A schematic of how astronomers envisioned the motion of the planets before Copernicus. The Earth sat near the center of the universe. The planets moved around a small circle (the epicycle) which in moved along a larger circle (the deferent). The deferent was centered on a point (X) midway between the Earth and another spot called the equant. This complicated setup was needed to explain the complex motions of the planets. Credit: Wikipedia user Fastfission.