In the Syntaxis (or Almagest), Ptolemy (second century A.D.) synthesised mathematical astronomy into a coherent whole which rendered his predecessors obsolete and would dominate Western and Islamic thought until the sixteenth century. He also wrote works on astrology, geography, optics and music.
An image of Ptolemy, crowned.

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Nothing reliable is known of Claudius Ptolemy's life that cannot be deduced from his surviving works; only a few brief and unsupported biographical statements are made by much later sources. 'Claudius' suggests he held Roman citizenship, 'Ptolemy' that he was of Greek descent and lived in Egypt. The astronomical observations that he listed as having himself made cover the period 127-141 AD, from which it may be inferred that he was active in the first and into the second half of the second century AD, and all of those observations are listed as made in Alexandria, so it is likely that he lived in or near that city, still a great centre of learning at that time. In the Middle Ages, before the twelfth century, when his work was being discovered and studied in detail by Islamic scholars, little more than his name was remembered in the Latin West; as early as the Encyclopedia of Isidore of Seville (c.600 AD) he was confused with the dynasty of Ptolemies who had ruled Egypt after Alexander the Great, and from that sprang an iconographic tradition, lasting a thousand years, in which Ptolemy was regularly represented wearing a crown.

Recommended Reading

M.Hoskin (ed.) The Cambridge Illustrated History of Astronomy, Cambridge 1997

J.North The Fontana History of Astronomy and Cosmology, London 1994

G.J.Toomer "Ptolemy", pp.186-206 in The Dictionary of Scientific Biography, New York 1970

Full Bibliography