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LEFT ASTRONOMY 316 ASTRONOMY ations, and the effect of these radiations upon other bodies, and all allied ques- tions arising out of these. Its princi- pal instrument, the spectroscope, has likewise furnished data otherwise un- attainable ini the field of mathematical astronomy, viz., the determination of the motion to or from us of the heavenly bodies by displacement of the lines of their spectra due to this motion. COELOSTAT- 2>i?. MIRROR, ISO M FOCUS LBNS OBSERVfiiTION HOUSE F^OTO&RAPHIC F>LPrr£ m^ •RAY5 OPW/^RD TO PLATE RfitYS POWn/WARD LENS .^ ORATING DIAGRAM OF COELOSTAT OF MT. WILSON OBSERVATORY. THE TOWER IS 164' HIGH History. — The Chinese, Hindus, Chal- deans, Egyptians, and Greeks investi- gated the heavens long before the Chris- tian era. In China, astronomy was intimately associated with state poli- tics; the Indians, Chaldeans, and Egyp- tians made it a matter of religion. The Greek historians attribute the earliest knowledge of astronomical science to the Chaldeans and Egyptians. They say that the former discovered the Saros or cycle of 223 lunations, nearly equal to 18 years, by which they predicted the return of previously observed eclipses and made use of other empirical cycles or periods. Thales (640 B. c), the founder of the Ionic school, laid the foundation of Greek astronomy. The successors of Thales held opinions which, in many re- spects, are wonderfully in accordance with modern ideas. Anaximander, it is said, held that the earth moved about its own axis, and that the moon's light was reflected from the sun. To him is also attributed the belief in the plural- ity of worlds. Pythagoras (500 B. C.) promulgated the true theory that the sun is the center of the planetary world, and that the earth revolves round it. But the views of Pythagoras met with little or no support from his successors until the time of Copernicus. Between Pythag- oras and the advent of the Alex- andrian school, nearly two centuries later, among the most prominent names in astronomical annals is that of Meton, who introduced the Metonic Cycle, con- sisting of 125 months of 30 days each, and of 100 of 29 days, making a period of 6,940 days, nearly equal to 19 solar years. To the Alexandrian school, owing its existence to the Ptolemies, we are in- debted for the first systematic observa- tions in astronomy. Hipparchus of Bithynia (160-125 B. c), was a theorist, a mathematician, and observer. He catalogued no less than 1,081 stars. He discovered the precession of the equinoxes; he deter- mined the mean motion as well as the inequality of the motion of the sun, and the length of the year; also the mean motion of the moon, her eccentricity, the equation of her center and the in- clination of her orbit; and he suspected the inequality afterward found by Ptol- emy (the evection). After the death of Hipparchus, astronomy languished for nearly three centuries. Ptolemy (130-150 A. D.), besides being a practical astronomer, was accom- plished as a musician, a geographer, and a mathematician. His most im- portant discovery in astronomy was the evection of the moon. He also was the first to point out the effect of refraction. He was the founder of the false system known by his name, and which was uni- versally accepted as the true theory of the universe until the researches of Copernicus exploded it. The Ptolemaic system placed the earth, immovable, in the center of the universe, making the entire heavens revolve round it in the