Geocentric model

The geocentric model of the cosmos is a paradigm which places the Earth at the center of the universe. Common in ancient Greece, it was believed by both Aristotle and Ptolemy. Most Greeks assumed that the Sun, Moon, stars, and planets orbit Earth. Similar ideas were held in ancient China.

The geocentric model was gradually replaced by the heliocentric model of Copernicus and Galileo due to the simplicity and predictive accuracy of that newer model.

In this model, a set of fifty-five concentric crystalline spheres were considered to hold the Sun, the planets, and the stars. These spheres (called deferents) revolved at varying velocities around the Earth to account for the rising and setting of celestial objects every day.

However, this simple model of the revolutions of spheres could not explain all astronomical phenomena. In particular, planets were observed to wander across the fixed fields of stars over time; mostly they wandered in one direction, but occasionally they seemed to reverse course. To explain this strange retrogradation, Aristotle claimed that planets were attached, not directly to deferents, but to smaller spheres called epicycles. The epicycles were themselves attached to the deferents; the simultaneous revolution of both sets of spheres created an occasional apparent reversal of the planets' motions across the skies of the Earth.

Ptolemy further modified this model to more accurately reflect observations by placing epicycles upon epicycles, creating an extraordinarily complicated--but fairly accurate--depiction of the cosmos. He also displaced the Earth from the center of the universe, claiming that, while Earth was enclosed by the celestial spheres, the spheres actually revolved around a point called an eccentric, which was near the Earth but not quite on it.

This elaborate theoretical system stemmed largely from two deeply held Greek beliefs: that the Earth was the center of the universe, and that all heavenly objects move in a uniform circular motion.

This view of a geocentric universe held sway for well over a millennium, until the publication of Copernicus's De Revolutionibus Orbium Coelestium in 1543.

See also: celestial sphere, heliocentric model