Geocenter

Geocenter is the origin of a geocentric reference frame used in geodesy and astronomy. It is typically defined as the instantaneous center of mass of the Earth as a whole, including the solid Earth, oceans, atmosphere, and cryosphere. In some contexts, the term may refer specifically to the center of mass of the solid Earth itself. The distinction matters because the mass distribution among ocean, atmosphere, and land can shift the geocenter relative to the planet’s crust and mantle.

The geocenter serves as the origin of the International Terrestrial Reference Frame (ITRF) and the Geocentric Celestial Reference System (GCRS). Its position is not fixed; it moves as mass is redistributed by tides, atmospheric pressure changes, ocean currents, hydrological cycles, and large-scale geophysical processes. Observations from satellite laser ranging, very long baseline interferometry, GNSS, and Doppler/DORIS networks are used to estimate geocenter offsets and its drift.

Determining the geocenter is important for precise orbit determination, satellite geodesy, and Earth science research because even small shifts can affect coordinate systems used for navigation, mapping, and Earth observation. The concept also underpins models of the Earth's gravity field and load Love numbers that describe how mass redistribution deforms the planet.

In sum, the geocenter is the reference point that anchors geocentric coordinates to the Earth’s mass distribution, and it is defined and refined by international geodetic standards.