Equatorial bulge

If Earth were scaled down to a globe with an equatorial diameter of 1 metre (3.3 ft), that difference would be only 3 mm (0.12 in).

While too small to notice visually, that difference is still more than twice the largest deviations of the actual surface from the ellipsoid, including the tallest mountains and deepest oceanic trenches.

Earth's rotation also affects the sea level, the imaginary surface used as a reference frame from which to measure altitudes.

But since the ocean also bulges, like Earth and its atmosphere, Chimborazo is not as high above sea level as Everest is.

A way for one to get a feel for the type of equilibrium involved is to imagine someone seated in a spinning swivel chair and holding a weight in each hand; if the individual pulls the weights inward towards them, work is being done and their rotational kinetic energy increases.

Matter first coalesces into a slowly rotating disk-shaped distribution, and collisions and friction convert kinetic energy to heat, which allows the disk to self-gravitate into a very oblate spheroid.

When the equilibrium state has been reached then large scale conversion of kinetic energy to heat ceases.

[1] Estimates of how fast the Earth was rotating in the past vary, because it is not known exactly how the moon was formed.

Estimates of the Earth's rotation 500 million years ago are around 20 modern hours per "day".

Measurements of the acceleration due to gravity at the equator must also take into account the planet's rotation.

The acceleration that is required to circumnavigate the Earth's axis along the equator at one revolution per sidereal day is 0.0339 m/s2.

In summary, there are two contributions to the fact that the effective gravitational acceleration is less strong at the equator than at the poles.

If the reference z axis of the coordinate system adopted is aligned along the Earth's symmetry axis, then only the longitude of the ascending node Ω, the argument of pericenter ω and the mean anomaly M undergo secular precessions.

[5] Such perturbations, which were earlier used to map the Earth's gravitational field from space,[6] may play a relevant disturbing role when satellites are used to make tests of general relativity[7] because the much smaller relativistic effects are qualitatively indistinguishable from the oblateness-driven disturbances.

for the equilibrium configuration of a self-gravitating spheroid, composed of uniform density incompressible fluid, rotating steadily about some fixed axis, for a small amount of flattening, is approximated by:[8]