Claudius Ptolemy, a Helleno-Egyptian mathematician, astronomer, and geographer, said to have been born in Pelusium, flourished at Alexandria in the 2d century A. D. Scarcely any particulars of his life are known. His MClaudius Ptolemy 140027 'A Claudius Ptolemy 140028 , or "Great Astronomical Construction," contains nearly all that is known of the astronomical observations and theories of the ancients, and is generally cited under the Latin titles Syntaxis Mathematica and Constructio Mathematica. The most important part of this work is a catalogue of stars, deduced from that constructed by Hipparchus. (See Precession.) The Syntaxis treats of the relations of the earth and heavens; the effect of position upon the earth; the theory of the sun and moon, without which that of the stars cannot be undertaken; the sphere of the fixed stars; and the determination of the planetary orbits. He places the earth in the centre of the universe, and the Ptolemaic system, based on the theories of Hipparchus, was universally received till the time of Copernicus. During all that interval the history of astronomy presents scarcely anything more than comments on Ptolemy's writings. But for the Arabians the Syntaxis would probably have perished. It was translated by them in the reign of the caliph Al-Mamoun, son of Haroun al-Rashid (about 827), and handed down under the title of Almagest. Translations from the Arabic were made into Latin, but the Greek text was subsequently also discovered in Byzantine manuscripts.

Ptolemy left a copious account of the manner in which Hipparchus established his theories, and in most of the branches of the subject gave additional exactness to what that astronomer had done. He computed, notwithstanding the fundamental errors and the inaccuracies of his system, the eclipses of the next six centuries; determined the planetary orbits; and is commonly said to have discovered the moon's second inequality or evection, though it is probable that Hipparchus really detected this inequality. Three observations cited by Ptolemy in support of his theory were borrowed from Hipparchus, and the nature of one of them suggests that they were taken from a great mass of observations, though Ptolemy himself says nothing to that effect. The astronomer who took a predecessor's star catalogue, and adding a constant correction to each star published it as the result of his own observations, would have left unnoticed all lunar observations by Hipparchus not absolutely necessary to establish his own theory.

As a geometer Ptolemy has been ranked as certainly the fourth among the ancients, after Euclid, Apollonius, and Archimedes. He caused light to pass through media of unequal density, and thus discovered refraction, and he is said to have first recognized the alteration of the apparent position of a heavenly body which is due to this cause; but here again it is probable that Hipparchus anticipated him. Ptolemy wrote a universal geography, which continued to be the standard text book till the 16th century. He was the first to use the terms latitude and longitude, by which he laid down the position of each country and town. He proved the earth to be a globe, and calculated its inhabited parts to extend from the meridian of Thinae, lon. 119° 30' E. of Alexandria, to the meridian of the Islands of the Blessed, 60° 30' W.; and from the parallel of Meroė, about lat. 16° 30' N., to that of Thule (Iceland or the Shetland islands), 63° N. The maps of this geography have been preserved with it. After him no one attempted for many centuries to reform geography except in the improvement of details. He was distinguished also as a musician, and wrote treatises on music, mechanics, chronology, and astrology; but probably most of these works were mere compilations.

The best edition of the Almagest is by Halma (Greek text with French translation, 2 vols. 4to, Paris, 1813-'16).