In geodesy and geophysics, theoretical gravity or normal gravity is an approximation of Earth's gravity, on or near its surface, by means of a mathematical model. The most common theoretical model is a rotating Earth ellipsoid of revolution (i.e., a spheroid).

Other representations of gravity can be used in the study and analysis of other bodies, such as asteroids. Widely used representations of a gravity field in the context of geodesy include spherical harmonics, mascon models, and polyhedral gravity representations.

The type of gravity model used for the Earth depends upon the degree of fidelity required for a given problem. For many problems such as aircraft simulation, it may be sufficient to consider gravity to be a constant, defined as:

based upon data from World Geodetic System 1984 (WGS-84), where ${\displaystyle g}$ is understood to be pointing 'down' in the local frame of reference.

If it is desirable to model an object's weight on Earth as a function of latitude, one could use the following:

where

Neither of these accounts for changes in gravity with changes in altitude, but the model with the cosine function does take into account the centrifugal relief that is produced by the rotation of the Earth. On the rotating sphere, the sum of the force of the gravitational field and the centrifugal force yields an angular deviation of approximately

(in radians) between the direction of the gravitational field and the direction measured by a plumb line; the plumb line appears to point southwards on the northern hemisphere and northwards on the southern hemisphere. ${\displaystyle \Omega \approx 7.29\times 10^{-5}}$ rad/s is the diurnal angular speed of the Earth axis, and ${\displaystyle R\approx 6370}$ km the radius of the reference sphere, and ${\displaystyle R\sin \varphi }$ the distance of the point on the Earth crust to the Earth axis.