Epicycles are small circular orbits that were used in ancient astronomy to explain the complex movements of planets. In the Ptolemaic system, which was developed by the Greek astronomer Claudius Ptolemy, planets were thought to move in these smaller circles while simultaneously orbiting the Earth in larger circles. This model aimed to account for the observed retrograde motion of planets, where they appear to move backward in the sky. The concept of epicycles was eventually replaced by the heliocentric model proposed by Nicolaus Copernicus, which placed the Sun at the center of the solar system. This shift simplified the understanding of planetary motion and eliminated the need for epicycles, leading to more accurate predictions of celestial movements.