An epicycle is a small orbit within a larger orbit that was used (mistakenly) to describe the movements of celestial objects in the Ptolemaic system (about AD 150). In Ptolemy's model of the Solar System, the Sun, the Moon, or a planet moved in an epicycle, the center of which traveled along a bigger circular orbit, known as the deferent, whose center was offset from Earth. Many layers of epicycles were needed to approximate real orbits with their retrograde motion.


Nicolaus Copernicus also used epicycles in his heliocentric (Sun-centered) representation of the Solar System in the mid 1500s and they were only superceded following Johann Kepler's discovery of the elliptical nature of orbits in the early 1600s.