44 ASTRONOMY the sun's distance, with the true length of the precession al period, and with other astronomi- cal elements the discovery of which has re- warded the exact methods and the profound mathematical researches of modern times. As to Chinese astronomy, we have abundant evi- dence to show that it was inexact, though un- doubtedly very ancient. Its antiquity may be inferred from the circumstance that the em- peror Chwen-hio adopted as an epoch a con- junction of the planets Mercury, Mars, Jupiter, and Saturn, which has been shown by M. Bailly to have occurred no less than 2449 years B. 0. In a remarkable work on the subject of Chinese astronomy, recently published by Mr. Williams, assistant secretary of the astronomical society of England, we are told that the instruments at present used by Chinese astronomers, as well as their principal methods of calculation, were introduced by Jesuit missionaries. Yet the ancient Chinese must have possessed some familiarity with the celestial motions. They could calculate eclipses ;. .for we learn that " in the reign of the emperor Chow-kang, the chief astronomers Ho and Hi were condemned to death for failing to announce a solar eclipse which took place 21 69 B. C. ; " a clear proof that the prediction of eclipses was a part of the duty of the imperial astronomers. The Chinese were also acquainted with the Metonic and Callippic cycles. The earliest Greek school of astronomy was that founded by Thales of Mi- letus (600 B. C.) and termed the Ionian school. Thales appears to have been acquainted with the motions of the sun and moon, with the ex- Elanation of seasonal changes, and with the sngth of the year. It has been said that he taught mariners to regard the Lesser Bear rather than the Greater as the polar constel- lation; but Manilius ascribes the selection of the Lesser Bear as the cynosure to the Phoe- nicians. To Pythagoras, who also belonged to the Ionian school, a knowledge of the true theory of the earth has been ascribed, though on insufficient grounds. According to the statement of his pupil Philolaus, he taught that " the earth and planets move in oblique circles (or ellipses) about fire, as the sun and moon do " a statement which certainly does not as it stands indicate exact knowledge respecting the constitution of the solar system. Nicetas of Syracuse is said in like manner to have taught that the diurnal motions of the celestial bodies are caused by the rotation of the earth upon her axis. 'Theophrastus," says Cicero, "nar- rates that Nicetas of Syracuse held that the sun, moon, and stars are at rest, and the earth alone moves, turning about its axis, by which the same phenomena are produced as if the contrary were the case." Eudoxus of Cnidus first endeavored to explain the looped paths of the planets, solving the problem by the inven- tion of the theory of concentric spheres. But it was by the Alexandrian school, founded under the Ptolemies, that exact and systematic observation of the celestial bodies was first undertaken. Hipparchns of Nictea (160 B. C.) surpassed all the astronomers of antiquity in skill and acumen. He made the first catalogue of the stars, and was the first to calculate the motions of the sun and moon. He also made a series of observations of the planets, and rep- resented their motions by the famous theory of epicycles a theory which, though unsound, was in so far in advance of previous ideas, that it was intended to be brought into comparison with the real motions of the celestial bodies. Hipparchus also invented plane and spherical trigonometry. Ptolemy is another distinguished member of the Alexandrian school. Some of the theories and observations which have been ascribed to him were indeed due to the labors of Hipparchus. Thus the Ptolemaic system of astronomy was wholly based on the theories of his predecessor ; and the star places indi- cated in his works seem to have been simply deduced from Hipparchus's catalogue of 1,081 stars by introducing a correction for precession. Yet Ptolemy's labors were unquestionably im- portant. He detected the inequality in the moon's motions called the evection, and was the first to recognize the effect of refraction in altering the apparent places of the heavenly bodies. His work, the Almagest (or the Syn- taxis), contains nearly all that we know of the astronomy of the ancients. The school of Alexandria ceased to exist when Egypt was invaded and conquered by the Mohammedans, and the celebrated Alexandrian library de- stroyed, in the 7th century. The Arabians, however, formed no contemptible astronomers. They even surpassed the Greeks in the depart- ment of practical astronomy ; and they handed down to the Europeans the system which they had derived from their predecessors. In the 13th century European astronomy may be said to have had its origin or revival, though nearly two centuries elapsed before any important advance was effected. Toward the close of the 15th century the labors of Purbach and Regio- montanus prepared the way for the work of Copernicus, the founder of the true system of astronomy; while Waltherus revived the art of astronomical observation, and thus indi- rectly supplied the means of establishing the theories of Copernicus, Kepler, and Newton. Copernicus (born in 1473) found that by pla- cing the sun instead of the earth at the centre of the scheme, there resulted a simple and rational explanation of all the chief motions of the planets. He was not able to show, however, that the epicycles of Hipparchus and Ptolemy could be wholly removed. According- ly, many astronomers, who might have been attracted to the Copernican system if it could have been presented as it is known in our day, were found in the ranks of its opponents. Among these was Tycho Brahe, the Dane, who pointed out that the apparent fixity of the stars is opposed to the Copernican theory, unless the distances of all the stars be assumed to exceed enormously the distance of the earth
