Subsequently to Hipparchus, we find the astronomers Geminus and Cleomedes; their fame, however, is totally eclipsed by that of Ptolemy, A.D. 138, the author of the great work "Syntaxis," or the mathematical construction of the heavens--a work fully deserving the epithet which has been bestowed upon it, "a noble exposition of the mathematical theory of epicycles and eccentrics." It was translated by the Arabians after the Mohammedan conquest of Egypt; and, under the title of Almagest, was received by them as the highest authority on the mechanism and phenomena of the universe. It maintained its ground in Europe in the same eminent position for nearly fifteen hundred years, justifying the encomium of Synesius on the institution which gave it birth, "the divine school of Alexandria." The Almagest commences with the doctrine that the earth is globular and fixed in space; it describes the construction of a table of chords and instruments for observing the solstices, and deduces the obliquity of the ecliptic. It finds terrestrial latitudes by the gnomon; describes climates; shows how ordinary may be converted into sidereal time; gives reasons for preferring the tropical to the sidereal year; furnishes the solar theory on the principle of the sun's orbit being a simple eccentric; explains the equation of time; advances to the discussion of the motions of the moon; treats of the first inequality, of her eclipses, and the motion of the node. It then gives Ptolemy's own great discovery--that which has made his name immortal--the discovery of the moon's evection or second inequality, reducing it to the epicyclic theory. It attempts the determination of the distances of the sun and moon from the earth, with, however, only partial success, since it makes the sun's distance but one-twentieth of the real amount. It considers the precession of the equinoxes, the discovery of Hipparchus, the full period for which is twenty-five thousand years. It gives a catalogue of 1,022 stars; treats of the nature of the Milky Way; and discusses, in the most masterly manner, the motions of the planets. This point constitutes Ptolemy's second claim to scientific fame. His determination of the planetary orbits was accomplished by comparing his own observations with those of former astronomers, especially with those of Timochares on Venus. [Sidenote: His geography.]