Epicycle

A term employed by Ptolemy, in whose astronomical system the Earth was regarded as the centre, to indicate a small orbit around a central deferent (q.v.). He assumed that the orbits of all the other planets formed epicycles around the Earth's orbit. It was involved in an attempted solution of the phenomenon of retrograde motion. Assuming that the Sun pursues an orbit, the Earth's orbit is an epicycle, which while pursuing its own orbit is carried forward in the larger orbit of the Sun. The Moon's orbit is an epicycle upon the orbit of the Earth around the Sun. The Sun, which is never retrograde, was the only solar system body which, according to Ptolemy, did not have an epicycle.