The geoid, however, is a very special kind of oblate spheroid. It isdefined as follows:
The geoid is the shape that the surface of theoceans would take under the influence of Earth'sgravitation androtation alone, in the absence of other influences such as winds andtides.
It was defined byGauss, in 1828.[1] It is often described as the true physical shape of the Earth.[2] The study of Earth's measurements and shapes is calledgeodesy.
For many practical purposes, a simpler shape is used because that makes calculations easier. That shape is called areference ellipsoid. In basic education, the shape of the Earth is described as being like an orange, a sphere but wider round the equator. This shape is produced by allcelestial spinning spheres, such asstars andplanets. The details differ somewhat in each individual case.
