Monday, 11 November 2013

Lecture 1: Introduction to and History of Astronomy, from Thales to Copernicus

Lecture 1

Introduction to and History of Astronomy, from Thales to Copernicus


            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.

            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.

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.

            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.

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.

            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.

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.

            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.

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.

Quitting however the regions of fabulous history let us endeavour to trace the progress of this Science by the light of more authentic records.

            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.

            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.

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.

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.

            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.

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.

            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.

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.

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".

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.

            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.

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;  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.

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.

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.

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.

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.

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.

            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.

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.

            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.

"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.

            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.            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.

[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.]

            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.

            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.

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.

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.

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.

            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.

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.

            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.

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.

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.

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.