ligible, we shall premise an account of the state which astronomical knowledge had attained at the close of the seventeenth century, compiled from a former work of the same author,-his Histoire de l'Astronomie Moderne.

The origin of Astronomy is hidden from us in the most remote ages of antiquity. Man became an observer as soon as he turned his eyes to the heavens ; and successive observations, gradually increasing in accuracy, and handed down by tradition from age to age, have been the basis of the information we now possess. But even the embryo of that accuracy of knowledge, and of that exactness of observation, which are the pride of modern astronomy, cannot be traced back much beyond the beginning of the century preceding that whose history is before us. Previous to this epoch, the methods, and instruments of observation, were rude and imperfect; the means of calculation, laborious and difficult. If, then, we are to look to the earliest ages of man's history for the rise of this science, we are to seek for the origin of all that is valuable in theory, as late as the sixteenth century; and for all that is correct and definite as knowledge, to the celebrated and immortal names that have embellished the eighteenth of our era.

The visions of fanciful writers have discovered, in the astronomy of the Greeks, whence we derive the little that can be called valuable, that has come down to us from the ancients, the fragments and imperfect remains of a more complete system, existing in some more civilized and remote nation. From this fancied people, they conceive, that not only did the Greeks derive what they knew of astronomy, but that it was the common source of that of the Hindoos and Chinese. Delambre, in his former works, -the histories of the astronomy of the ancients, and of the middle age, has effectually dispersed this splendid vision. He has in them conclusively shown, that the astronomy of Ptolemy is in fact that of the Asiatic nations, who, so far from giving any thing to the Greeks, or from having drawn with them from a common spring of knowledge, are in truth their scholars and imitators. Wherever, indeed, we apply the test of real science to these systems of astronomy and methods of calculation, which pretend to so lofty and remote an origin, we find at every step the traces of Ptolemy and Hipparchus; their astronomy is not only the concentration of the discoveries of the Greeks, but the basis of that of the Arabs, the Persians, the Tartars, the Hindoos, and the Chinese, as well as of that known by the Europeans previous to the time of Copernicus.

In all these systems, we find the Earth placed immovable in the centre of the Universe, and of all the planetary motions. By means of improbable hypotheses, all these nations are enabled, in the words of Ptolemy, "to save the appearances. "They all are able to predict phenomena, and the position of bodies, within a few

degrees of the truth, and appear never to have discovered these. hypotheses to be erroneous, or at least to have suspected that the mistake arose from a fundamental defect in the system itself.

The first astronomer who appears to have entertained any doubt of the truth of the hypotheses of Ptolemy, is Alphonso, king of Castile. But so far from making any valuable use of the doubt that arose in his mind, he contented himself with saying, that, "had he been called to the council of the deity when the Universe was created, he could have ordered it to greater advantage."

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In ages more remote than those of Ptolemy, we are told, that some Grecian philosophers placed Fire in the centre of the Universe, and made the earth turn around the sun in the year, and on its own axis in twenty-four hours. Others, less bold, had ascribed to the earth only the latter of the two motions, and left to the sun his annual motion. But we find these ideas in no work on astronomy, nor in the pages of any geometer. Ptolemy hardly deigns to notice them; he intimates indeed, that, to ascribe to the earth a motion of rotation, would facilitate the explanation of some phenomena; but all the rest appeared to him, too absurd to merit a serious refutation.

It is to the school of Pythagoras that the ideas of the motion of the earth are usually attributed; and this opinion has derived strength from the acts of Copernicus himself. This distinguished, and we must say original, inventor of the system we now know to be true, aware of the prejudices he was about to encounter, endeavoured to support himself, against the authority of the schools, and the dicta of Ptolemy, by the quotation of opposite opinions, from ancients of equal authority. But these opinions are so vague, that although they might in that age of darkness serve as a support to a new hypothesis, they could not have answered in any degree as a foundation on which to build it. Archimedes indeed informs us, that Aristarchus deviated from the received opinion of the age, and supposed the earth to revolve around the sun in a circle of a radius equal to that usually ascribed to the whole celestial sphere; but he takes care to show that he is of a different opinion; and when he constructed his celebrated Planetarium, he made the earth the centre of all the motions of the other bodies of the Universe.* Plutarch informs us, that the idea of Aristarchus was with him purely conjectural, but that it had received demonstrative evidence from Seleucus; but this demonstration has not been handed down to us, nor indeed is it possible that the knowledge of the time could have furnished a correct one. Seneca says, that it is important to inquire, whether the heavens or the

See Cicero "de Republica."

earth were immovable; but this inquiry, so important in his view, receives no farther notice from him.

The Greeks were an acute and lively people; fond of argument and metaphysical discussion. Their sects of philosophers were divided on every possible question. It was sufficient that one school should profess any given doctrine, for it to find opposers among the neighbouring sects. The most ancient philosophers, no doubt held, in conformity with appearances, that the earth was the centre of the universe, and that the sun, by its various motions, caused the alternations of day and night, and the vicissitudes of the seasons. They were then content to imagine a mechanism by which the several less obvious and collateral phenomena might be produced. Some, of the school of Pythagoras, or perhaps that philosopher himself, for the mere sake of contradiction, placed the sun in the centre of the system, and launched the earth into the ecliptic, there to perform an annual revolution. But in what did the merit of this consist, as compared with the system of their opponents? They alleged, as their sole and simple reason, that the sun was the most noble of all bodies. To this it might have been replied, with equal force, and more of popularity, that man is the most important of beings, that all has been created for his use, that it was fit that his abode should be fixed and permanent, and proper that the heavenly bodies should turn around him to give him light, and afford him vital heat. Such reasons, if in truth no better than those of the Pythagoreans, carried with them at least more of probability, particularly as they are consistent with the impressions derived from our senses. And what motive can we suppose the people of Greece would have to reject the apparent evidence of their senses, and believe in the motion of the earth? They had not observed a single phenomenon which could not be accounted for on the other hypothesis; even the stations and retrogradations of the planets had been explained by means of it. They were ignorant of the great size and distance of the heavenly bodies; the improbability of their being created especially and solely for the use of the inhabitants of our earth, could not therefore have occurred to them. In truth, it is almost within our own days, that the astronomic phenomenon, which shows conclusively the annual motion of the earth, has been detected; and the truth of its diurnal motion of rotation was supported by no known fact, not equally applicable to the hypothesis of Ptolemy, before the latter end of the seventeenth century.

Until these dates, and before the discovery of the universal influence of gravitation, even the most determined of the followers of Copernicus, were obliged to resort, in defending his theory, to the evidence of probabilities alone. They were compelled to sustain their opinion by exhibiting the simplicity of the views of

their master, when compared with the complex absurdity of the system of Ptolemy. But this very argument was wanting to the ancients for the complexity of that system, as we find it in the hands of its last supporters, had not yet been introduced; for the simple reason that observation had not yet become so nice as to show the necessity of the innumerable encumbrances with which, in a more advanced state of practical astronomy, it became necessary to load it.

We find, in truth, in ancient authors, but few traces of an opinion analogous to that of Copernicus; and even had it been taught in the school of Pythagoras, of which we discover evidence in the works of Cicero and Vitruvius alone, it is evident that it had met with few followers, and had been finally entirely forgotten. If a true view of the subject had ever been brought forward, all traces of it had been obliterated, and it was left to Copernicus to discover it anew.

It is, then, by Copernicus, that the motion of the earth has been introduced as a fact in astronomy; he it is that first demonstrated how the revolution of our planet around the sun was capable of explaining the succession of the seasons, and the precession of the equinoxes; who showed with what simplicity motions performed, at unequal rates, in orbits concentric at the sun, give rise to the phenomena of stations and retrogradations. He placed astronomy upon a new and consistent basis, and by the important change he introduced, opened the way for all subsequent investigations.

It is to the enthusiasm excited in the mind of Kepler, by the new truth given to the world by Copernicus, that we owe the discovery of the figure of the planetary orbits, and of the laws of their motions. The bare and naked idea of the earth's motion, had been unproductive among the ancients, because it had never been seriously entertained by astronomers. Its publication forms the epoch of modern astronomy.

But if Copernicus be entitled to the glory of being the founder of modern astronomy, that of being its legislator was reserved for a genius of even higher and more daring order. Copernicus seems to have been dismayed at his own boldness, and to have wanted the courage to put the finishing hand to his work. He in truth delayed its publication so long, that he did not receive a complete copy until the very day on which he died.

The system imagined by Copernicus, is one of extreme beauty and simplicity. By his introduction of circular orbits, supposed by him at first to be concentric at the sun, he suppressed at one blow all the epicycles with which Ptolemy and his followers had been compelled to load the path of their planets; the phenomena of stations and retrograde motions, become simple corollaries of the different radii of the orbits, and different rates of motion

of the planets. All the parts of the system are in close connexion with each other, the mutual relations are determinate, and all the distances are commensurable. On the other hand, in the ancient system, all is incoherent and vague; each of the planets might be considered as nearer, or farther, indifferently, provided the order of distances were not inverted, by bringing closest to the earth a planet of the longest zodiacal revolution.

These advantages of the system of Copernicus, are of themselves highly important. Of them, none of the ancients had the least suspicion, for had they been aware of them, it is impossible they could have avoided dwelling upon them. If, however, in its general features, the system of Copernicus be thus brilliant and imposing, in its detail it was far less complete. The author lays down as an axiom, that all the motions are circular and uniform, while observation makes us acquainted with none that are not constantly varying. To account for this, Copernicus was finally compelled to give to each of his circular orbits a different centre ; the sun is enclosed within all the orbits, but does not occupy the centre of any; it has no other apparent duty, but to distribute light, and appears unconnected with any of the motions. In fine, in order to reconcile the appearances with the theory, he was compelled to recur again to new epicycles, after having suppressed those of Ptolemy.

An important step was however made by him, without which a farther progress was impracticable. But if the reformation had proceeded no farther, practical astronomy would have gained little by the change. The founder of modern astronomy was not in possession of a sufficient series of good and authentic observations; he had not the taste or fitness for long calculations. To have done all, would have required more years than fall to the lot of man, and three whole lives were employed before the task begun by Copernicus was completed. Tycho Brache made the observations for which the life and strength of Copernicus were insufficient, and dying, left Kepler in possession of all that was necessary to complete the revolution in the system of astronomy.

Plutarch tells us, that an ancient philosopher had said, that the Greeks ought to have brought to trial as impious, him that had dared to disturb the sanctuary of Vesta, by ascribing motion to the earth. Such was the fate Copernicus feared for himself, and which caused him to defer for thirty-six years the publication of his book.

Tycho, who is entitled to the gratitude of astronomers for his observations, made a retrograde step in the theory, by proposing a system intermediate between those of Copernicus and Ptolemy. It is uncertain whether he were actuated by the theological scruples of his day, or ambitious of the glory of creating a new