geocentrism

Geocentrism is the view that Earth sits at the center of the universe, with the Sun, Moon, planets, and stars orbiting it. In many geocentric systems, Earth is assumed to be fixed and unmoving.

Historically, geocentric cosmologies were standard in ancient Greek and medieval astronomy. Early Greek thinkers such as Eudoxus and Aristotle proposed a system of concentric, crystalline spheres rotating around a stationary Earth, with celestial bodies attached to outer spheres. The Ptolemaic model, formalized in the 2nd century CE by Claudius Ptolemy in the Almagest, added deferents and epicycles to explain retrograde motion and varying speeds. To reconcile observation with uniform orbital motion, Ptolemy also used the equant: a point offset from Earth from which planets appear to move at a constant angular speed, introducing a further departure from a strictly geocentric, uniform rotation.

The geocentric view began to lose ground after the Copernican Revolution. In 1543 Copernicus proposed heliocentrism, placing the Sun near the center and reassigning the planet order. Subsequent observations—the phases of Venus, Jovian moons, stellar parallax, and Kepler’s laws—favored heliocentrism and a solar-centered geometry, leading to the decline of geocentrism in scientific practice.

Today geocentrism is regarded as a historical model and is discussed mainly for educational or religious/philosophical reasons. In modern astronomy, Earth is not treated as the universe’s center, though “geocentric” can describe coordinate systems with Earth as the reference frame for observational calculations.