Editing Equatorial bulge (section)

==On Earth==
{{main|Earth radii}}
{{further|Earth ellipsoid|Figure of the Earth}}

The planet [[Earth]] has a rather slight equatorial bulge; its equatorial diameter is about {{cvt|43|km|mi}} greater than its polar diameter, with a difference of about {{fract|1|298}} of the equatorial diameter. If Earth were scaled down to a globe with an equatorial diameter of {{convert|1|m|ft}}, that difference would be only {{cvt|3|mm|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 [[list of mountain peaks by prominence|tallest mountains]] and deepest [[oceanic trench]]es.

[[Earth's rotation]] also affects the [[sea level]], the imaginary surface used as a [[reference frame]] from which to measure [[altitude]]s. This surface coincides with the mean water surface level in oceans, and is extrapolated over land by taking into account the local [[gravity of Earth|gravitational potential]] and the centrifugal force.

The difference of the [[radius|radii]] is thus about {{cvt|21|km|mi}}. An observer standing at sea level on either [[geographical pole|pole]], therefore, is {{cvt|21|km|mi}} closer to Earth's center than if standing at sea level on the Equator. As a result, the highest point on Earth, measured from the center and outwards, is the peak of Mount [[Chimborazo]] in [[Ecuador]] rather than [[Mount Everest]]. But since the ocean also bulges, like Earth and [[atmosphere of Earth|its atmosphere]], Chimborazo is not as high above sea level as Everest is. Similarly the lowest point on Earth, measured from the center and outwards, is the [[Litke Deep]] in the [[Arctic Ocean]] rather than [[Challenger Deep]] in the [[Pacific Ocean]]. But since the ocean also flattens, like Earth and its atmosphere, Litke Deep is not as low below sea level as Challenger Deep is.

More precisely, Earth's surface is usually approximated by an ideal [[oblate ellipsoid]], for the purposes of defining precisely the [[latitude]] and [[longitude]] grid for [[cartography]], as well as the "center of the Earth". In the [[World Geodetic System|WGS-84]] standard [[Earth ellipsoid]], widely used for map-making and the [[GPS]] system, [[Earth radius|Earth's radius]] is assumed to be {{cvt|6378.137|km|mi|comma=gaps}} to the Equator and {{cvt|6356.7523142|km|mi|comma=gaps}} to either pole, meaning a difference of {{cvt|21.3846858|km|mi|comma=gaps}} between the radii or {{cvt|42.7693716|km|mi|comma=gaps}} between the diameters, and a relative [[flattening]] of 1/298.257223563. The [[ocean surface]] is much closer to this standard ellipsoid than the [[Earth's crust|solid surface]] of Earth is.