Epicycles are small circular orbits used in ancient astronomy to explain the complex movements of planets. The concept was developed by Claudius Ptolemy in the 2nd century AD as part of the geocentric model, which placed the Earth at the center of the universe. According to this model, planets moved in small circles, called epicycles, while simultaneously orbiting the Earth in larger circles. This system allowed astronomers to account for the observed retrograde motion of planets, where they appear to move backward in the sky. Although epicycles were eventually replaced by the heliocentric model proposed by Nicolaus Copernicus, they played a crucial role in the development of astronomical theories.