moon. He published some mathematical and geographical treatises, and made a number of observations to determine
the obliquity of the ecliptic and the precession of the equinoxes.
We come now to the period of the overthrow of the Ptolemaic system and the total renovation of the science of astronomy, which was due to the labours of Copernicus. The system which is associated with the name of Copernicus is now so familiar to every one, that it is almost unnecessary to describe it. The heaven, composed of stars perfectly at rest, occupies the remotest bounds of space, then the orbit of Saturn, next Jupiter, Mars, the Earth (accompanied by its moon), Venus, Mercury, and, lastly, the Sun immovable at the centre. By this arrangement the stations and retrogradations of the planets became simple mathematical corollaries, following from the differences of the radii of their orbits and their unequal motions. The diurnal rotation of the earth explained more simply and rationally the apparent daily revolution of the heavens ; and the precession of the equinoxes was referred to a small variation in the inclination of the earth s axis to the plane of the ecliptic. But the simplicity of the system, and its consequent probability, were the only arguments which Copernicus was able to bring forward in proof of its reality. The motion of the earth can, indeed, never be made an object of ocular demonstration ; but after Richer s discovery of the diminution of gravity towards the equator, it was impossible to doubt longer of the existence of its rotatory motion ; and when Roemer had measured the velocity of light, and Bradley observed the phenomena of aberra tion, the evidences of its annual revolution were rendered equally convincing. Great, however, as were the merits of Copernicus, it must.be acknowledged that he left his system in a very imperfect state. After the example of the ancients, he assumed as an axiom the uniform circular motion of the planets ; and as the only motions which are observed are in a state of incessant variation, he was obliged, in order to explain the inequalities, to suppose a different centre to each of his orbits. The sun was placed within the orbit of each of the planets, but not in the centre of any of them, consequently he had no other office to perform than to distribute light and heat; being excluded from any influence on the system, he became as it were a stranger to all the motions. Yet notwithstanding these and other imper fections, the establishment of the doctrine of the earth s motion, with an evidence which dissipated the illusions of sense, was a great step towards the true knowledge of the planetary system ; and when we consider the ignorance and prejudices of the age, we cannot hesitate to admit his claim to a high rank among philosophers. But whether the actual services which he rendered to astronomy are commensurate with the great fame he has obtained, may admit of doubt. He revived an ancient opinion opposed to the prejudices and religious dogmas of his times, and fortified it with new and strong, though not absolutely convincing, proofs. It seldom happens, however, with regard to those sciences which ultimately appeal to experi ence, that general reasoning, even of the soundest kind, tends much to their real advancement ; and there is little reason for thinking that astronomy would have been less perfect, or that any discoveries since made in it would have been retarded a single day, even if Copernicus had never lived. His great merit, like that of Lord Bacon, consists in the sound views which he took of nature, and in advanc ing so far before the general attainments of his age. For the events of his life see Copernicus.
Tycho Brahe stands next in chronological order on the roll of those who have contributed to the progress of astro nomy. As an indefatigable and skilful observer, he is justly considered as far superior to any astronomer who had preceded him since the revival of the science in Europe. His ample fortune gave him the means of pro curing the best instruments which the age could produce ; and by his ingenuity and persevering application, he was admirably qualified to employ them to the best advantage. He computed the first table of refractions, and if it ex tended only to 45, the reason was, that the effects of refraction, at a higher altitude, were altogether insensible to his instruments. His solar tables were brought to so great a degree of exactness, that he affirms he could never detect an error in them exceeding a quarter of a minute ; but there is reason to suspect some exaggeration in this statement, particularly as Cassini, a century after, with much better means, could scarcely answer for errors of a whole minute. He contributed greatly to the improvement of the lunar tables, and detected a considerable inequality in the moon s motion in longitude, to which he gave the name of the Variation, by which it has ever since been distinguished. He also discovered an equation in latitude similar to the evection which had been observed by Hip- parchus, and fixed its amount with great accuracy. He remarked the fourth inequality of the moon in longitude, although he failed in his attempt to ascertain its amount, or assign its law. He represented the inequalities of the motions of the nodes, and in the inclination of the lunar orbit, by the motion of the pole of that orbit in a small circle round the pole of the ecliptic. He demonstrated that the region of the comets is far beyond the orbit of the moon, and determined the relative and absolute positions of 777 fixed stars with scrupulous exactness, which gave his catalogue an immense superiority over those of Hip- parchus and U lugh Begh ; and he left to his successors a regular series of observations of the planets, amassed for the purpose of establishing- Ihe truth of his own system, but of which Kepler made a better use by employing them to establish the system of Copernicus. For an account of his life, see Brahe, Tycho.
Kepler may be said to have constructed the edifice of the universe. The observations of the Danish astronomer had furnished the latter with the means of establishing with certainty the truth or inaccuracy of the various hypotheses which he successively imagined ; and the diligence with which he laboured in comparing and calculating these observations during 20 years, was finally rewarded by some of the most important discoveries which had yet been made in astronomy. Deceived by an opinion which had been adopted by Copernicus, and had never been called in question by the ancients, that all the celestial motion* are performed in circles, he long fruitlessly endeavoured to represent by that hypothesis the irregular motions of Mars; and after having computed with incredible labour the observations of seven oppositions of that planet, he at length discovered that the motions could only be accurately represented by supposing the planet to move in an ellipse, having the sun in one of its foci. Having arrived at this important result, he next proceeded to consider the angular motion of the planet, and finding that it was not uniform in respect of any point situated within the orbit, he concluded that uniform motion, till then universally received as an axiom, had no existence in nature. He perceived, however, that the areas described in equal times by the radius vector of the planet, at its greatest and least distances, were equal ; and subseqiient observations enabled him to demonstrate that this equality extended to every point of the orbit. It was therefore discovered that Mars moves in an elliptic orbit, of which the sun occupies a focus, and in such a manner, that the area described by
a line drawn from the centre of the planet to that of the