Physical geodesy

Physical geodesy determines Earth’s shape and gravity field from measurements interpreted through physical laws, especially Newtonian gravitation and potential theory.

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Geopotential: The gravitational potential energy per unit mass in a gravitational field, often combined with centrifugal potential for Earth. Its level surfaces provide the mathematical framework for relating gravity to Earth’s shape.
Geodesy: The science of measuring Earth’s shape, orientation, gravity field, and changes over time. Physical geodesy is the gravity-based branch of this broader measurement science.
Geoid height: The separation between the geoid and a reference ellipsoid, measured along the ellipsoid’s normal. It converts the gravity-derived geoid into a correction used with ellipsoidal heights.
Isaac Newton: An English mathematician and physicist who formulated the laws of motion and universal gravitation. His rotating-Earth theory predicted polar flattening and made Earth’s figure a physical problem.
Geoid: An equipotential surface of Earth’s gravity field that approximately coincides with mean sea level. It is the physical reference surface whose shape physical geodesy seeks to determine.
Reference ellipsoid: A mathematically defined oblate ellipsoid used to approximate Earth and support geodetic coordinates. It supplies the geometric baseline against which geoid height and gravity departures are expressed.
Orthometric height: The distance from the geoid to a point measured along the curved plumb line. It expresses elevation relative to the gravity-defined surface rather than an ellipsoid.
Alexis Clairaut: A French mathematician whose work connected Earth’s rotation, flattening, and surface gravity. His theorem related gravity at different latitudes to the shape of a rotating Earth.
Gravity anomaly: The difference between observed gravity and a specified reference value after defined corrections. Anomalies expose departures in Earth’s mass distribution from the chosen reference model.
Earth's gravity field: The spatial distribution of gravitational acceleration around and within Earth, modified by its rotation. Its variations are the central physical observations and unknowns of the discipline.