An epicycle is a small circle whose center moves along the circumference of a larger circle, known as a deferent. This concept was used in ancient astronomy to explain the complex movements of celestial bodies, such as the planets. The idea was part of the Ptolemaic system, developed by the Greek astronomer Claudius Ptolemy in the 2nd century AD, to account for the observed retrograde motion of planets. In this model, planets move in small circles (epicycles) while simultaneously orbiting the Earth along larger circles (deferents). Although the epicycle model was eventually replaced by the heliocentric model proposed by Nicolaus Copernicus, it played a significant role in the history of astronomy and our understanding of planetary motion.