PTOLEMAIC SYSTEM of astronomy, so called from Ptolemy (q.v.), its chief expounder, was originated, however, long before his time, and was, in fact, merely an attempt to reduce to a scientific form the common and primitive notions concerning the motions of the heavenly bodies. It was implicitly adopted by Plato, Aristotle, Hippar elms, and (with the exception of the Pythagoreans, and probably of Pythagoras himself) all the eminent physicists and philosophers of ancient times; passing from them to the Byzantines and Arabs, who, especially the latter, were the means of disseminating it through western Europe, where it continued to be the universally established doctrine till ilk: 16th century. The primary and fundamental doctrines of this system are that the earth is the center of the universe, and that the heavenly bodies revolve round it in circles, and at a uniform rate. These notions, which are naturally suggested by the first general aspect of things, having, previous to any accurate observation, established them selves as unquestionable axioms, phenomena which were found, on closer examination, to be inconsistent with them, were explained by the introduction of additional hypo theses. The belief that the earth is the center of the universe was supported by its tieing in accordance with the relation of the primary elements of which the material world was supposed to be composed. Thus, earth, the most stable of the elements, held the lowest place, and supported water, the second in order; above water was placed air, and then lire, ether being supposed to extend indefinitely above the others. In or beyond the ether element were certain zones or heavens, each heaven containing an immense crystalline spherical shell, the smallest inclosing the earth and its superincumbent elements, and the larger spheres inclosing the smaller. To each of those spheres was attached a heavenly body, which, by the revolution of the crystalline, was made to move round the earth. The first or innermost sphere was that of the moon, and after it in order came those of Ifercury, Venus, the, Sun, Mars, Jupiter, Saturn, and the fixed stars, eight in all. To this system later astronomers added a ninth sphere, the motion of which should produce the precession of the equinoxes (q.v.), and a tenth, to cause the alternation of day 'and night. This tenth sphere, or primum nzobzle, was supposed to revolve from e. to w. in
;14 hours, and to carry the others along with it in its motion; but the Ptolemaic astrono mers do not venture to explain how this was done, although since the axis of motion of the primum mobile was that of the equator, its extremities being the poles of the heavens, while that of the ninth sphere was the axis of the ecliptic, some explanation was certainly necessary. As observations of the heavens became increased in accuracy, it was found that the heavenly motions were apparently not uniform, and this was explained as fol lows: The acceleration of the sun on one side, and etardation on the other side of his orbit, is only apparent, and results from the earth not being in the center of his sphere. C (see Fig.), but at E, and consequently his motion appears to be slowest at P, and quickest at R. The alternate progression and regres sion of the planets was accounted for by supposing them to move, not directly with their crystallines, but in a small Circle, whose center was a fixed point in the crystalline, and which revolved on its axis as it was carried round with the latter; thus (fig.) the planet was carried round the small circle ABD, as that circle was carried round 1'QR (now supposed to represent the planetary crystalline). The planet, while in the outer portion of its small circle, would thus have a forward, and in the inner portion a backward motion. The larger circle was called an eccentric, and the smaller an epicycle. This theory of eccentrics and epicycles satis fied the early astronomers; but further investigation showed its incompleteness, and ill later times it was found necessary to explain newly discovered discrepancies by heaping epicycle upon epicycle, till such a complication of the system had been produced as drew from Alfonso X. of Castile, to whom the Ptolemaic system was being explained. the humorous though somewhat blasphemous remark, that "if the Deity were now to recon struct the world, be (Alfonso) could give him a few useful hints." As soon as astrono mers came to understand and test the Copernican theory (q.v.), the venerable and dis orderly pile of hypotheses, the then representative of the Ptolemaic system, which had received the papal seal of infallibility, and had in various forms held supreme sway over the minds of men for twenty centuries, at once crumbled to atoms, and sunk into oblivion.