2 articles for "Epicycle"

**Epicycle**[Astro*Index]

An artifact of the System of Planetary Motion advanced by Ptolemy, in which each of the planets revolves in a small circle called an epicycle about a point which, in turn, revolves about the Earth in a large circle, called a *deferent*. The system advanced by Copernicus replaced these complex motions with elliptical paths-[WRONG] for the planets and a heliocentric framework. Note, however, that by adding a heirarchy of epicycles, it is possible to describe a planet's motion to whatever accuracy is desired. In fact, modern astronomical "theories" express a planet's motion in the form of *Poisson-Fourier series*, which are little more than a mathematical representation of epicyclic motion.

See also: ♦ Ptolemy ♦ Deferent

**Epicycle**[DeVore]

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.

See also: ♦ Ptolemy ♦ Deferent

Astro*Index Copyright © 1997 Michael Erlewine