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 course of 24 hours. The work by which he is best known, however, is the col lection and systematic arrangement of the ancient observations in his great work, the "Megale Syntaxis," which gives a complete résumé of the astro nomical knowledge of the day. The most important part of it is the seventh and eighth books, which contain the catalogue of stars which bears Ptolemy's name, though it is only a compilation of the catalogue of Hipparchus with the positions brought up to the time of Ptol moon's motion, the variation, and de termined its amount.
The revival of astronomy in Europe may be said to have begun with George Purbach, who translated the "Alma gest" at Vienna. His pupil, John Mul ler, translated into Latin the works of Ptolemy and the conics of Appolonius, built an observatory at Nuremberg, and equipped it with instruments of his own invention. He died in 1476.
Copernicus (1473-1543) exploded the Ptolemaic idea, and promulgated a cor emy. These latter are in use to-day, though the gaps between them have been filled up in some cases by more modern asterisms.
To the Arabs we owe the next ad vances in astronomy. The most illus trious of the Arabian school were Alba tegnus, or Al Batani (880 A. D.), who dis covered the motion of the solar apogee, and who was the first to make use of sines and versed sines instead of chords; and Ibn-Yunis (1000 A. D.) , an excellent
mathematician, who made observations of great importance in determining the disturbances and eccentricities of Jupi ter and Saturn, and who was the first to use cotangents and sectants. Like wise, at about the same time, Abul Wefa discovered the third inequality in the rect theory. It makes the sun the im movable center of the universe, around which all the planets revolve in concen tric orbits, Mercury and Venus within the earth's orbit, and all other planets without it.
Decidedly the most industrious ob server and eminent practical astronomer from the time of the Arabs to the latter half of the 16th century was Tycho Brahe (1546-1601). He made the first table of refractions, and discovered the variation and annual equation of the moon, the inequalities of the motion of the nodes, and of the inclination of the lunar orbit. He also demonstrated that the region of the comets is far beyond the orbit of the moon, and he deter mined the positions of 777 stars with an accuracy far surpassing anything before done in that line. He left behind him a mass of observations of the sun and planets which he had made to demonstrate the truth of his system of the universe, but which afterward be came, in Kepler's hands, the means of haps the greatest of these was the in vention of logarithms by Lord Napier. In 1603, John Bayer, of Augsburg, pub lished his "Uranometria," or maps of the 48 constellations which had been handed down from Hipparchus through Ptolemy in the "Almagest," and, on its overthrow and the final and perma nent establishment of the truth of the Copernican system. Kepler's brilliant discovery of the three laws of planetary motion made his name immortal.
Galileo Galilei was the contemporary of Kepler, and, as his discoveries were of a more popular character, he obtained a more immediate fame and reputa tion. In the interval between the great discoveries of Kepler and Galileo and those of Newton various astronomers made valuable additions to astronomical knowledge or invented new apparatus for observing the heavenly bodies. Per these maps, he for the first time assigned to the individual stars the letters that are used to-day. The researches of Descartes gave a new help to mathe matical analysis. Horrox observed the transit of Venus in 1639, the first ever seen by man.