Definition
The geocentric model is the pre-Copernican cosmology that placed a stationary Earth at the center of the universe, with the Sun, Moon, planets, and stars carried around it by a system of concentric celestial spheres.
It was the standard scientific model in the Greek, Roman, Islamic, and medieval European traditions for nearly two thousand years, most fully developed by Claudius Ptolemy in the second century in his Almagest.
Why it matters
How it works
Aristotle's fourth-century-BCE version had Earth at the center surrounded by nested transparent spheres carrying the Moon, Mercury, Venus, the Sun, Mars, Jupiter, Saturn, and finally the fixed stars. The spheres rotated uniformly with their own characteristic periods. Below the Moon was the realm of corruptible matter (Earth, water, air, fire); above, the unchanging quintessence.
Ptolemy's Almagest refined this into a quantitative predictive tool. To account for the retrograde motion of the outer planets — the periods when Mars or Jupiter visibly reverse direction against the stars — he introduced epicycles (small circles whose centers ride on larger circles, the deferents) and the equant (an off-center point about which the deferent's motion is uniform). With enough epicycles the model could match observation to about 1°, accurate enough to predict planetary positions for centuries.
The model was wrong in its core claim, but not naïve. It was overturned by a combination of forces: Copernicus's 1543 De revolutionibus proposed the heliocentric alternative on grounds of mathematical elegance; Tycho Brahe's late-16th-century observations gave the precision required to distinguish the models; Kepler's elliptical orbits (1609, 1619) replaced epicycles with three clean laws; Galileo's 1610 telescopic observations of Jupiter's moons and Venus's phases provided decisive physical evidence; and Newton's 1687 Principia supplied the underlying dynamical theory.