The geocentric theory of the universe describes the universe with Earth at its center and puts the other celestial bodies in circular orbits around it. The usual ordering placed the Moon, Mercury, Venus, the Sun, Mars, Jupiter, Saturn, and the fixed stars on a series of concentric spheres further and further away from the stationary Earth.

The heavens revolved around Earth once every 24 hours. While an outer sphere carried the Sun around Earth every day, it rotated on an inner sphere about an axis attached to the outer one. This second effect accounted for the Sun's yearly transit along the ecliptic, a plane held at a 23.5° angle with respect to the celestial equator (taking the North Star as the North Pole). Because the Sun's tilted orbit placed it at different angles relative to the equator at various times of the year, this model provided a...