.
His principle of philosophising would now lead him to consider the general rule
and omit for the present these exceptions. But before we leave the newly
discovered bodies our astronomer must have some means of distinguishing them
from other stars lest by this mistake he should be led into errors. After some
trials he will probably find out this accurate and infallible method: by
viewing these wanderers, which from this circumstance he calls planets, with a
telescope possessing a tolerable magnifying power he will see them much
increase in magnitude, but if he direct this same glass to any of the fixed
stars he will see them exactly of the same size as they appear without its
assistance, or indeed rather smaller.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">The
planets will appear small luminous circular bodies; the fixed stars will seem
brilliant points. Whatever may be the magnifying power made use of the result
will always be the same. The fixed stars viewed through the most powerful
telescopes yet constructed always appear as mere luminous points. This then is
a certain criterion by which they can always be distinguished from other
bodies.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>Having now discovered that the stars
all move in the same direction and always preserve their relative distances an
important question will naturally suggest itself. What becomes of these stars
which sink below the horizon? how does it happen that they reappear in the
opposite part [of the sky]? An attentive consideration of the phenomena we have
remarked will be sufficient for the solution to this question.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>It was observed that some of the
stars described circles round an imaginary point called the Pole, and never
disappeared below the horizon. These were situated at various distances from
the Pole; some of them might in the lowest part of their circles almost touch
the horizon. At a small distance beyond these other stars would sink below and
be hid for a short time but<span style="mso-spacerun: yes;"> </span>would soon
reascend. Now those stars which remained but a short time below the horizon did
while above it always continue at the same fixed distance from those which
described circles. There is nothing which can induce us to suppose this
distance changed while they are out of sight for they reappear with the same
distance between them that there was when they parted from us. The conclusion
is inevitable. They could not have altered that distance when out of our sight.
From this it necessarily results that they likewise described circles around
the same imaginary point or Pole.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">The same
train of reasoning may be applied to all other stars whether nearer or more distant
from the Pole. -As important consequences result from it I will repeat the
process- It is shortly this: certain stars which we see during the whole of
their revolution move in circles round the Pole. Certain other stars which we
see only in a part of their course, because they sink below the horizon, always
preserve a fixed distance from the first mentioned ones, while we have the
power of observing them. The consequence drawn from this is that they preserve
the same distance when we have not the means of viewing them and therefore they
also move in circles round the Pole. This law happily connects together a
variety of appearances observed in the heavens. We see clearly from it why some
stars appear to move fast [and] others more slowly. Those which move swiftly
have larger circles to describe than those which have a slow motion. And
therefore, that they may all finish their revolution in the same time, the
velocity of the former must be proportionally larger than that of the latter.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Another
very important result of this law is that the Earth is insulated in space; that
it is not as was supposed by some of the ancients an immense place surrounded
by an interminable ocean. It has limits for the stars move round it. But what
may be its figure? or what is its magnitude? are enquiries which our imaginary
observer must for the present postpone. Other investigations of more importance
which present themselves in crowds at present demand his attention.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>It is now obvious that somewhere
beneath or on the other side of the Earth there must be another stationary
point or Pole. One these two the whole heavens will appear to turn. This latter
is called the South Pole, and the line which joins the two is called the Axis.
At an equal distance from these two points will be a great circle extending
round the heavens which is called the Equator. There are points and also a
circle similarly situated on the surface of the Earth, which also possess the
same name.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">It will
now be convenient for our astronomer to form some hypothesis to connect
together the results he has arrived at. It is of little consequence whether
that which for the present he assumes be true or false, provided only that it
explains the few phenomena that he has observed. The ancient one of the crystalline
sphere will answer his present object very well. Let him then suppose that at
an immense distance from the Earth there is placed a transparent sphere in
which the stars are fixed. Its revolution round the Earth will explain all the
appearances he has yet observed.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>The next questions he would be
solicitous to determine would be whether this starry sphere moves uniformly on
its axis and whether its revolutions are always performed in the same time. To
determine this point is not a matter of much difficulty, he places the transit
instrument in any position he chooses and observes the passage of a star. He
notes the time indicated by his clock, leaving the instrument and repeating
this observation on the same star several successive evenings; and for the sake
of greater accuracy he performs it on many different stars. From this he finds
that the star always takes the same time to return to the instrument in
whatever situation it is fixed, and consequently, he considers their motion as
uniform.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Having
now discovered that the time of the revolution of the stars is always
invariable our astronomer may make a very advantageous use of it in future for
the purpose of regulating his clock. Another point which will now be necessary
to determine is the inclination of this sphere to the horizon, or in other
words to find the height of the Pole. We have before observed that there is no
star situated precisely in that point; how then is its altitude to be
discovered? Indirect means must be made use of, and the following method will
supply them. Those stars which never set are called Circumpolar Stars. Observe
one of these when it arrives at its greatest height above the horizon. Measure
its altitude by means of Hadley's Quadrant. And do the same when it arrives at
its least height. The difference of these two altitudes will give the diameter
of the circle which it describes round the Pole. Half this quantity added to
the lowest of the two altitudes of the star will give the height of the Pole.
When a star is at its greatest or least altitude above the horizon a
perpendicular circle passing through it likewise passes through the Pole, and
as this great circle is very frequently made use of a name is appropriated to
it, and it is called the Meridian. All stars which are not circumpolar are,
when they arrive at this line, at their greatest height above the horizon. And
when a star passes it is said to "Transit the Meridian".</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">The
purpose of finding a meridian line by which the transit instrument may be
rectified, the equable revolution of the stars may be made use of: let the
instrument be placed nearly in the plane of the meridian by means I have
described at my last lecture and let some circumpolar star, that is one that
never sets, be made choice of. This star revolves in a circle round the poles
and as it moves equally it is obvious that it will take just as much time to
descend from its highest elevation above the pole to its lowest depression
below as it requires to ascend from this latter point to its greatest altitude.
If now the transit instrument is placed nearly in the plane of the meridian,
and if we note the time which elapses during its progress from its greatest to
its least altitude, and again the interval of time which it occupies in
returning to its greatest height, these two intervals will be equal if the
instrument is accurately adjusted; but if they should be unequal the plane of
its motion must be altered by moving the proper adjusting screws.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">We may
now suppose our astronomer to have determined accurately a meridian line and to
have fixed his instrument in that plane. His next care will be to find some
easy method of regulating his clock. This likewise will be readily afforded by
the revolution of the heavens. He has observed that the same star always returns
to the meridian after the same interval of time. If therefore he adjusts his
clock by means of the pendulum so that when a circumpolar star comes to the
meridian above the pole it shall point to twelve o'clock and the when this same
star comes to the meridian below the pole it shall after having performed one
revolution return to the same hour. It is obvious that if the clock goes
correctly whenever this star is on the meridian the hands ought to point to the
hour of 12. A clock regulated by these means is said to be adjusted to sidereal
time. The difference between this and mean solar time will be explained when we
consider the motions of the Sun. It has been observed that the great circle
which divides the heavens into two equal parts and which is equally distant
from both poles is called the Celestial Equator or the Equinoctial. It is by
means of this circle and others called meridians which are perpendicular to it
that the situation of a star is determined. If a great circle be drawn through
any star and likewise through the pole it will cut the equinoctial line. And
the distance of the point of intersection from some certain fixed point assumed
in this line is called the Right Ascension [or R.A.] of a star. The point from
which Right Ascension is generally measured is the intersection of the
equinoctial line with another great circle called the Ecliptic. But as we must
for the present suppose our observer unacquainted with this point he must
employ some other means of reckoning it.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">For this
purpose he chooses any remarkable star and measures the R.A. of all other stars
by their distance from this when referred to the equinoctial. If, on this
supposition, he determines the R.A. of a multitude of stars and should
afterward, from any discovery, wish to alter the point from which it is
reckoned, it may be readily effected. Thus suppose the difference of R.A.
between the new point<span style="mso-spacerun: yes;"> </span>from which he
proposes to commence his reckoning and the point from which it was counted to
be 10 degrees then the R.A. of each star he has observed must be increased or
diminished by these 10 degrees according as the new point is situated to the
east or west of the point from which it was originally reckoned.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>For the convenience of calculations
it is usual to divide the circle into a number of equal parts called degrees:
there were generally 360, a number probably selected from its being nearly
equal to that of the number of days in the year. Each degree was again
subdivided into sixty equal parts called minutes, and these were also
subdivided into 60 parts called seconds. This is the oldest method of division.
Another has lately been substituted by the French. It consists in dividing the
quadrant in 100 degrees and each degree into 100 minutes. This is called the
decimal division of the circle. The former or sexagesimal division is usually
adopted in this country.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">To return
however to the determination of the positions of the stars, each star during
about 24 hours appears to describe a circle round the earth or to move through
360 degrees. It will therefore during a less space of time move over a
proportionally less arc of the circle. From this circumstance we may measure
the difference of R.A. between two objects. If a star is observed to transit
the meridian at a given hour and another star passes it 2 hours after we know
that the difference in R.A. is 30 degrees for 2 hours is the 12th part of 24,
the time in which a star moves through 360 degrees. Therefore we must allow 30
degrees which is the twelfth part of 360 degrees for its motion in two hours.
Another method will however answer this purpose with much less trouble. Let the
pendulum clock be so adjusted that the hour hand may move round the dial once
in 24 hours. Instead of marking the face of the dial with the hours let it be
divided into 360 parts or degrees. From this arrangement it is evident that the
heavens and the clock will make one revolution or pass over 360 degrees in the
same time. Suppose now that when a particular star transits the meridian the
hands of this timekeeper be set to the commencement of the divisions, it is
obvious that after any interval of time the star and the index of the clock
will have passed over the same angle. And consequently we may know exactly how
far this star is from the meridian. If therefore at the moment we observe the
transit of a star over the meridian the number of degrees indicated by the
clock be observed, this is the R.A.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Our
astronomer has now an easy and expeditious method of determining one of the
elements which fix the place of stars. He observes in his transit instrument
the moment a star is bisected by the middle wire and makes a signal to an
assistant who immediately observes the degrees indicated by the timekeeper. By
this means he may determine the R.A. of some hundred in the course of a night.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>An observation of a different nature
will be necessary for ascertaining the other element which determines the
position of a fixed star. This is called its Declination. Declination is the
distance of a star from the equinoctial measured on a great circle which passes
through the object and the pole. If therefore we can find its distance from the
pole the declination may be easily found by subtracting this from 90 degrees or
the quarter of a circle. But the polar distance may be readily found by means
of the astronomical quadrant. Its altitude must be observed when it comes to
the meridian. Let us suppose it a circumpolar star, and that we have observed
its altitude when it transits the meridian below the pole. As our observer has already
found the elevation of the pole, he has only to subtract from it the altitude
of the object and the remainder will be its polar distance.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">These two
quantities R.A. and declination determine the position of a star. Our
astronomer for the purpose of discovering whether there exist any minute
motions among them may be supposed to form a catalogue of an immense number and
to have ascertained accurately their R.A. and Declination.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>We have seen that the first labour
of this kind was undertaken by Hipparchus. But the number in his catalogue as
extended by Ptolemy only amounted to 1022. Succeeding observers devoted much
attention to this subject. Flamsteed the first who made use of a telescope for
this purpose gave a catalogue of 3000 stars.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>Dr. Maskelyne published a catalogue
of 36 of the most brilliant fixed stars. The situation of each was determined
by the mean of several hundred observation, and this list though small in
number was the result of several years labour.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>The catalogue of greatest extent in
point of number is that began in 1789 by Jerome Lalande and continued by his
nephew. By their joint labours they had in less than 6 months observed 3,000
stars, a number equal to that which had occupied Flamsteed during twenty eight
years.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">About the
end of 1790 the number observed amounted to 8,000 when the elder Lalande was
unable to support the fatigue it required and gave up the completion of the
task to his nephew which occupied him without intermission until 1799 when he
had determined the position of 50,000 stars.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>Our fictitious astronomer having now
ascertained that the motion of the stars is uniform and having determined their
situations may readily find by calculation the time at which they ought to rise
and set. He will therefore wish to put his theory to the test by observing if
these times correspond with those he has calculated. Upon trying various stars
he finds that they all appear to rise sooner and set later than they ought from
theory and that this happens indifferently to every star in whatever part of
the heavens it may be situated. This will induce him to try whether their
observed altitudes at different times correspond with those derived from
calculation. He will find that they do not accurately agree but that this disagreement
is always greatest near the horizon and diminish up to the zenith where it
disappears.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">On
considering what may be the cause of this difference he will observe that it
seems to have some connection with the Earth because it is much the greatest when
near the horizon. He will then be naturally led to enquire whether it may not
be owing to the air with which the Earth is surrounded. When an object is in
the zenith its rays will pass through but a small part of the atmosphere,
whereas when it is situated in the horizon they must traverse a large portion
of dense vapours. Analogy will confirm the conjecture. It is an established
principle in Optics that as soon as rays of light enter obliquely into a medium
the density of which differs from that of the medium from which thay came they
are bent from their rectilineal direction. If the medium into which the rays
enter is equally dense throughout they are only bent at their entrance, but if
its density increases in proportion to its depth the rays of light will be more
and more curved their curvature following a law that is correspondent to that
proportion.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">This
takes place in the rays of light passing from the heavenly bodies to the eye,
for the atmosphere through which they must pass before they arrive at the eye
being unequally dense causes the rays to be bent and arrive at they eye in a
different direction from that in which they would come to it were it not for
the effect of the intervening medium. The difference between the real and
apparent place of the heavenly bodies as affected by the passage of the rays of
light through the atmosphere is by astronomers called Refraction. When rays of
light pass from air into a fluid or vice versa this refraction is very evident.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>The appearance of a stick or an oar
partly immersed in water is familiar to everyone it appears broken. This is
owing to the part within the fluid appearing raised by refraction. It is from
the same cause that the real depth of water is always one third more than it
appears to be. The practical application of this property is well known to
fishermen who when they wish to destroy a salmon by the spear or by shooting it
always aim considerably below its apparent place. These are strong arguments
from analogy that refraction exists in the atmosphere, but if it does it might
perhaps be expected that distant land objects should appear elevated above
their real situation. This in fact does take place in particular states of the
atmosphere. It is called Terrestial Refraction and as the circumstances which
sometimes accompany it are very remarkable some account of them may not be
uninteresting.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">A
singular phenomenon of this kind is related to have been discovered at the town
of Modbury in Devonshire which is situated at 12 miles distance in a straight
line from Plymouth. On the 4th December 1793 a gentleman viewing the
surrounding country with an acromatic telescope descried an object like a
perpendicular pole standing up in the chasm of a hedge which bounded his view
at about 9 miles distance which from its direction was conjectured to be the
flagstaff on Maker near Plymouth. Directing the glass the next morning to the
same part of the horizon a flag was perceived on the pole which corroborated
the conjecture of the preceding day. This day's view also discovered the
pinnacles and part of the shaft of the tower. Viewing the same spot at 8 am in
the morning on the 9th January 1794 the whole tower and part of the roof of the
church with other remote objects not before noticed became visible.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">The singularity
of this phenomenon has occasioned repeated observations of it. From all which
it appears that the summer season and wet windy weather are unfavourable to
this refracted elevation, but that calm frosty weather with the absence of the
Sun are favourable to it.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>Another curious effect of refraction
is the appearance of two or three images of the same object. Sometimes the
additional images appear above sometimes below the real one. At one time it is
erect; at another it is inverted. An instance of this nature was mentioned in
the Philosophical Transactions for 1795 by Mr Dalby who relates that he
observed the top of a hill on the horizon to appear detached and the sky to be
seen under it. Another instance was observed at Malta. There appeared rising out
of the sea at the distance of about 8 miles an island in the shape of a conical
mountain. It excited the surprise of hundreds collected to view it and several
fishwomen put off to take possession of this newly risen island. But long
before they could arrive at the spot it was imagined to have occupied the
illusion had vanished and no traces remained. This appearance was seen from the
observatory, but the telescope through which it was viewed followed the
stranger home again and discovered at an immense distance in the same direction
the lofty summit of Mount Etna, which in clear weather is just visible to the
inhabitants of Malta. These and various similar phenomena have been explained
by Dr. Wollaston in Phil. Trans. 1801. He refers them all to two causes. The
first is a considerable difference of density between different strata of air
in contact with each other. The other cause is rapid evaporation which
increases the refractive power of the lower strata of the atmosphere. To the
first of these causes may be attributed the appearances of double images and
the inversion of objects; to the second may be ascribed<span style="mso-spacerun: yes;"> </span>their extraordinary elevation. This latter
is illustrated by the appearances of Maker tower.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">I shall
only mention one other phenomenon which has been accounted for by the
refractive power of the atmosphere. It is well known that the caravans which
travel through the deserts of Egypt are obliged to carry water with them as
they meet with no supply of this very necessary article during their journey. it
sometimes happens that their stock is exhausted and they are compelled to
search for one of the wells of brackish water which are thinly scattered
through the desert while thirsty travellers are thus employed it sometimes
occurs that they perceive at a short distance the appearance of a lake. If
unacquainted with the deception they rush forward to cool their burning lips
the tempting beverage like the rainbow flies their pursuit.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>It is in vain they accelerate their
pace. However they advance it still preserves the same provoking distance thus
realising to them the fictitious sufferings of Tantalus. Sometimes though more
rarely a city appears at a small distance and gradually removes as they
advance. Both these phenomena were observed by the French army in Egypt and
both are explained by refraction. The following solution was given by Monge, a
member of the Egyptian Institute. The sand with which the desert is covered is
heated by the rays of the Sun to a very considerable degree. This causes the
lower strata of the air close to the surface of the sand to be much rarified.
If now a ray of light proceeds from a town a great distance it will enter the
lower strata very obliquely and the effect of refraction being here very strong
will turn the ray into a direction nearly parallel with the surface; but still
continuing to suffer the refractive influence it will be bent slightly upwards
and then reach the eye of a spectator still more distant instead of being lost
by striking against the earth. If there is no city at a distance the rays
proceeding from the clouds will suffer the same operation and in that serenity
of an eastern sky might be readily mistaken for water. This phenomenon is from
the resemblance of the operation of the heated air to the effect of a looking
glass called the Mirage.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">This
explanation is an ingenious one, but there are some circumstances which it does
not explain. I am inclined to believe that the theory of Dr. Wollaston is more
applicable to it and that it arises from the evaporation during the day of the
copious dew which falls in the night. One strong reason against the hypothesis
of Monge is that, if it were true, there ought to be a city at about 6 to 8
times the distance of the apparent one. This however is not the case for similar
appearances are asserted to have taken place at many days' journey from any
town. Dr. Wollaston's theory will account for this. He has shown that if the
evaporation from the surface of water be sufficient to cause a refraction of
about one minute of a degree in every mile the ray of light will have an equal
curvature with the surface of the Earth and consequently an object may become
visible at any distance.</span></div>
<span lang="EN-GB" style="font-family: "Arial Black"; font-size: 14.0pt; mso-ansi-language: EN-GB; mso-bidi-font-family: "Times New Roman"; mso-bidi-font-size: 10.0pt; mso-bidi-language: AR-SA; mso-fareast-font-family: "Times New Roman"; mso-fareast-language: EN-US;"><br clear="all" style="page-break-before: always;" />
</span>Unknownnoreply@blogger.comtag:blogger.com,1999:blog-1290567400778113129.post-57057421983016905262013-11-11T09:56:00.002+00:002013-11-11T18:26:59.766+00:00Lecture 2: On Astronomical Instruments<h1>
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Lecture
2</span></h1>
<h1>
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">On Astronomical
Instruments</span></h1>
<h1>
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"> <table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhu7DLDDeG6Xx92AHbkBOCH6R4eOAMCgYQIuZdBh74zaJbs4Z8Db2XAqAn7gvgHnuCwUkGdXAYnADWV3KFxxfXruca1h-8jAhr6KOzWedxElc2XGDO3H4F1Ro0Yme1fIzulcRonFlLSu1s/s1600/Sextant.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="320" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhu7DLDDeG6Xx92AHbkBOCH6R4eOAMCgYQIuZdBh74zaJbs4Z8Db2XAqAn7gvgHnuCwUkGdXAYnADWV3KFxxfXruca1h-8jAhr6KOzWedxElc2XGDO3H4F1Ro0Yme1fIzulcRonFlLSu1s/s320/Sextant.jpg" width="255" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">Sextant</td></tr>
</tbody></table>
</span></h1>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Advertisement
for the lecture taken from the Royal Institution's archives:-</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Guard
Book Volume 1 f139</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Thursday
March 2nd 2 o'clock Mr Babbage Astronomy </span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Lecture
II.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">The
difficulties peculiar to Astronomy, and the means of obviating them. Plan to be
pursued. Description of Instruments. The Pendulum the Measure of Time; its
irregularities; the means of correcting them. Astronomical Quadrant and
Circular Instruments; Hadley's Quadrant; Artificial Horizon; Transit
Instruments.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">[Pages 1
- 18 incl. of the manuscript for the lecture are missing]</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-spacerun: yes;"> </span>... other instrument, and as our imaginary
astronomer will have occasion to make use of it, some description may not be
uninteresting. The principle on which it depends is the apparent coincidences
of two images:<span style="mso-spacerun: yes;"> </span>one seen by reflexions,
the other viewed directly. For this purpose there are two mirrors. One of these
has a part of the quicksilver scraped off; through this vacant part one of the
objects is viewed directly, and the other object is reflected from the largest
mirror to that part of the small one, which is silvered, the index being moved
until they coincide. For the purposes of more accurate observation a small telescope
magnifying a few times is usually added. By this means the objects become
better defined, and the results are more to be relied on.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">The great
circumstance on which the vast utility of this instrument depends is this. That
during an observation it may be held in the hand. There is no necessity for any
steadiness any firm support. It is this which makes it valuable to the
navigator admidst the unsteadiness and fluctuations of the element which he
traverses. On this he securely relies.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>Notwithstanding the motions to which
the ship is necessarily subjected, he can make the most accurate observations
when he has once, by means of these glasses, brought the edge of the Sun to an
apparent coincidence with the distant horizon. It will appear immoveably fixed
whatever may be the agitations of the vessel on which he is placed. It is this
stability which is so desireable. It enables him readily to measure their
angular distance, an object he could have effected by no other instrument. He
has no occasion either for a spirit level or a plummet; both would be useless
in such a situation. And no other instrument can be rectified without them.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">There is
another advantage peculiar to the sextant. It is the only instrument by which
the distance of two celestial objects can be immediately ascertained without
the trouble of any calculation. This circumstance renders it of frequent use in
ascertaining the distance of the Sun from the Moon, or of the Moon from a star.
These means are constantly employed by seamen for determining the longitude.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>When this instrument is used to
ascertain the altitude of an object at sea, it is made, by reflections from the
mirror, to coincide with the horizon, which is always at the same distance if
the observer is at the same altitude above the level of the water. But when an
observation is to be made on shore, the case is altered. There is no horizon to
which the object can be compared. At least there is no certain and regular one.
The termination of our prospect is generally broken and uneven, intercepted by
trees or by hills. From this cause it would, at first sight, appear that the
sextant would be useless in observations on the altitude of the heavenly bodies
when made on land. There is however, a method by which this inconvenience experienced
by the want of an accurate horizon may be obviated. It was observed that, when
we look at a smooth piece of water, all the objects around appear represented
in it, but in an inverted position. The houses or trees which are adjacent to
it are reflected in the water of exactly the same magnitude the objects really
appear to the eye. In fact, the water acts just in the same manner as a looking
glass would if laid perfectly horizontal. If now the Sun is above the horizon,
it will be reflected in the water and its apparent depression below the surface
will be exactly equal to its real elevation. If therefore, we have any means of
measuring the angular distance of the Sun from its reflected image in the
water, it is clear that its real altitude above the surface of the water must
be equal to half this measured angular distance. The reason is evident. The
Sun's image appears just as much below the surface of the water as it itself is
actually above that surface.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Now by
means of the sextant this distance may be easily measured. The image of the Sun
in the water may be viewed directly and the reflected image may be made to
coincide with it. This was doubtless the object which first suggested the idea
of an artificial horizon, but it was soon found that the surface of the water
exposed to the active air was too unsteady to admit of much accuracy in the
observations: various others were substituted, but that which is now
universally adopted is quicksilver.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>This brilliant fluid reflects more
light than any other we are acquainted with and is on that account admirably
adapted to the purpose. The instrument in which it is employed, is of very
simple construction, being nothing more than a small wooden tray filled with
mercury. This is covered by a kind of roof formed of two bits of plate glass;
the use of this roof is merely to protect the quicksilver from the undulations
which might arise from the action of the air on its surface. There are but few
cautions to be observed either in its construction or use. It is only necessary
that the two surfaces of each glass ...</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">[remainder
of lecture missing]</span><br />
<br />
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="http://www.longcamp.com/gifs/artificial_horizon22.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" src="http://www.longcamp.com/gifs/artificial_horizon22.jpg" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">Artificial Horizon</td></tr>
</tbody></table>
</div>
Unknownnoreply@blogger.comtag:blogger.com,1999:blog-1290567400778113129.post-88225473021288029032013-11-11T09:54:00.000+00:002013-11-11T14:34:45.015+00:00Lecture 1: Introduction to and History of Astronomy, from Thales to Copernicus<h1>
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Lecture
1</span></h1>
<h1>
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Introduction to and
History of Astronomy, from Thales to Copernicus</span></h1>
<div>
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><br /></span></div>
<div class="separator" style="clear: both; text-align: center;">
<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh70rPVfD2HyuctmHZKJ8hbIR3Atwt3K7EG3wfR5vkYRT7b608BMgfA_3j1fAq0oNYPmA837oWPVOgQIUqkNtTvZNH9rJFJ9sStX8TowwhL4GbtHLGc0jqdrDm2YUgc9yVvACdO0dpJGbE/s1600/Thales.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="200" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh70rPVfD2HyuctmHZKJ8hbIR3Atwt3K7EG3wfR5vkYRT7b608BMgfA_3j1fAq0oNYPmA837oWPVOgQIUqkNtTvZNH9rJFJ9sStX8TowwhL4GbtHLGc0jqdrDm2YUgc9yVvACdO0dpJGbE/s200/Thales.jpg" width="131" /></a> to <a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEigukrERc_jXkfJQ_m1zfeYwdLLMgnwy-PLnv5c6iO_L3JYIqBMbyoot3cJk3BAoPQ8z4t05qKKHSsT9eYmUpgjTqHn3LltRwOFiW-pcXVABHYLoT1Sz7JLA9eIvwptjVZnTJyf0J9o610/s1600/Copernicus-Boissard.gif" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="200" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEigukrERc_jXkfJQ_m1zfeYwdLLMgnwy-PLnv5c6iO_L3JYIqBMbyoot3cJk3BAoPQ8z4t05qKKHSsT9eYmUpgjTqHn3LltRwOFiW-pcXVABHYLoT1Sz7JLA9eIvwptjVZnTJyf0J9o610/s200/Copernicus-Boissard.gif" width="156" /></a></div>
<div>
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><br /></span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>Of all the phenomena of nature the
celestial appearances are, by their greatness and beauty, the most universal
objects of the curiosity of mankind. To ascertain the distance and motions of
those luminaries which are scattered with apparent irregularity through the
regions of space, to discover the laws by which they are regulated so as to predict
with unerring accuracy the various appearances they will assume and their
precise situations in the heavens are the objects of Astronomy.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>Commensurate with the magnitude and the
difficulties of the subject have been the application the industry and the
talents devoted to its cultivation. Ages have elapsed since its infancy first
occupied the attention of mankind; each has been marked by some name rendered
illustrious by its pursuit, nor has its maturer progress diminished the number
of its ardent followers. By such combined assistance it has gradually risen to
its present state. It now presents itself the grandest monument of human
reason: it stands the first amongst the physical sciences unrivalled in the
accuracy of its results, in the certainty of its conclusions.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">The
discovery of the grand principle which connects it into one uniform system
which embraces in its grasp the minutest atom, and the most ponderous globe,
forms the finest specimen of the inductive philosophy of Bacon. And the truth
of this law and of the system of which it forms the basis rests on the surest
foundations, on a mass of evidence than which we can conceive nothing greater
short of demonstrative knowledge.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>On the utility of Astronomical
Science it is almost needless to enlarge. It may be sufficient to observe that
by its assistance the intercourse between the most distant nations is carried
on with ease and safety. Commerce is indebted to this source almost for its
existence. And thus by supplying the wants of one people from the superfluities
of another it contributes to the happiness and civilisation of mankind. There
is another point of view in which the calculations of the astronomer are of
great importance in the rectifications of the dates assigned to various events
of history. As this application is a curious one, an illustration of the method
may not be uninteresting.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Sir Isaac
Newton has applied it with singular felicity to the determination of some very
important eras. The manner in which he discovered the date of the Argonautic
expedition is peculiarly ingenious. It was founded on these considerations.
Astronomers have from the earliest ages conceived certain imaginary circles to
be described in the heavens. They have also distributed the groups of stars
which it exhibits into various figures termed constellations. This was the case
in the time of the Argonauts, and a sphere was formed by two artificers, Chiron
and Musaeus, to represent their respective situations.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>On the longest day in the year the
Sun is situated precisely in one of these circles, which is named the
solstitial colure. More modern observations have discovered that this
fictitious circle moves backwards with a nearly uniform motion. On this point
the success of Newton's method depended. He reasoned thus. The sphere invented
for the use of the Argonauts represented the state of the heavens at the time
of its formation. If we could see this sphere, the position of the colure would
readily be determined, but unfortunately it had long been lost. The only hope
then which remained was to consult those ancient authors who had given
descriptions of this sphere, and to examine if from anything they had mentioned
the position of this circle might be determined. Fortunately Eudoxus, in an
account he has given of this instrument, relates that the solstitial colure
passed through certain stars, which he names. This was sufficient for the
purpose of Newton. The fixed stars always retain their relative position; he
had only therefore to measure the distance of the stars through which it passed
in the time of the Argonauts from those which it cut in his own time. This
would give the space through which it had receded in the interval, which he found
to be nearly 37 degrees. Now it was well known that this colure requires 72
years to advance through the space of one degree. Newton therefore found by a
very simple calculation that it must have occupied 2,645 years in receding the
37 degrees. This he counted back from the year 1689 in which he wrote and
placed the Argonautic expedition about 43 years after the death of Solomon.</span></div>
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<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">The
period at which various other events recorded in history have happened has also
been fixed by their proximity to some great solar or lunar eclipse. And as
these are but of rare occurrence, and as the times at which they have happened
can be calculated with the greatest accuracy, the dates determined by this
means are worthy of very considerable credit.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>Thus the expedition of Xerxes
against Greece is generally thought to have taken place in the year 480 before
Christ. But from a circumstance mentioned by Herodotus this date would seem to
require correction. He relates that Xerxes marched from Sardis in the spring
and that a great solar eclipse happened, which terrified the army, who regarded
it as an evil omen. That Pytheas requested that his son might be dismissed from
serving any longer. But Xerxes refused the request and ordered the young man to
be cut to pieces and that the army should march between the parts. Now it is
certain from calculation that no solar eclipse did happen in the spring of the
year 480 BC. But there arrived a very considerable one about two years
afterwards, in the spring of 478 BC. From this it appears that Xerxes came into
Europe two years later than the period assigned by the common chronology.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">When we
endeavour to trace the history of Astronomy in the earliest ages of the world
we find it involved in the greatest obscurity. We can with difficulty separate
some fragments which bear the semblance of truth from volumes that are filled
with fabulous history and allegorical representations. The constellations into
which the heavens are divided and particularly the twelve which constitute the
Zodiac are perhaps the oldest remains we possess of ancient Astronomy, and with
respect to the origins of these, how various and how contradictory are the
opinions which have been entertained. By some it is asserted they are purely of
Egyptian origin, while others have asserted their invention entirely to the
Chaldeans. One author has displayed a profusion of learning in the northern
languages of Europe in endeavouring to prove that we are indebted to the wild
inhabitants of Lapland for the constellations which form the Zodiac, and even
for some of the more modern discoveries of Astronomy. Many have attributed them
to an Indian origin, but this seems improbable from several reasons. The
inhabitants of India confess that their system of Astronomy is not of their own
invention, but was borrowed from some other nation. And indeed their Zodiac
only differs from that of the Greeks in four of the signs. Besides a sphere of
Indian origin would have exhibited those characteristic marks which peculiarly
distinguish that people. We should have found among their constellations Brahma
or Vishnu, their gods. Surely they would have given a place in the heavens to
these the objects of their veneration: but if we examine the sphere we shall
find nothing which bears any analogy to the objects of their worship, the
instruments of their arts, or the animals with which they are familiar, nothing
which indicates an eastern origin.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Quitting
however the regions of fabulous history let us endeavour to trace the progress
of this Science by the light of more authentic records.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>Thales of Miletus appears to be the
first who transplanted the Sciences into Greece. Passionately fond of the study
of nature and entirely destitute of the means of pursuing it from the ignorance
of his countrymen, he travelled into Egypt in search of instruction. Finding
the priests the sole depositories of learning he applied himself to them, and
with such success that he soon became an adept in all their mysteries. A
circumstance which is related of him and which probably contributed to the
success of his application may serve to show the state of Science at this
period.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>Plutarch informs us that he measured
the height of the pyramids in the presence of Amasis, king of Egypt, who was
much pleased with the novelty and ingenuity of the method he used, which was
nothing more than this. He stuck a rod of known length upright in the earth,
and waited until its shadow became of the same length with the rod. He then
measured the length of the shadow projected by the pyramid, and concluded that,
as that of the rod was just equal to its length, the shadow of the pyramid must
be equal to its height. This is certainly the simplest method that could be
proposed, but we could hardly suppose so profound an observer to have been
ignorant of the other means which this contrivance supplies. It is obvious that
he need not have waited until the length of the rod's shadow became equal to
its height, but might have chosen any other proportion. Thus, for instance, if
the rod projected a shadow equal to twice its length, he might have found the
height of the pyramid, by halving the length of its shadow, and similarly with
any other ratio.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Thales
returned into Greece with all the science of the Egyptians, enriched and
matured by his own reflections. His countrymen flocked to him for instruction
and he became the founder of the Ionian school. His astronomical knowledge
seems particularly to have excited their attention; it was for that period of
time very considerable. If we may believe all that is related of him, he taught
that the earth is round; he explained the true cause of eclipses; he is even
said to have foretold one, and that event justified the prediction. This is
difficult to believe, but, if true, he must most probably have employed some artificial
method devised by the Egyptians. They possibly were aware that, after a certain
period of time, the eclipses return nearly in the same order. The true cause of
the phases of the Moon and a knowledge of the obliquity of the ecliptic are
also said to have been taught by the philosopher of Miletus. He was likewise
the first who attempted to measure the apparent diameter of the heavenly
bodies: the success of his plan and the accuracy with which it was executed
will be a lasting monument of his skill in the practical part of Astronomy.
Thales, however, did not confine himself merely to theoretical speculation
which, however praiseworthy and difficult, was not calculated to make much
impression on a people just emerging from a state of barbarism. He exerted
himself to apply Astronomy to objects of public utility. The Grecian calendar
was at this time in the greatest disorder, from an ignorance of the lengths of
the solar and lunar revolutions. This he rectified considerably, but, from a
want of observations made at distant intervals of time, it was impossible with
the instruments he possessed to arrive at any very considerable degree of
accuracy.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Anaximander
succeeded Thales in the direction of the Ionian school. He seems to have
entertained nearly the same opinions as his master, but the novelty of such
doctrines and the almost total want of proof contributed much to prevent their
diffusion. We are indebted to him for two very considerable inventions. The
first of these was the gnomon, which, it seems probable, was from his
construction nothing more than an upright wire placed perpendicularly on a
plane, which marked by the extremities of its shadow the hours of the day. This
rude beginning was sufficient for the common concerns of life, and was received
by his countrymen with admiration and gratitude.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>The gnomon was one of the first
astronomical instruments made use of by the ancients, and is certainly the best
calculated of any they possessed for exact observations on the altitude of the
Sun. But they did not pay all the attention requisite to the circumstances
which contribute to its accuracy. The shadow projected by a point is not
distinctly marked. There is always a fainter shadow round the interior and
darker one. It appears probable that they used this latter, and, if that were
the case, their observations ought to be corrected by subtracting the
semi-diameter of the Sun, in order to have the height of its centre. But it
must be confessed that we are by no means certain that this correction was not
applied.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">It
appears that Manilius was not ignorant of this circumstance when he had
direction of the gnomon erected by Augustus. This was an obelisk built near
Rome, and on its summit he placed a round ball. Its centre was accounted the
top of the gnomon, and the middle of the oval shadow, which it was easy to
determine, might be reckoned the extremity of the shadow. Great pains appear to
have been taken in the construction of this instrument: its height was upwards
of 70 feet, and the meridian line on which its shadow fell was formed of bronze
fixed into blocks of stone. Unfortunately its foundation was not sufficiently
secure, and in about 30 years after its erection it became useless.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>The other invention for which we are
indebted to Anaximander is the construction of maps or charts. We learn on the
authority of Strabo, that he produced a map on which was represented the whole
of Greece with its cities and rivers, and also most of those countries
frequented by Grecian navigators. This seems to have been accounted the origin
of Geography. But I am inclined to believe that it might be traced to a much
higher origin, as we find in our sacred writings an accurate account of the
land of Canaan with its divisions and boundaries. But it seems hardly probable
that Joshua should have executed this so correctly without some map or
representation of the country to which it alludes.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">The next
teachers of the Ionian school were Anaximenes and Anaxagoras. Of their
particular labours history has left us few remains. We know little more than
that the study of the heavens continued to occupy a principal station among
their pursuits. The accounts we have of them appear mingled with many errors,
and their doctrines were expressed in a poetical and mysterious language. Of
this latter circumstance, however, it is not difficult to find an explanation.
The origin of persecution seems to have been almost coeval with that of
Philosophy, and the votaries of the latter were obliged in enigmatical language
discoveries which, if published, would expose them to the fury of the people.
Anaxagoras furnishes us with an illustration: towards the latter period of his
life he was induced to make public his opinions relative to eclipses. The
ignorant multitude accused him of impiety in diving into the secrets of the
gods, and he was with difficulty saved from the honour of being the first
martyr of Philosophy by his friend and disciple, Pericles.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">While
Greece was thus enlightened by a succession of philosophers, another and not
less brilliant school was established in Italy by Pythagoras. He appears to
have held Astronomy in the highest estimation, and to have cultivated it with
much success. Many ideas which sprang from this school have received
confirmation from time and experience. The doctrines of Pythagoras nearly
resembled those of Thales. He likewise explained the true cause of the light of
the Moon, and of eclipses. He maintained the rotation of the Earth and placed
the Sun at the centre of the System. His opinions approached more nearly to the
true explanation of the Universe than those of any of the ancient philosophers.
His ideas on Comets were very just, and his disciple, Artemedorus, explained
their disappearance and reappearance with singular felicity. He taught that
there were more than five planets, but that they had not all been observed, on
account of the position of their orbits, which only suffered them to be visible
in one of their extremities. It is honourable to the sagacity of Seneca that he
embraced this opinion with avidity and ventured to foretell that a time should
arrive when the laws which regulate these singular planets should be known and
their calculations understood. Speaking of this subject he adds, "Our
posterity shall wonder that these things which are so well known to them were
not understood by us".</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">From a
philosopher of this school we have one of the first physical hypotheses to
account for the formation of the Universe which deserves notice, chiefly from
the strong resemblance it bears to the more modern and more celebrated one of
Descartes. Democritus attributed the motion and formation of the heavenly
bodies to whirlwinds of atoms, some of which becoming compressed together
formed the planets, the Earth and the Sun. He explained the various motions
which would result from this theory, but it seems to have been neglected until
remodelled by Epicurus. It derived celebrity from the elegant pen of Lucretius.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>The only other astronomical labour
that deserves notice during the first centuries of Grecian learning is the
improvements in the calendar. The most obvious division of this, which nature
presents to the attention of Man, is by means of the revolutions of the Moon,
and we find accordingly that this was the first he made use of. It possesses
two advantages very desirable for Man in an uncultivated state: its motions and
changes are equally simple and apparent.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">The
different appearances of this luminary are themselves a sufficient indication
of the divisions of its revolution. It is not therefore surprising that it was
used by many of the ancient nations to regulate the returns of their religious
ceremonies or the period of their political assemblies. Such was the case with
the Jews, the Arabs, and the Gauls;<span style="mso-spacerun: yes;"> </span>and
even at this time, the greater part of the tribes of America reckon the
duration of time by the number of lunations elapsed. This division was however
by no means the most convenient. The return of the same temperature of the air,
and of the same seasons indicates a much more natural one. And this is entirely
regulated by the motion of the Sun. They endeavoured to adopt this, and as
twelve revolutions of the Moon nearly take place during one of the Sun, they
divided the year into twelve months. Such an arrangement was soon found to be
defective owing to a difference of about eleven days in the two modes of
reckoning, by solar and by lunar revolutions, and the means of reconciling
these two methods became a great difficulty. Some nations as the Egyptians
[did], avoided it by confining themselves to the solar year, while others
[such] as the Arabs, gave up the direction of time to the luminary of the
night. But the Greeks, trusting to the reply of their oracles, persisted in
their endeavours to reconcile them; and to this circumstance we may perhaps
attribute much of the progress which they made in Astronomy.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">It would
be tedious to detail the many fruitless attempts which were made, and the
numerous plans which were rejected as insufficient. The point was at last
accomplished by Meton. He discovered that in 19 solar years there were almost
exactly contained 235 lunar revolutions, and that by adopting this period in
the calendar, the New Moon would, at the end of every 19 years, be brought back
to the same day of the year, and nearly to the same hour of the day; and the
two luminaries would, at the end of the term, be nearly in the same part of the
heavens, and in the same position as they were at the beginning of the period.
Meton explained to the Grecians assembled at the Olympic Games his alteration
of the calendar. It was immediately adopted and received with so much applause
that it was called by way of eminence, the cycle or the golden number, a name
which has been universally adopted by all those nations who make use of a luni-solar
year.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Amongst
the philosophers who flourished at this period Eudoxus is celebrated for the
theory of concentric spheres, which he invented to explain the motions of the
heavenly bodies. He conceived the stars placed in a solid transparent sphere,
which formed the boundary of the Universe, and which, by its revolution, caused
the rotation of the stars round the Earth. He made use of many of these
transparent spheres to account for the motion of the Sun and Moon, and his
successors were obliged to augment their number very considerably: each new
inequality in their motions requiring the supposition of a new crystalline orb.
The brittle fabric soon, however, became more confused than the motions it
pretended to account for. It vanished, but only to make room for other
theories, which [for] a while engaged the attention and received the applause
of Mankind. These in their turn fled before the scrutinising eye of truth,
leaving to their authors the renown of splendid errors, the frequent fate of
misdirected genius. Such will ever be the result when imagination is taken for
our guide in philosophical enquiries. Theory unfounded on induction,
unsupported by facts, though it may dazzle for a while, will ultimately
disappoint the hopes of its too credulous believers.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Among the
illustrious list of philosophers to whom Astronomy was indebted for her
progress during the few centuries antecedent to the Christian era we meet with
the names of Archimedes and Aristarchus. To the first we are indebted for
discoveries and inventions in every science which is subservient to the
improvement of Mankind. Many of these have descended to us, but of his
astronomical observations unfortunately we possess no remains, and this is more
to be regretted as from his great skill in practical mechanics he probably
possessed the means of performing them with considerable accuracy. Concerning
Aristarchus, little is recorded but that he made a long series of interesting
observations on the motions of the planets. One circumstance has, however,
escaped the ravages of time and forms a lasting monument to his glory.
Aristarchus was the first who attempted to measure the relative distances of
the Earth, the Moon and the Sun. The method he made use of is one of the most
elegant and ingenious that has ever been invented, but unfortunately it was not
susceptible of any very considerable degree of accuracy when applied to
practice, and therefore it is not surprising that he made but little progress
in the solution of this most difficult problem.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">The name
of Hipparchus will be ever celebrated in the History of Astronomy. Possessed of
a rare union of talent with indefatigable industry he devoted himself entirely
to its cultivation, and was well merited, by his discoveries and observations,
the title of the father of this science. His youth was spent in endeavouring to
determine the length of the solar year, which he fixed with greater accuracy
than any of his predecessors. He discovered that the Sun is not situated
exactly in the centre of the circle which the Earth describes round it, and he
likewise made the same remark respecting the Moon. He also found that this
latter body describes a path which forms an angle with that of the Earth.
Having determined these quantities with all the accuracy his instruments would
admit, he calculated very extensive tables of their motions. He likewise formed
a plan for determining the relative distances of the Moon and the Sun, and
displayed in its execution that readiness of invention, that fertility of
resource, which ensure success. But if from those delicate observations which
it necessarily requires we find his results considerably different from those
of modern calculation, we must remember that great allowances should be made
for the imperfection of the instruments he was obliged to use, at a time when
their construction was but little attended to or understood.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">The
appearance of a new star during the time of Hipparchus determined him to
undertake one of the grandest projects which had yet been imagined by the enterprising
spirit of Man. In order that posterity might be able to determine whether the
face of the heavens remained the same, or whether new stars might not appear,
and others decay and be lost, he undertook the immense task of numbering them,
giving to each a name and finding its relative situation. He executed this
project to a considerable extent and made a catalogue of most of the principal
fixed stars. This subsequently formed the basis of the more extensive one of
Ptolemy.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>The next observer of the heavens
whose name has descended to us is Posidonius, celebrated for his measure of the
magnitude of the Earth. He determined its circumference as 240,000 stadia. But
owing to our ignorance of the length of the stadium, it is impossible to
estimate the accuracy of ancient measurements. They do not appear generally to
have been made with much attention, and must have had considerable errors from
their ignorance of several circumstances on which they materially depend.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Two
conjectures of Posidonius deserve mentioning from the confirmation they have
subsequently received. It was the general opinion at the time he lived that
countries situated under the equator were burnt up by the Sun, and were
therefore uninhabited deserts. This he opposed and said that countries situated
near the tropics were known to be inhabited; therefore the human species could
bear a very considerable degree of heat, and that it was probable those under
the equator would not be much hotter for this reason, that, as the days and
nights would be equal during the whole year, the Earth would always have as
much time to cool in the night as it had to acquire heat in the day, which is
not the case in the tropics, where the Sun is sometimes above the horizon for
16 hours out of the 24. This fortunate conjecture is now confirmed by
experience: it is well known that countries situated near the tropics are
generally more troubled with heat than those immediately under the equator.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>The other circumstance observed by
Posidonius is that the Moon and Sun appear larger when situated near the
horizon than they do when nearer the zenith. He attributed it to certain gross
vapours floating in the air that divided the rays of light. This seems to be
the first hint of that property of the atmosphere known by the name of
refraction. It was enlarged and improved by his disciple Cleomedes. An accident
appears to have turned his attention to this subject. Cleomedes taught that the
eclipses of the Moon were caused by the Earth passing between that luminary and
the Sun. To this it was objected by one of his disciples, that in certain lunar
eclipses both the Sun and the Moon are above the horizon at the same time.
Cleomedes at first denied the possibility of this phenomenon and founded his
opinion on the circumstance that in a lunar eclipse the Sun is in direct
opposition to the Moon. Those who had noticed this singular appearance,
attributed it to the height of the eye above the surface of the Earth. But
Cleomedes now convinced of the fact was dissatisfied with the explanation, and
endeavoured to invent some other reason which should explain the phenomenon.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">"It
is possible," said he, "that the rays of light which proceed from
these luminaries meeting with the vapours with which the atmosphere is loaded may
be turned out of their course and thus reach the eye, though the objects
themselves are below the horizon just in the same manner as an object invisible
at the bottom of a cup becomes visible when it is filled with water."
This, in fact, is the explanation afforded by refraction, and the illustration
he makes use of is of the most familiar of those which are now exhibited as
proof of it.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>Shortly after the commencement of
the Christian era Egypt produced the celebrated Ptolemy. One of the first
undertakings to which he applied himself was to complete the vast project begun
by Hipparchus, the formation of a catalogue of the fixed stars. He has left us
in his great work an account of more than a thousand whose situations he has
determined. Ptolemy is however better known by the system which bears his name,
though this is probably his least valid title to the gratitude of posterity. He
imagined the Earth to be placed in the centre of the Universe and the heavenly
bodies to revolve round in this order: the Moon, Mercury, Venus, the Sun, Mars,
Jupiter and Saturn, and lastly at an immense distance the fixed stars. To
accommodate to his system the various irregularities of the heavenly bodies he
was obliged to make them revolve in curves called epicycles. These are
described by a body which revolves in a circle. The centre of this circle is
itself carried round in the circumference of another. Such would be the path of
the Moon if seen from the Sun. But this was not sufficient to account for all
the appearances. The centre of each epicycle was again obliged to revolve
around some other point, and in some cases this last point was denied repose.
The complexity and confusion of this system was wonderful, but the name of
Ptolemy was its passport to belief, and coinciding with the prejudices of
Mankind, that the Earth is at rest in the centre of the Universe, it long
usurped an authority which was, with difficulty, wrested from it by the
simplicity and truth of that of Copernicus.<span style="mso-tab-count: 1;"> </span>Notwithstanding
the deficiencies felt, and even acknowledged in some instances, he constructed
tables of the solar and lunar motions which were tolerably accurate, and
continued so for a short period, but being founded on a wrong hypothesis, they
soon became useless.</span></div>
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<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">[Insert:
It was to this hypothetical construction of the Universe that Alphonsus
referred in the impious speech which his recorded of him and to which it is
here sufficient to allude.]</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>The great work of Ptolemy, in his
Almagest, in which he has transmitted to us all the observations of the
ancients that he thought worthy of credit, has treated of his own system at
great length. It likewise contains all his own discoveries and observations,
the labour of many years. But the most valuable part is the table of Fixed
Stars which he gives us as determined by his own observations.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>After the death of Ptolemy the
sciences appear to have remained stationary. Nature, as if exhausted by the
production of so many sages, now rarely soared above mediocrity. The few whose
names have descended to us seem to have founded their highest claim to science
on the commentaries and explanations they wrote on the works of their
predecessors. Nearly two centuries elapsed, and the stock of human knowledge
remained the same. But an event now happened which seemed for a while to roll
back the course of civilisation. The collective wisdom of the ancients
contained in all their most celebrated writings were deposited in the library
of Alexandria. This was destroyed by the eruption of the Arabs, and the few who
yet made science their pursuit were driven from this venerable abode of so many
philosophers, and lamented in solitude the loss of those treasures it was
impossible to restore.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">A long
period of darkness and ignorance succeeded this lamented catastrophe. We are
indebted to this very people who had at one blow so nearly annihilated every
trace of ancient learning for the small remains of what escaped their fury. The
throne which had been disgraced by a bigot was now adorned by a monarch the
patron of the sciences. The Caliph Almamon, on ascending the throne of Baghdad,
determined to restore the cultivation of the sciences throughout his dominions.
He procured the best of the Grecian authors which remained and ordered them to
be translated. He assembled together the learned and assisted at their
conferences. Astronomy participated largely in his cares. Numerous observations
were undertaken and executed with the greatest care, sometimes in the presence
of Almamon and frequently by himself in person. Two observations of the
obliquity of the ecliptic are recorded as having been made in his presence. But
the grandest undertaking during his reign was the mensuration of two degrees of
the meridian. He confided its execution to the most skilful observers in his
kingdom. They selected one of the immense plains of Mesopotamia and divided
into two parties, one travelling to the north, and the other southward. The
results of this mensuration are preserved. But as there still remains a great
doubt concerning the length of the Arabian mile, we have not the means of
appreciating its accuracy. This is much to be regretted, as it is the first and
only instance in which the whole arc has been measured by the actual
application of a rule to the surface of the Earth.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">The
example of Almamon spread widely the taste for science, and we find at this
period numerous Arabian authors who wrote on the subject of Astronomy. These
contributed much to diffuse the study of this science, and the serenity of the
atmosphere and immense magnitude of the instruments with which they observed,
added to the care and precautions they took in making their observations,
render them of considerable value. Most of their treatises have descended to
us, but unfortunately few have met with a translator. The Bodleian library
alone is said to possess upwards of 400. About two years since the National
Institute of France ordered the translation of a fragment of a work of the
Arabian author, Ibn Junis, containing 19 observations of solar and lunar
eclipses. These were of considerable importance, as by calculating the time at
which they ought to have happened by means of modern tables, they were found to
agree very nearly, thus affording a strong proof of the accuracy of these
tables and also showing us what degree of credit might be give to Arabian
observers.</span></div>
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<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">From
Arabia the sciences appear to have travelled into Spain, for we find about the
13th century Alphonsus, King of Castille, inviting the learned of whatever
religion or nation to assist him in the formation of new astronomical tables:
after several years of continued application they produced those which were
called, in compliment to their patron, Alphonsine. But these were inaccurate
even at their birth, and merited the severe criticism they met with from an Arabian
writer. The justice of this as acknowledged by their authors, who immediately
undertook the task of revising them, and four years afterwards they produced
another set which corresponded more accurately with the phenomena of the
heavens. These maintained their reputation a few years, and then gave place to
others, which alike enjoyed a transitory fame.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>The two succeeding centuries furnish
few cultivators of Astronomy. The 15th produced Purbach, who with his disciple
Regiomontanus are justly considered as its restorers. Purbach visited most of
the universities of Europe in search of improvement, and on his return was
appointed professor of Astronomy at Vienna. The first undertaking he engaged in
was a new translation of the Almagest, a work much wanted, as those which
existed at that time were full of inaccuracies.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Purbach
particularly employed himself in making observations. He felt that this was the
only means of correcting or confirming the hypotheses of the ancients. With
this view he constructed several new instruments and improved those already in
use. We are indebted to him for one contrivance which is still adopted in most
modern and most costly instruments. He was the first who made use of the plumb
line to adjust an instrument. The result of his observations was the
application of several corrections to the theory of Ptolemy, which seemed now,
for the first time, to have begun to totter.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span>Regiomontanus, the pupil of Purbach,
continued the labours of his master with great reputation. He made a numerous
series of observations on the planets, in order to compare the various theories
on the subject of their motions. He travelled into Italy to acquire a knowledge
of the Greek language and on his return employed himself in translating many of
their best writers on scientific subjects. Indeed the number of works on which
he was employed at the same time is almost incredible. Many of these he
completed, but a premature death prevented the conclusion of by far the larger
portion.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">We have
now nearly arrived at the era of Copernicus; but before we speak of that great
man to whom Astronomy is so much indebted, it may be well to notice an
observation on the altitude of the Sun which probably exceeds in accuracy any
we have yet recorded. Its author was Paul Toscanelli, who was born about the
beginning of the 15th century. He was a pupil of the celebrated architect,
Phillip Brunelleschi, who completed the cupola of the church of St. Maria at
Florence against the opinion of the most famous architects of the time. It was
this cupola which Toscanelli converted into a gnomon. We have observed in
considering that erected by Manilius at Rome the causes which impeded its
accuracy. These appear to have been noticed by Toscanelli, and he avoided them
in the following manner. He pierced a small hole in the cupola at the height of
[not stated] from which he hung a plummet which reached the pavement of the
cathedral. On this he drew a meridian line. When the Sun arrived at its
greatest height, a ray from it passed through this aperture and falling on the
line caused an oval image several feet in length. He measured the distance of
its centre from the plummet with exactness, and by comparing this with the
height of the aperture, he determined the Sun's altitude with the greatest
accuracy.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">We are
now arrived at a period when Astronomy was to undergo one of its grandest
revolutions: a theory which had received the sanction of the learned for nearly
two thousand years was to receive its final overthrow. For this great attempt
we are indebted to Copernicus. Born of a noble family at Thorn in Prussia he
quitted his native country to indulge in the study of Astronomy. His progress
was rapid and he was shortly elected to the professorial chair at Rome. At the
beginning of the 16th century he quitted Italy at the request of his uncle, the
bishop of Wurms, who made him a canon of his cathedral. This fixed him for the
remainder of his life, and it was at this period that he began those
observations and reflections which concluded by demonstrating the insufficiency
of the ancient system of the world, and obliged him to establish another on its
ruins.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">The
inconvenience of the Ptolemaic hypothesis of the Universe appeared to him in a
striking point of view. The great embarrassment which resulted from it, the
total want of symmetry and order which was manifest in this pretended
arrangement of the Universe, these and other reasons added to its discordance
with his observations, induced him to conclude that men were far from having
arrived at the true explanation of the phenomena of Nature. He therefore
searched among the opinions of the ancient philosophers and, from their
scattered hints he met with in their writings, constructed that beautiful
system which still bears his name. He placed the Sun immovable in the centre
and conceived the planets to revolve round at fixed distances. He gave to the
Earth a rotation round its axis. To satisfy himself of the truth of this
arrangement, he undertook a series of observations which occupied him during 36
years before he ventured to give it to the public; and even after this
lengthened investigation, he was with difficulty induced at the urgent request
of his friends and protectors of the highest rank to print his great work on
the Celestial Revolutions. Copernicus did not live to witness its reception: he
died suddenly on the 24th May 1543, the very day he had received from Nuremburg
the first copy of his work. He was interred without any pomp or even an
epitaph. But he has left in the admirable system he restored, the most durable
monument to his memory.</span></div>
<span lang="EN-GB" style="font-family: "Arial Black"; font-size: 14.0pt; mso-ansi-language: EN-GB; mso-bidi-font-family: "Times New Roman"; mso-bidi-font-size: 10.0pt; mso-bidi-language: AR-SA; mso-fareast-font-family: "Times New Roman"; mso-fareast-language: EN-US;"><br clear="all" style="page-break-before: always;" /></span>Unknownnoreply@blogger.comtag:blogger.com,1999:blog-1290567400778113129.post-15752949714557209982013-11-11T09:52:00.002+00:002013-11-12T00:04:17.670+00:00Synopsis<h1>
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Synopsis</span></h1>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">British
Library Additional Manuscript Vol. 37203</span></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Babbage's
Papers on Astronomy</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">[Note
Babbage did not provide titles for the lectures. These have been ascertained
from their content.]</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Lecture
1: <span style="mso-tab-count: 1;"> </span>BL Add. Ms. 37203 ff34-69</span></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Introduction
to<span style="mso-spacerun: yes;"> </span>and History of Astronomy, from Thales
to Copernicus</span></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">The
primacy of Astronomy amongst the physical sciences - Utility of Astronomy:
commerce and navigation - Newton's astronomical method of ascertaining
historical dates, e.g. dating of the expedition of Jason and the Argonauts,
Xerxes invasion of Greece - Origins of Astronomy: the Chaldean Zodiac - The
Ancient Greeks - Thales of Miletus: heights of pyramids, calendars and
prediction of eclipses - Gnomons: Anaximander and Manilius - Anaximenes and
Anaxagoras - Pythagorean school - On the Greek calendar and the Metonic cycle -
Eudoxus and his theory of concentric spheres - Archimedes and Aristarchus -
Hipparchus: the length of the solar year, path of the Moon, distances of the
Moon and Sun from the Earth, cataloguing of the Stars - Posidonius and his
disciple Cleomedes: magnitude of the Earth, terrestrial climates, refraction -
Ptolemy: his catalogue of the Stars, on the Ptolemaic system of the Universe,
his Almagest - The Dark Ages - Arabic Astronomy - Caliph Almamon - Importance
of Arabic astronomical records even for present-day study of Astronomy -
Spanish Astronomers - Alphonsus, King of Castille: Alphonsine Tables - The
Renaissance - Purbach and Regiomontanus: new translations of Ptolemy's Almagest
and the Greek Astronomers - Toscanelli's plumb line and the measurement of the
altitude of the Sun - Copernicus: his life and work, his recognition of the
inconvenience of the Ptolemaic hypothesis, his observations leading to the
publication of Celestial Revolutions.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Lecture
2: <span style="mso-tab-count: 1;"> </span>BL Add. Ms. 37203 ff73-77</span></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">(Ms pages
1-18 of script of lecture missing, begins page 19, ends page 23, remainder of
lecture missing)</span></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">On
Astronomical Instruments</span></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">[Difficulties
peculiar to Astronomy, and the means of obviating them - Plan to be pursued - Description
of Instruments. On the Pendulum and the Measure of Time, its irregularities and
the means of correcting them - The Astronomical Quadrant and Circular
Instruments - Hadley's Quadrant - ] On the Sextant and its use for measuring
angular distance - On the use of a mercury artificial horizon - [Transit
Instruments].</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">[ff78-82
Unused printed subscription forms for the Cambridge Analytical Society the
backs of which Babbage has used for notes.]</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Lecture
3: <span style="mso-tab-count: 1;"> </span>BL Add. Ms. 37203 ff83-103</span></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Observing
and Cataloguing the Heavens</span></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">On what
Astronomers observe, and their hypotheses and the methods used by them<span style="mso-spacerun: yes;"> </span>to confirm these - Division of the heavenly
bodies into three types: Sun, Moon and Stars - On the apparent motion of the
Stars in the heavens, circumpolar stars, Pole Star, Constellations,
Planets<span style="mso-spacerun: yes;"> </span>- On the isolation of the Earth
in space: North and South Pole, Axis and Equator - The hypothesis of the
crystalline sphere and its motion round a fixed Earth -Determination and use of
a Meridian, 'Transiting the Meridian' - Sidereal Time - Mean Solar Time -<span style="mso-spacerun: yes;"> </span>Right Ascension and Declination of an
astronomical body - Origin of Degrees - History of star catalogues:
Hipparchus', Flamsteed's, Maskelyne's and Lalandes' - On the Refraction of
Light, its effect on observations - Terrestrial Refraction: two reports in
Phil. Trans. - Dr. Wollaston's<span style="mso-spacerun: yes;"> </span>and
Monge's theories - Mirages.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">[ff104-112
mathematical notes. Copy made by CB of Newton's Principia, section 11 from
Corollary 6 proposition 66 to the end of the 17th Corollary.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Lecture
4[?]: (labelled "3rd" with comment by CB as "too short by
¬") BL Add. Ms. 37203 ff113-142</span></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Ascertaining
the Figure of the Earth</span></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Lecture
is principally on the discovery of the shape and magnitude of the Earth - On
the rotation of the Earth on its axis and its isolation in space - Examples of
a ship appearing from over the horizon and how canal diggers noted that,
because of the curvature of the Earth, there is dip of 8" in the horizon
every mile in length along the canal - "If Man had confined himself solely
to the collection of facts science would have presented a barren detail of
arbitrary names and he would never have attained the knowledge of the great
laws of Nature. It is by comparing phenomena together and by endeavouring to
trace their mutual connection, by gradually correcting his theory that he has
succeeded in discovering these laws the existence of which may be perceived
even in their most complicated effects." - That the diurnal motion of the
heavenly bodies is only an apparent motion - Newton: his laws of motion and
hypothesis that the Earth is not a perfect sphere, but flattened at the poles.
Newton's laws of motion -<span style="mso-spacerun: yes;"> </span>Galileo and
his adoption of the Copernican theory: his subsequent trial for heresy at the
hands of the Inquisition - 16th to 18th century attempts to measure the shape
of the Earth: Fernel and the magnitude of Earth; Snellius (Snell) of Holland,
his measurement of the distance of one degree of latitude between Alcmaer and
Bergen op Zoom; Richard Norwood, his measure of the distance between London and
York; Riccioli, his fraudulent measurements and numerous errors; Abbé Picard.
Richer's pendulum measurements at Paris and Cayenne - Newton's versus
Descartes' views respectively on whether the Earth was flattened at the poles
or not - French Academy of Science's expeditions to settle the question of the
length of a degree of latitude - Mapertuis' expedition to Lapland - La
Condamine's measurement of the length of 3« degrees of latitude at Quito in
1739 - Adventures of the expeditionists - As a result of these expeditions, </span></div>
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<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-spacerun: yes;"> </span>"It was decided that the degrees
diminish as we approach the equator and consequently that it (The Earth) is a
flattened spheroid."</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Lecture
5: <span style="mso-tab-count: 1;"> </span>BL Add. Ms 37203 ff143-178 </span></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">On the
Moon</span></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">On months
- On the appearances and phases of the Moon - On the reflectivity of the Moon's
surface - On the motion of the Moon and its orbit - On Eclipses of the Sun and
Moon, their causes -<span style="mso-spacerun: yes;"> </span>On the dating of
historical events using eclipses - How Dr. Halley estimated the date of
Caesar's invasion of Britain as being the 26th August 3950 world time (55 bc)
-<span style="mso-spacerun: yes;"> </span>How people in France and Spain, even
as late as the eclipse on 12th May 1706, were still superstitious about them -
On other 18th century eclipses - On the use of the micrometer for measuring the
apparent size of an astronomical body - On the Moon's apparent diameter and its
variations in its orbit round the Earth - The fact that the Moon always shows
the same face to observers on the Earth - The Moon's Libration and its causes -
The fact that there are two librations: a latitudinal and a longitudinal motion
- On the diurnal libration caused by optical parallax -<span style="mso-spacerun: yes;"> </span>On the theory of tides - On the effect of
the Moon and Sun on tides - On Spring tides and tidal bores - On the stability
of the equilibrium of the oceans -<span style="mso-spacerun: yes;">
</span>Laplace's analysis of tides - On deluges - Distance of the Moon from
Earth - Diameter of the Moon - Force of Gravity on its surface - Appearance of
the surface of the Moon: craters, on their possibility of being volcanoes - How
to measure the width of craters - How Sir W. Herschel had observed, in May
1783, a volcano in action on the Moon, and again in March 1794 - On the
question of whether there could be life on the Moon - The fact that the power
of telescopes was insufficient to answer this question.</span></div>
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<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Lecture
6: <span style="mso-tab-count: 1;"> </span>BL Add. Ms. 37203 ff179-213</span></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">On the
Sun</span></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">On the
proper motion of the Sun versus its apparent motion - Measurement of proper
motion - On the observational origin of the Laws of Nature - That the proper motion
of the Sun is retrograde with respect to the daily rotation of the Earth - On
the Plane of the Ecliptic - How it gives rise to the seasons of the year - On
the seasons: their climatic differences, Solstices and Equinoxes - Variations
in the apparent diameter of the Sun - Distance from the Sun and<span style="mso-spacerun: yes;"> </span>its measurement by parallax - On Sun Spots,
their size and time of revolution - On the Sun Spot Belt - On the variations in
the brightness of the Sun's disc - On the Sun's atmosphere - Zodiacal light:
Cassini the elder's and Mairan's ideas - Measurement of Time - Astronomical or
Solar Day - Equation of Time - On the question of whether the Earth moves round
the Sun or vice versa - Arguments for - Rmer's discovery of the velocity of
light from the occultation's of Jupiter's sattelites - Man's propensity to
consider himself and the Earth at the centre of the Universe - Tidal effects in
the Earth's atmosphere caused by the Sun - Patterns of the prevailing winds on
the Earth - Halley's theory on the origin of the Trade Winds - The grazing
comet theory of the origin of the planets, Buffon's views - Errors of this
theory - On the cause of Sun Spots, Lalande's and others' theories - On the
question of the origin of the Sun's power - Herschel's observation's of the Sun's
Spots and its atmosphere and his methods of observing the Sun.</span></div>
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<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Lecture
7: <span style="mso-tab-count: 1;"> </span>BL Add. Ms. 37203 f214 verso</span></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">The Inner
Planets</span></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Script
Missing. No notes made other than it was to be on Observations on the planets Mercury,
Venus, the transit of Venus, and Mars.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Lecture
8: <span style="mso-tab-count: 1;"> </span>BL Add Ms. 37203 ff215-251</span></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">On the
Minor Planets (Asteroids) and also the History and Development of the
Reflecting Telescope</span></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">The
singularity of interest of the 4 smaller planets beyond the orbit or Mars
recently discovered in Babbage's time - The need for a telescope to observe
them - On the History of the Titius[-Bode's] Law - Kepler's suppositions - That
there appeared to be a gap or vacancy in-between the orbits of Mars and Jupiter
suggeesting a new planet - Astronomers agree to make a planned search of the
heavens - Piazzi's discovery of Ceres: confirmed by Gauss and Olbers - Orbit of
Ceres - Impact of discovery on Astronomy - Olbers discovery of Pallas, its
orbit - Harding's discovery of Juno - Olbers hypothesis on the origin of the
Minor Planets: exploded planet theory - Consequences of this hypothesis -
Discovery of Vesta - On the eccentricity and obliquity of the orbits of the
Minor Planets - Final abandonment of the Zodiac of the ancients - Lagrange's
calculations and likelihood of the exploded planet hypothesis - Size of the
Minor Planets - Herschel's experiments and measurement, his estimates of their
size.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">History
of the Reflecting Telescope - Mersenne's idea - Gregory's Optica Promota 1660 -
Newton's design and models - Hadley's Reflecting Quadrant - James Short's
telescopes - Herschel's work and his quantification of the power of a
telescope: (i) Magnifying Power (ii) Space Penetrating Power - Effects of the
weather and climate on the performance of a telescope.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Lecture
9: [Missing?]</span></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">The Outer
Planets [conjectured title of lecture]</span></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Presumably
on the planets Jupiter, Saturn and Uranus, and the history of the discovery of
the latter by Herschel.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Lecture
10:<span style="mso-spacerun: yes;"> </span><span style="mso-tab-count: 1;"> </span>BL
Add. Ms. 37203 ff252-281 <span style="mso-spacerun: yes;"> </span>Draft,
ff282-301 Top Copy</span></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">On the
Scientific Method and the Theory of Universal Gravitation</span></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">"By
observation and experiment we gain the first materials of our knowledge. These
are the foundations of all philosophy." - The reason for scientific theory
as a means of organising knowledge - Bacon's arguments against hypothesis
countered by Newton - That the theories of Universal Gravitation and the
Copernican System are examples of useful hypotheses - That the origin of the
Copernican hypothesis lay in its beauty and simplicity - That the utility of
hypothesis lay in its predictive power and testability by experiment - That
hypotheses which had been superseded had been useful in their time - That some
hypotheses were a historical necessity for progress - Kepler and his discovery
of the elliptical shape of the paths of the Earth and planet round the Sun -
Kepler's law: that the square of the tiem of any planet's revolution always
bore a certain proportion to the cube of its distance from the Sun and that
this ratio is a constant - Table for the planets - Kepler's view of the Sun's
propensity for moving bodies, a power which spreads throughout space and moves
rapidly - His hints that a force of gravity might exist - Descartes theory of
vortices - Utility of Descartes' theory even though wrong - Newton's principles
of Universal Gravitation - Origin of the theory lie with the Ancient Greeks:
Anaxagoras, Democritus and Epicurus - Kepler and the Moon's tendency to fall
towards the Earth - Hooke's letter to the Royal Society in 1666 anticipating
Newton's theory - Newton's discovery of the theory when he was contemplating
the fall of the Moon towards the Earth and asking whether the same law applied
with the fall of bodies at the surface of the Earth - His first estimates in
error because estimate of the size of the Earth in error, corrected when
Picard's value for this was known - Newton's predictions published in Principia
- On the admiration of his contemporaries - On Newton's personality - On his
genius - Consequences of the Law of Universal Gravitation - Irregularities in
the motion of planets Lagrange's and Laplace's extensions of the theory - On
the stability of the Solar System - Argument for the existence of God from
design.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Lecture
11: <span style="mso-tab-count: 1;"> </span>BL Add. Ms. 37203 ff302-333</span></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">On Comets</span></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Comets as
distinct from stars and planets as astronomical phenomena - Ideas and theories
of the Greeks - Tycho Brahe's<span style="mso-spacerun: yes;">
</span>observations - Kepler's theories - The straight line motion hypothesis -
Cassini - Helvetius and Hooke - The parabolic motion hypothesis - The Great
Comet of 1680/81 - Doerfell and the elliptic motion hypothesis - Newton's
theory of universal gravitation applied to Comets - On the composition of the
tails of Comets - De Mairan - Halley's treatise on Comets (1705) - On the
periodic return of various comets - Historical records - Origin of the Great
Flood - Halley's prediction of the return of the comet named after him -
Observations on the comet of 1759 - Calculations of the gravitational
perturbation effects of Jupiter and Saturn on the path of the comet by Lalande,
Clairaut and Mme. Lepante - Lexell and the discovery of another periodic comet
- Terror caused by the comet of 1773 - Estimate of the tidal force caused by a
close encounter of the Earth with a comet - Laplace's calculations -
Composition of the nuclei of comets - Schroeter and Herschel views - The comet
of 1811 - On the return and non-return of comets -Speculations on what happens
to those which do not return.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Lecture
12: [Missing?] Beyond the Solar System [conjectured title of lecture]</span><br />
<br />
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="http://collectionsonline.nmsi.ac.uk/grabimg.php?wm=1&kv=106957" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="320" src="http://collectionsonline.nmsi.ac.uk/grabimg.php?wm=1&kv=106957" width="228" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">William Herschel's 20 ft Reflecting Telescope</td></tr>
</tbody></table>
</div>
Unknownnoreply@blogger.comtag:blogger.com,1999:blog-1290567400778113129.post-70378177387018746812013-11-11T09:50:00.001+00:002013-11-11T14:28:22.645+00:00Preface<h1>
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Preface</span></h1>
<div>
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><br /></span></div>
<div class="separator" style="clear: both; text-align: center;">
<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjJZ_b91ExyT6MX-uXUoDFbg9W4GbACKTOUgVOY0VISmxrH1WT86T4V4fRiXRFXYId6ihi67DnJw00IbVynKNILylo0WjykZyuK1AKe0CZWpqDvJlCwKx6AOXdZjUmC-c7P4_pVOFbNXy4/s1600/rigb.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjJZ_b91ExyT6MX-uXUoDFbg9W4GbACKTOUgVOY0VISmxrH1WT86T4V4fRiXRFXYId6ihi67DnJw00IbVynKNILylo0WjykZyuK1AKe0CZWpqDvJlCwKx6AOXdZjUmC-c7P4_pVOFbNXy4/s1600/rigb.jpg" /></a></div>
<div>
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><br /></span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Early in
1815 Charles Babbage was invited to give a series of twelve general lectures on
Astronomy at the Royal Institution</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Managers'
Minutes, Royal Institution: Vol VI f30 30-Jany-1815</span></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">(4) Mr.
[Daniel] Moore having stated that Mr. Babbage would be willing to deliver a
course of lectures on Astronomy, he was introduced by Mr Moore to the Board and
stated that he would be ready to commence a Course of Lectures on Astronomy on
Thursday the 16th of February next- the terms of which are 12 lectures for 50
guineas and after that rate in case the course did not consist of that number.</span></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Resolved:
That Mr. Babbage be engaged to give a Course of Lectures on Astronomy on the
above terms, at its lecture theatre in the house of the Royal Institution,
Albemarle Street, London, the large colonnaded building which still exists
today. They were his first professional engagement. He was paid 50 guineas for
them</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Managers'
Minutes, Royal Institution: Vol. VI f62. June 5th 1815</span></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">(2) Read
and approved the minutes of the Committee of Accounts of this day and signed an
order for payment of the following sum, vizt.</span></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span><span style="mso-spacerun: yes;"> </span>Mr Babbage for a Course of<span style="mso-tab-count: 1;"> </span>}<span style="mso-tab-count: 1;"> </span></span></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 1;"> </span><span style="mso-spacerun: yes;"> </span>Lectures on Astronomy in 1815}<span style="mso-tab-count: 1;"> </span></span></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="mso-tab-count: 5;"> </span>£52-10-0.</span></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">He was
only 23 when he delivered them.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">The Royal
Institution of Great Britain, to give it its full name, was founded some 15 years
prior to this through the joint efforts of Count Benjamin Rumford and Sir
Joseph Banks. They had intended it to have been a kind of college for the
education of artisans and apprentices, to teach them the latest skills in and
the application of the sciences to technology and the arts. However, not long
afterwards it gave up this philanthropic purpose. Owing to a lack of funds the
board of managers opted to turn it instead into an institution for the
presentation of popular, public lectures on science <span style="mso-spacerun: yes;"> </span>for middle class, fee-paying<span style="mso-spacerun: yes;">
</span>audiences.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Both Sir
Humphry Davy and Michael Faraday were launched upon their scientific careers at
the Royal Institution. Sir Humphry Davy, in particular, was one of its first
public lecturers and it was especially he who popularised its function as a
platform for promoting science.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">These
lectures on Astronomy were perhaps the only public lectures which Babbage ever
gave, and they form probably the only instance where he practised the art of
public speaking, something which he personally detested. The only other
occasions where he had to speak in public were perhaps those on the hustings of
the two parliamentary election campaigns in the Finsbury constituency during
the 1830s, in which he stood for the Whig cause and the reformed parliament,
and lost.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Babbage
had many academic interests during his long life. After Mathematical Analysis
Astronomy was his first love. He was perhaps encouraged in this by his close
friendship with John F.W. Herschel and through him the other members of the
Herschel family. He was a frequent visitor to their household in Slough and
became personally very well acquainted with J.F.W. Herschel's father, Sir
William Herschel. It is possible he may have acquired much of the material
contained in these lectures at firsthand from the grand master himself: a great
deal of the content of the later lectures clearly indicates this to be so.
Babbage greatly admired Sir Wiliam, and the relationship must have been close
for he was later asked, following Sir William'sdeath in 1822, to act as the
executor to the latter's will. He would have certainly been introducedto all
the famous astronomers of the day by Sir William, and would also have had
access to Sir William's astronomical library and personal papers, and of course
the many instruments and telescopes at Slough, the latest of their kind. And
whilst preparing these lectures he would have no doubt debated much with the
Herschels on all matters both scientific and speculative relative to the
subject of Astronomy.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">The
manuscript original for these lectures is to be found amongst the British
Library's collection of Babbage's scientific papers and correspondence; these
were deposited at the library by his youngest son, Henry Prevost Babbage, in
1905. They are contained in folder Add. Ms. 37203 (ff32-332), along with a few
of his other papers on Astronomy. As a set they are almost complete, with the
following exceptions: Lectures 9 and 12 are wholly missing, the synopsis for
Lecture 7 is found in the collection but its content is missing, about one
fifth of Lecture 2 has survived, together with an<span style="mso-spacerun: yes;"> </span>synopsis for it forming part of an original advertisement for the
lecture, the last manuscript page of lecture 5 is also missing: otherwise the
series is complete.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">Taken as
a whole they give a very clear picture of the extent to which knowledge in
Astronomy had evolved to by the beginning of the 19th century. Modern readers
may find them somewhat verbose, but the ideas seem to have presented reasonably
clearly. As editor I have taken a few liberties, making a number of minor
amendments to Babbage's spelling, grammar and punctuation, and I also have
'modernised' one or two obsolete words and expressions in certain instances to
make them more acceptable to today's reader, especially where the original
meaning of a word has changed drastically during the past 173 years. Given
their quality one is surprised Babbage did not arrange for their publication
during his lifetime.</span></div>
<div class="DefaultText">
<br /></div>
<div class="DefaultText">
<span lang="EN-GB" style="mso-ansi-language: EN-GB;">West
Hampstead November 1988</span></div>
Unknownnoreply@blogger.com