PTOLEMY, a. celebrated astronomer and geographer, whose proper name is CLArDius PrroLEmzEus, was a native of Egypt, though it is uncertain whether he was. born at Pelu slum or Ptolemais in the Thebaid. Nothing is known of his personal history, except that he flourished in Alexandria in 139 Am. and there is probable evidence of his hay.infir been alive in 161 A.D. The chief of his writings are: liqyaVe te8 Astronomias, which, to distinguish it from the next-mentioned, was probably denominated by the Arabs ?negate, the greatest, whence was derived the name Almagest (Arab. article al, the), by which it is generally known; Tetrabiblos Signta.ris, with which is combined another work, called Karpos or Centiloguium, from its containing a hundred aphorisms, both works treating of astrological subjects, and held by some on this account to be of doubt ful genuineness; Phaseis aplanen asteron •ai onagege episemaseion, a treatise on the phenomena of the fixed stars, or a species of almanac; the Getigraphike IlyphegTsis, his great geographical work, in eight books. The rest of his works are of inferior impor tance, and consist of descriptions of various kinds of projections (q.v.), the theory of the musical scale, chronological and metaphysical treatises, and a summary of the hypo theses employed in his great work, the Almagest. Others of Ptolemy's works have been lost, and it is still a moot-point whether or not they contained a treatise on optics, as a Latin version of what is said to have been an Arabic translation of Ptolemy's original treatise on that subject is still in existence.
Ptolemy, both as an astronomer and geographer, held supreme sway over the 'minds of almost all the scientific men from his own time down till about the 15th c.; but, and in astronomy specially, he seems to have been not so much an independent investigator as a corrector and improver of the work of his predecessors. In astronomy he had the labors of Hipparchns to guide him; and, indeed, scrupulously distinguishes between Ilipparchus's labors and his own. To Ptolemy belongs the invention of a planetary theory, the discovery of the moon's evectiou (q.v.), and the singular distinction of being the sole existing authority on the subject of ancient astronomy. From this last-mentioned fact,, the system of astronomy which he sets forth in the Almagest received his name; and, as the Ptolemaic system (q.v.), obtained • the homage of succeeding generations till the time of Copernicus. His great work, the Almagest, is divided into 13 books. Ptolemy
seems to have been little of an independent observer, trusting implicitly to his prede cessor, Hipparchus; but his geometrical powers were of a very high order, unless, as Delambre suggests, but with little probability, the elegant demonstrations here and there occurring in the Almagest were borrowed from other sources.
As ageo,grapher, Ptolemy occupies a similar position to what he holds in astronomy; ho appears before his readers as the corrector and improver of the works of a predecessor, Manlius cf Tyre, about whom, except from Ptolemy's writings, little is known. Ptolemy here appears to more advantage as an independent investigator, and his improvements and suggestions are at once more valuable and correct; but it is sometimes difficult to separate his data from those of Marinus. His geography is divided into eight books, all of which, with the exception of the first, eighth, and a portion of the seventh, are nothing more than a catalogue of places, with their latitude and longitude (to 12ths of a degree), with a brief general description prefixed to each continent and country or tribe, and interspersed here and there with remarks of a miscellaneous character on any point of interest, The rest of the work contains details regarding, his mode of noting the posi tions of places—by latitude (meltos) and longitude (platos)—with the calculation of the size of the sphere of the earth, and of the extent of surface then known. Ile also describes the mode adopted by him of projecting the surface of a hemisphere on a that surface, and shows its superiority over the projections of Eratosthenes, Hipparelms, and Marinas. He also constructed a series of twenty-six maps, together with a general map of the world, in illustration of his work.
The Almag8st and the Geography were the standard text-books to succeeding ages, the first till the time of Copernicus, the second till the great maritime discoveries of the 15th c. showed its deficiencies. They have passed through numerous editions, the best of which are, for the Almagest and the most of Ptolemy's minor works, that by Halma (Paris, 1813-16-19-20, quarto); and for the Geography, the Latin versions of 1482 and 1490, published at Rome, the ,editio priuceps of the Greek text by Erasmus (Basel, 1533, 4to), and the Elzevir edition (Lugd. Bat.1619, fol.). The catalogue of stars has been fre quently reprinted separately, the last and best edition being that of Francis Bally, in vol. xiii. of the Memoirs of The Royal Astronomical Society (London, 1843).