Editing Equatorial bulge (section)

== Effect on gravitational acceleration ==
{{main|Theoretical gravity}}
[[File:Forces oblate spheroid2.gif|frame|right|The forces at play in the case of a planet with an equatorial bulge due to rotation.<br/>
Red arrow: gravity<br/>
Green arrow: the [[normal force]]<br/>
Blue arrow: the resultant force<br/>
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The resultant force provides required centripetal force. Without this centripetal force frictionless objects would slide towards the equator.<br/>
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In calculations, when a coordinate system is used that is co-rotating with the Earth, the vector of the notional [[centrifugal force]] points outward, and is just as large as the vector representing the centripetal force.]]

Because of a planet's rotation around its own axis, the [[gravitational acceleration]] is less at the [[equator]] than at the [[geographical pole|poles]]. In the 17th century, following the invention of the [[pendulum clock]], French scientists found that clocks sent to [[French Guiana]], on the northern coast of [[South America]], ran slower than their exact counterparts in Paris. Measurements of the acceleration due to gravity at the equator must also take into account the planet's rotation. Any object that is stationary with respect to the surface of the Earth is actually following a circular trajectory, circumnavigating the Earth's axis. Pulling an object into such a circular trajectory requires a force. The acceleration that is required to circumnavigate the Earth's axis along the equator at one revolution per [[sidereal day]] is 0.0339&nbsp;m/s<sup>2</sup>. Providing this acceleration decreases the effective gravitational acceleration. At the Equator, the effective gravitational acceleration is 9.7805&nbsp;m/s<sup>2</sup>. This means that the true gravitational acceleration at the Equator must be 9.8144&nbsp;m/s<sup>2</sup> (9.7805&nbsp;+&nbsp;0.0339&nbsp;=&nbsp;9.8144).

At the poles, the gravitational acceleration is 9.8322&nbsp;m/s<sup>2</sup>. The difference of 0.0178&nbsp;m/s<sup>2</sup> between the gravitational acceleration at the poles and the true gravitational acceleration at the Equator is because objects located on the Equator are about {{cvt|21|km|mi}} further away from the [[center of mass]] of the Earth than at the poles, which corresponds to a smaller gravitational acceleration.

In summary, there are two contributions to the fact that the effective gravitational acceleration is less strong at the equator than at the poles. About 70% of the difference is contributed by the fact that objects circumnavigate the Earth's axis, and about 30% is due to the non-spherical shape of the Earth.

The diagram illustrates that on all latitudes the effective gravitational acceleration is decreased by the requirement of providing a centripetal force; the decreasing effect is strongest on the Equator.