geocentric

Geocentric describes a viewpoint, coordinate system, or model that uses the Earth as its center. In astronomy and geodesy, a geocentric reference frame places the origin at the Earth's center of mass, with positions of objects measured relative to that point. The term is often used to distinguish Earth-centered descriptions from heliocentric or other perspectives.

Historically, geocentric models assumed the Earth was at rest at the center of the universe. The most influential form was the Ptolemaic system, which used deferents and epicycles to explain planetary motions. The geocentric view was challenged by Copernicus and later by observational data, leading to the heliocentric model and the understanding that the Sun is at the center of the solar system. Nevertheless, geocentric descriptions persist in practical calculations and in specific coordinate systems.

Today, geocentric coordinates are used in satellite tracking, navigation, and Earth observation. Geocentric equatorial coordinates, geocentric latitude and longitude describe positions with the Earth’s center as origin, while geocentric celestial coordinates describe directions to celestial objects from Earth’s center. In spaceflight, Earth-centered inertial (ECI) and Earth-centered, Earth-fixed (ECEF) frames are common; the former is non-rotating with respect to the distant stars, the latter rotates with the Earth. The term also appears in discussions of geocentric versus geodetic references, where geocentric coordinates reference the Earth’s center rather than its surface.

See also: geocentric model, geodesy, celestial coordinates, orbital mechanics.