PTOLEMY (CLArraus PTOLEALEUS). An an cient astronomer and geographer. He was a native of Egypt, though it is uncertain whether he was born at Pelusium or at Ptolemais in the Thebaid. Nothing is known of his personal history, except that flourished in Alexandria in A.D. 139 and there is probable evidence of his having been alive in A.D. 161. Ptolemy, both as an astronomer and geographer.
held supreme sway over the minds of almost all the scientific men from his own time down to the close of the Middle Ages; but, and in astron omy especially, 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 Ilipparchus to guide him; and, indeed, he scrupulously distin guishes between Hipparchus's labors and his own. To Ptolemy belongs the invention of a planetary theory, the discovery of the moon's evection • (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 Meyd/oi Zi/vraEts rip 'Aerpopoplas, commonly known by the mediaeval title Almagcst (q.v.), received his name. and. as the Ptolemaic system (q.v.), obtained the homage of succeeding generations till the time of Copernicus.
The Almagest is divided into thirteen books, and the trigonometry of the Greeks is known almost entirely through this work of the • second century. It shows that the Greeks were then confined to the sexagesimal sys tem and used tables of whole chords instead of half-chords. Book i. contains all that can he regarded as pure theory. It contains a method for calculating chords: the fundamental theorem in rectilinear trigonometry, that the rectangle of the diagonals of an inscribed quadrilateral equals the sum of the rectangles of the pairs of opposite sides; and the principles of spherical geometry and trigonometry sufficient to construct a table of the sun's declination to each of longitude. The further mathematical work of the Almagest consists of applications of the geometry and trigonometry of Book i.
As a geographer, Ptolemy appears as the cor rector and improver of the works of a predeees sor, Mullins of Tyre, about whom, except from Ptolemy's writings, little is known. His resrypa Oucii 70)-yncrts 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 positions of places, by latitude (4sos) and longitude (rXdros) with the calculation of the size of the sphere of the earth, and of the extent of surface then known. He also describes the mode adopted by him of projecting the surface of a hemisphere on a flat surface, and shows its su periority over the projections of Eratosthenc;, Hipparchns, and 'Marinus. He constructed a series of twenty-six maps, together with a gen eral map of the world, in illustration of his work.
The chief of his writings. besides those already mentioned, are: Terpd/343Xos Mh•raEls, with which is combined another work, called Korpos or Col tiloquium, from its containing a hundred apho risms, both works treating of astrological sub jects, and held by some on this account to he of doubtful genuineness; 43clorets dart ave7w derepow xal crtmcryaryii ertanlhacea:M, a treatise on the phenomena of the fixed stars, or a species of al manac. The rest of his works are of inferior ha portance, and consist of descriptions of various kinds of projections (q.v.), the theory of the mu sical scale. chronological and metaphysical treat ises, and a summary of the hypotheses employed in his great work, the .11magest. Others of Ptole my's works have been lost, mid it is still a moot point whether or not they contained a treatise on optics, as a Latin version of \\dint is said to have been an Arabic translation of Ptolemy's original treatise on that subject is still in existence.
The Jintagest and the Geography were the standard text-books to succeeding ages, the first till the time of Copernieus, the second till the great maritime discoveries of the fifteenth cen tury showed its deficiencies. They have passed through numerous editions, the best of which are, for the ..t/nuil/C8i and most of Ptolemy's minor works, that by Halma ( Paris, and for the (ieography, the Latin versions of l4S8 and 1490, published at Rome, the celitio princeps of the Greek text by Erasmus ( Basel, 1333), and the Elzevir edition (Leyden, 1619). The catalogue of stars has been frequently re printed separately, the last and best edition being that of Francis Baily, vol. xiii. of the Memoirs of the Royal Astronomieal Society (London, 1843).