Greek philosopher Claudius Ptolemy believed that the sun, planets and stars all revolved around the Earth. This belief gave way to the ancient Greek theory of a geocentric or Ptolemaic model of the universe. "Geocentric" refers to the belief that the Earth is the center of the universe.
Ancient Greek astronomers believed that planets followed perfect circular patterns. Astronomers observed patterns of celestial movement, such as the moon rising an hour later each day and the sun's change in path across the sky.
The Ptolemaic system was known for its series of complex circles. Ptolemy believed that each planet orbited around a circle or epicycle. An epicycle is a geometric model used to explain variations in speed and direction in relation to the moon, sun and other planets. The epicycle orbited around a larger circle, known as the deferent. The deferent is a circle, and its center is the midway point between the orbiting planet and Earth.
In 1514, Nicolas Copernicus, a Polish astronomer, refuted the geocentric system with his own theory of the universe, known as the heliocentric model. In Copernicus' model, the Earth and other planets orbited the sun. However, it was not until Johannes Kepler introduced his laws of planetary motion that the orbiting models of the geocentric system were finally debunked. Kepler proved that planets follow an elliptical orbit around the sun instead of a perfect circular orbit.