PTOLEMAIC (tiil'e-na'ik) SYSTEM. In astronomy, a planetary theory expounded by Ptolemy lq.v.). It was an attempt to reduce to a scientific form common and primitive no tions concerning the motions of the heavenly bodies. The primary and fundamental doctrines of this system are that the earth is the c•entae of the universe, and that the heavenly bodies revolve round it in circles, and at a uniform rate. These notions, which are naturally sug gested by the first general aspect of things, having. previous to any accurate observation, established themselves as unquestionable ax ioms, phenomena which were found, on closer examination, to be inconsistent with them were explained by the introduction of additional hypotheses. In the Ptolemaic system, the earth, the mast stable of the 'element:,' held the low est place, and supported water, the second in order; above water was placed air, and then fire, 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 superincum bent elements, and the larger spheres inelosing the smaller. To each of those spheres was at tached 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 Mercury. Venus, the sun, Mars, Jupiter, Saturn, and the fixed stars, eight in all. To this system later astronomers added a ninth sphere. the mo tion of which should produce the precession (q.v.) of the equinoxes, and a tenth, to cause the alternation of day and night. This tenth sphere, or primum mobile, was supposed to revolve from east to west in 24 hours, and to carry the others along with it in its motion ; hut the Ptolemaic astronomer- did 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 neces sary. As observations of the heavens increased in accuracy it was found that the heavenly mo tions were apparently not uniform, and this was explained as follows: The acceleration of the sun on one side. and retardation on the other side of his orbit, is only apparent, and results from the earth not being in the centre of his sphere, C (see Fig.), but at E. and consequently his um Lion appears to be slowest at P, and quickest at R. The alternate progression and regression of the planets was accounted for by supposing them to move. not directly with their crystallines, but in a small circle, whose centre 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 PQR (now supposed to represent the planetary crystalline). The planet while in the outer por tion of its small circle would thus have a for ward, and in the inner portion a backward mo tion. The larger circle was called an eccentric, and the smaller an epicycle (q.v.). This theory of eccentrics and epicycles satisfied the early astronomers; hut further investigation showed its incompleteness, and in later times it was found necessary to explain newly discovered dis crepancies by heaping epicycle upon epicycle, till such a complication of the system had been pro duced as drew from Alfonso X. of Castile, to whom the Ptolemaic system was being explained, the remark that. "if the Deity were now to recon struct the world, he (Alfonso) could give him a few useful hints." As soon as astronomers came to understand and test the Oopernican systrin the Ptolemaic system was discarded. See CO PERNICAN SYSTEM ; GALILEO.