epicycle

The term epicycle dates from geocentric systems, in which it indicated a supposed small circular motion of planets in their apparent large circular motion around Earth. It was presumed that a planet such as Mars moved exactly as if it moved in a small circular orbit around a non-existent object that orbits Earth, the small circular orbit-like motion termed an epicycle. This epicycle concept explained retrograde motion (motion in the opposite direction) of planets, that against the celestial sphere, the planets moved opposite their usual direction on occasion. The large circular motion/orbit was known as the deferent, and the planet itself apparently moved in a circular motion (the epicycle) around the point in space that follows the deferent. Ptolemy added the concept of equants, adjusting the center of the deferent orbit and explaining the observed orbital speed. This system came close to explaining the observed motions of planets, but was finally set aside when Kepler worked out the elliptical shape of orbits after the notion of heliocentricity (that planets orbit the Sun) was revived by Nicolaus Copernicus.

More recently, the term epicycle has been borrowed to indicate characteristics of some orbits of circumstellar disk material around stars and of stars around galaxies, and orbits around stable Lagrangian points, for analogous phenomena of objects or disk material following a relative circular motion around the Keplerian orbit (which, in the right situation, occurs according to Newton's laws). For stars orbiting around the center of disk galaxies, which can weave closer and further from the center, the term epicyclic frequency indicates the number of small somewhat-circular-motions occurring for each full orbit.