epicycles

Epicycles are a historical concept used to explain the apparent complex motions of planets in the sky. They involve small circles, called epicycles, which planets are thought to move along while themselves orbiting along larger circles, called deferents, around Earth. This model helped early astronomers account for irregularities and retrograde motion of planets before the acceptance of heliocentric models. Essentially, epicycles are steps in a mathematical system to describe and predict planetary positions, though we now understand planetary motions through the more accurate laws of celestial mechanics established by Newton and others.