Editing Earth radius (section)

===Physics of Earth's deformation===
{{further|Equatorial bulge}}

Rotation of a planet causes it to approximate an ''[[spheroid|oblate ellipsoid]]/spheroid'' with a bulge at the [[equator]] and flattening at the [[North Pole|North]] and [[South Pole]]s, so that the ''equatorial radius'' {{mvar|a}} is larger than the ''polar radius'' {{mvar|b}} by approximately {{mvar|aq}}. The ''oblateness constant'' {{mvar|q}} is given by
:<math>q=\frac{a^3 \omega^2}{GM},</math>
where {{mvar|ω}} is the [[angular frequency]], {{mvar|G}} is the [[gravitational constant]], and {{mvar|M}} is the mass of the planet.{{refn|This follows from the [[International Astronomical Union]] [[2006 definition of planet|definition]] rule (2): a planet assumes a shape due to [[hydrostatic equilibrium]] where [[gravity]] and [[centrifugal force]]s are nearly balanced.<ref>[http://www.iau2006.org/mirror/www.iau.org/iau0603/index.html IAU 2006 General Assembly: Result of the IAU Resolution votes] {{webarchive|url=https://web.archive.org/web/20061107022302/http://www.iau2006.org/mirror/www.iau.org/iau0603/index.html |date=2006-11-07 }}</ref>|group=lower-alpha}} For the Earth {{math|{{sfrac|1|''q''}} ≈ 289}}, which is close to the measured inverse [[flattening]] {{math|{{sfrac|1|''f''}} ≈ 298.257}}. Additionally, the bulge at the equator shows slow variations. The bulge had been decreasing, but since 1998 the bulge has increased, possibly due to redistribution of ocean mass via currents.<ref>[https://web.archive.org/web/20020810195620/http://www.gsfc.nasa.gov/topstory/20020801gravityfield.html Satellites Reveal A Mystery Of Large Change In Earth's Gravity Field ], Aug. 1, 2002, [[Goddard Space Flight Center]]. </ref>
[[Image:Lowresgeoidheight.jpg|400px|right]]
The variation in [[density]] and [[Crust (geology)|crustal]] thickness causes gravity to vary across the surface and in time, so that the mean sea level differs from the ellipsoid. This difference is the ''[[geoid]] height'', positive above or outside the ellipsoid, negative below or inside. The geoid height variation is under {{convert|110|m|ft|abbr=on}} on Earth. The geoid height can change abruptly due to earthquakes (such as the [[2004 Indian Ocean earthquake|Sumatra-Andaman earthquake]]) or reduction in ice masses (such as [[Greenland]]).<ref>[https://archive.today/20120529015755/http://www.spaceref.com/news/viewpr.html?pid=18567 NASA's Grace Finds Greenland Melting Faster, 'Sees' Sumatra Quake], December 20, 2005, [[Goddard Space Flight Center]].</ref>

Not all deformations originate within the Earth. Gravitational attraction from the Moon or Sun can cause the Earth's surface at a given point to vary by tenths of a meter over a nearly 12-hour period (see [[Earth tide]]).