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Claudius Ptolemaeus , Born AD 90 – Died AD 168), known in English as Ptolemy , was a Roman (probably of Greek or Egyptian ethnicity). He was a mathematician, astronomer, geographer and astrologer. He lived in Egypt and died in Alexandria around AD 168.

Ptolemy was the creator of many scientific treatises, several of which would be of continuing importance to later Islamic and European science, particularly Astronomy. The most important one probably is the astronomical text, the Almagest.

The Almagest is the only surviving astronomical text from antiquity. Babylonians had developed arithmetical techniques for calculating astronomical phenomena; Greek astronomers, including Hipparchus produced geometric models for calculating celestial motions; Ptolemy, however, claimed to have derived his geometrical models from selected astronomical observations by his predecessors spanning more than 800 years. Ptolemy presented his astronomical models in convenient tables, which could be used to compute the future or past position of the planets. The Almagest, which also lists forty-eight constellations, is ancestral to the modern system of constellations.

Ptolemy's model, like those of his predecessors, was geocentric and was almost universally accepted until the appearance of simpler heliocentric models during the scientific revolution during the Renaissance.

His Planetary model saw the universe as a set of nested spheres, which used epicycles (circular motions around a a point doing its own circular motion) to compute the dimensions of the universe ans calculate where the planets (which he thought circled the earth, and which included the moon and the sun) would be at any given time.

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