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Ptolemy**Born**: c. 100 CE Egypt, Roman Empire**Died**: c. 170 Alexandria, Egypt, Roman Empire

Ptolemy (Latin: Claudius Ptolemaeus) was an Egyptian astronomer, mathematician, and geographer of Greek descent who flourished in Alexandria during the 2nd century CE. In several fields his writings represent the culminating achievement of Greco-

The book that is now generally known as the Almagest (from a hybrid of Arabic and Greek, “the greatest”) was called by Ptolemy Hē mathēmatikē syntaxis (The Mathematical Collection) because he believed that its subject, the motions of the heavenly bodies, could be explained in mathematical terms. The opening chapters present empirical arguments for the basic cosmological framework within which Ptolemy worked. Earth, he argued, is a stationary sphere at the centre of a vastly larger celestial sphere that revolves at a perfectly uniform rate around Earth, carrying with it the stars, planets, Sun, and Moon-

How much of the Almagest is original is difficult to determine because almost all of the preceding technical astronomical literature is now lost. Ptolemy credited Hipparchus (mid-

Ptolemy was preeminently responsible for the geocentric cosmology that prevailed in the Islamic world and in medieval Europe. This was not due to the Almagest so much as a later treatise, Hypotheseis tōn planōmenōn (Planetary Hypotheses). In this work he proposed what is now called the Ptolemaic system, a unified system in which each heavenly body is attached to its own sphere and the set of spheres nested so that it extends without gaps from the Earth to the celestial sphere. The numerical tables in the Almagest (which enabled planetary positions and other celestial phenomena to be calculated for arbitrary dates) had a profound influence on medieval astronomy, in part through a separate, revised version of the tables that Ptolemy published as Procheiroi kanones (Handy Tables). Ptolemy taught later astronomers how to use dated, quantitative observations to revise cosmological models.

Ptolemy also attempted to place astrology on a sound basis in Apotelesmatika (Astrological Influences), later known as the Tetrabiblos for its four volumes. He believed that astrology is a legitimate, though inexact, science that describes the physical effects of the heavens on terrestrial life. Ptolemy accepted the basic validity of the traditional astrological doctrines, but he revised the details to reconcile the practice with an Aristotelian conception of nature, matter, and change. Of Ptolemy’s writings, the Tetrabiblos is the most foreign to modern readers, who do not accept astral prognostication and a cosmology driven by the interplay of basic qualities such as hot, cold, wet, and dry.