What Are the Three Cycles of the Ptolemaic System?

Earth

• Ptolemaeus' system dictated that the Earth was the center of the heavens, and that all celestial bodies orbited the earth. All of the planets, moons and other things in the sky circled the Earth, because they were all attached to the Earth by crystal spheres. The moon was on the crystal sphere closest to the Earth, which caused the occurrence of night and day and the phases of the moon. Since the moon did not create its own light, Ptolemaeus claimed that the moon was imperfect and tainted earth.

Epicycles

• Ptolemaeus theorized that the planets, and every other part of the heavens, was attached to Earth by two spheres. These spheres were centered on Earth, and the epicycles rotated around it. While the epicycles were rotating, they would come closer to Earth and recede. The order of the heavens, according to Ptolemaeus, was Earth, the moon, Mercury and Venus, which were centered on a line from Earth, the sun, Mars, Jupiter and then the stars.

Equant

• The Ptolemaic system had a disconnect between its theories and what actually happened in the solar system. Ptolemaeus developed the equant to help explain what happens to the planets at different points in their orbit. The equant is a point around the center of each planet that would appear to always be moving at the same speed. This seemed to contradict Ptolemaeus' theory of the planets orbiting around Earth in uniform, circular patterns.

Gallileo

• The Ptolemaic system was later disproved by Galileo, who observed the phases of Venus through his telescope. According to the Ptolemaic system, Venus was located between Earth and the sun, or on the opposite side of the sun, where it would have been impossible to witness any change to the planet during its phases. This caused Galileo to reexamine the Ptolemaic system and correctly theorize that Venus orbited the sun.