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Ptolemy (fl. 146—170) Greek astronomer and geographer

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(c. 190—120 bc) Greek astronomer and geographer

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Astronomer, (fl. second half of 2nd cent. bc). His recorded observations range from 147 to 127. His only extant work, the Commentary on the Phainomena of Eudoxus and Aratus, contains criticisms of their descriptions and placings of the constellations and stars (see aratus; eudoxus ), and a list of simultaneous risings and settings. Valuable information on Hipparchus' own star co‐ordinates has been extracted from it. Most of our knowledge of Hipparchus' other astronomical work comes from Ptolemy's (2) Almagest.

Hipparchus transformed Greek astronomy from a theoretical to a practical science, by applying to the geometrical models (notably the eccentric/epicyclic hypothesis) that had been developed by his predecessors numerical parameters derived from observations, thus making possible the prediction of celestial positions for any given time. In order to do this he also founded trigonometry, by computing the first trigonometric function, a chord table. He constructed viable theories for the sun and moon, and, using several ingenious methods for determining the lunar distance (which he was the first to estimate accurately), developed a theory of parallax. He was thus able to compute both lunar and solar eclipses. For the planets, however, he refused to construct a theory, contenting himself with compiling a list of observations from which he showed the insufficiency of previous planetary models. He is famous for his discovery of the precession of the equinoxes, which is connected both with his investigations of the length of the year and his observations of star‐positions.

Hipparchus was a systematic and careful observer, who invented several instruments. He had a critical and original mind and a fertile mathematical invention. But he could not have achieved what he did without the aid of Babylonian astronomy, of which he displays a knowledge far deeper than any Greek before or after him, and the success of which in predicting phenomena he evidently wished to emulate. Not only did he have access to the wealth of Babylonian observational records (which he seems to have been instrumental in transmitting to the Greek world), but he also adopted many numerical parameters directly from Babylonian astronomy, and used a number of Babylonian arithmetical procedures, which were only later replaced (by Ptolemy (2) ) with strictly geometrical methods. Hipparchus' skill in combining the Babylonian and Greek traditions in astronomy was crucial to the successful propagation of the science in that form for over 1,000 years.

Hipparchus' geographical treatise was a polemic against the Geography of Eratosthenes, criticizing descriptive and esp. mathematical details. He also wrote on astrology, and his establishment of methods for computation of celestial positions undoubtedly contributed to the enormous expansion of that ‘science’ in the Graeco‐Roman world soon after his time.

Subjects: Classical studies

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