*18.09.2011*
*Syn: Normal Earth, Reference Earth*

Mathematical earth models are described by stereometric bodies.

The main mathematical earth models are spheroids:

C) *Normal earth*, or *reference earth*, or *earth
model*. Mathematically the simplest model is an
*ellipsoid *of revolution, which therefor is practically almost exclusively used. Physically the
best
reference for describing the small, more or less elastic, temporal variations (free and forced
oscillations such as earth tides), is a hydrostatic *equilibrium figure*. Figures of hydrostatic
equilibrium for the earth are very close to ellipsoids, but do not exactly coincide with an ellipsoid...