PTOLEMAIC SYSTEM, in astronomy, the system by which Claudius Ptolemy tried to explain celestial motions. For 14 centuries this was the universally accepted and authorita tive system of astronomy, only being com pletely overthrown upon the development of the Copernican system by Kepler toward the middle of the 17th century. The very long time during which the system of Ptolemy was re ceived may perhaps be more clearly realized from the statement that though it was first pro posed so early as the 2d century, A.D., in the early days of our American universities it was still taught in connection with that of Coper nicus. This was doubtless because, so far as was evident from the rough, naked eye, obser vations of the sun, moon and planets, it seemed to account for all of their complicated apparent motions in a wholly satisfactory manner. Ac cording to this system the earth is the fixed centre of the universe; a vast sphere, the "primum mobile," carries all the clestial bodies, and rotates once a day; the sun and moon travel in eccentric circles round the earth, and this accounts for the annual motion of the sun, and the monthly motion of the moon; each planet moves in a circular path, termed its epicycle about a point, and this nowt travels in an eccentric circle round the sun; all mo tions in each order of circle being described uniformly, this explained the looped paths, the progressions, stations and retrogradations of the planets. But every new astronomical ob
servation required a new contrivance on this system, and at length the necessary cyclic and epicyclic combinations became excessively cumbrous. The discovery of the elliptical form of the orbits, in the 17th century, and especially the development by Newton of die principles of mechanics, which wholly govern all the motions of the planets, quickly made it evident that the reality of the Ptolemaic system was a manifest impossibility.