The most interesting circumstances connected with the early history of the Alexan drian school are the attempts made to determine the distance of the earth from the sun, and the magnitude of the terrestrial globe. Aristarchus of Samos—the pioneer of the Copernican system, as Humboldt calls him—is the author of an ingenious plan to ascer tain the former. See AIIISTARCHUS OF SA1109.
Among other eminent members of this school were Timocharis and Aristyllus, who made the observations which, together with observations of his own, enabled Hippar chus (q.v.) to discover the precession of the equinoxes; Eratosthenes (q.v.), who was the first who attempted to determine on true principles the magnitude of the earth, and to clear, as Humboldt expresses it, " the description of the earth from its fabulous tradi tions;" and Autolycus, whose books on A. are the earliest extant in the Greek language.
We have now arrived at by far the greatest name we have yet met in astronomical science—that of Ilipparchus of Bithynia (160-125 13c.), and here may he said to begin the real written history of scientific A.; for not until his era were there facts correct enough and sufficient in number upon which to build a system. Hipparchus was at once a theorist, a mathematician, and an observer. He catalogued nb less than 1081 stars. This is the first reliable catalogue we have. He discovered, as we have already mentioned, the precession of the equinoxes; he determined, with greater exactitude than his predecessors had done, the mean motion as well as the inequality of the motion of the sun; and also the length of the year. He also determined the mean motion of the moon, her eccentricity, the equation of her center, and the inclination of her orbit; and he suspected the inequality afterwards discovered by Ptolemy (the evection). • He invented processes analogous to plane and spherical trigonometry, and was the first to use right ascensions and declinations, which he afterwards abandoned in favor of lati tudes and longitudes.
For more than two centuries and a half after the demise of this indefatigable astron omer, we meet with no name of note. Ptolemy (130-150 A.D.) is the next who rises above the mass of mediocrities. Besides being a practical astronomer, he was accom plished as a musician, a geographer, and mathematician. His most important discovery
in A. was the libration or evection of the moon. He also was the first to point out the effect of refraction. He extended and improved many of the theories of Ilipparchus, and was the founder of the false system known by his name, and which was universally accepted as the true theory of the universe, until the researches of Copernicus exploded it. The Ptolemaic system, expounded in the Great Collection, or, as it was called by the Arabs, the which source most of our knowledge of Greek A. is derived —placed the earth immovable in the center of the universe, making the entire heavens revolve round it in the course of twenty-four hours.
With Ptolemy closes the originality of the Greek school. His successors were men of no mark, confining themselves for the most part to astrology, or to comments on earlier writers. It is to the Arabs that we owe the next advances in A. They com menced making observations 762 A.D., in the reign of the Caliph Al Mansur, who gave great encouragement to science, as did also his successors, the "good Haroun Alraschid" and Al Marnoum. both of whom were themselves diligent students of A. For four centuries the Arabs prosecuted the study of the science with assiduity, but they are chiefly meritorious as observers. They bad-little capacity for speculation, and through out held the Greek theories in superstitious reverence. They, however, determined with much more accuracy than the Greeks had done the precession of the equinoxes, the obliquity of the ecliptic, and the solar eccentricity; and the length of the tropical year was ascertained within a few seconds of the truth. The most illustrious of the Arabian school were Albategnius or Al Bataui (SSO A.D.), who discovered the motion of the solar apogee (see ANOMALISTIC YEAR), and who was also the first to make use of sines and versed sines instead of chords; he corrected the Greek observations, and was altogether the most distinguished observer between Hipparchus and the Copernican era; and Ibn Irunis (1000 A.D.), an excellent mathematician, who made observations of great impor tance in determining the disturbances and eccentricities of Jupiter and Saturn, and who was the first to use cotangents and secants.