Editing Equatorial bulge (section)

== The equilibrium as a balance of energies ==
{{Further|Hydrostatic equilibrium#Planetary geology}}

[[Image:Equatorial bulge model.png|frame|right|Fixed to the vertical rod is a spring metal band. When stationary the spring metal band is circular in shape. The top of the metal band can slide along the vertical rod.

When spun, the spring-metal band bulges at its equator and flattens at its poles in analogy with the Earth.]]

[[Gravity]] tends to contract a celestial body into a [[sphere]], the shape for which all the mass is as close to the center of gravity as possible. [[Rotation]] causes a distortion from this spherical shape; a common measure of the distortion is the [[flattening]] (sometimes called ellipticity or oblateness), which can depend on a variety of factors including the size, [[angular velocity]], [[density]], and [[Elasticity (physics)|elasticity]].

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 (physics)|work]] is being done and their rotational kinetic energy increases. The increase in rotation rate is so strong that at the faster rotation rate the required [[centripetal force]] is larger than with the starting rotation rate.

Something analogous to this occurs in planet formation. 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.

As long as the proto-planet is still too oblate to be in equilibrium, the release of [[gravitational energy|gravitational potential energy]] on contraction keeps driving the increase in rotational kinetic energy. As the contraction proceeds, the rotation rate keeps going up, hence the required force for further contraction keeps going up. There is a point where the increase of rotational kinetic energy on further contraction would be larger than the release of gravitational potential energy. The contraction process can only proceed up to that point, so it halts there.

As long as there is no equilibrium there can be violent convection, and as long as there is violent convection friction can convert kinetic energy to heat, draining rotational kinetic energy from the system. When the equilibrium state has been reached then large scale conversion of kinetic energy to heat ceases. In that sense the equilibrium state is the lowest state of energy that can be reached.

The Earth's rotation rate is still slowing down, though gradually, by about two thousandths of a second per rotation every 100 years.<ref>{{cite magazine|last=Hadhazy|first=Adam|title=Fact or Fiction: The Days (and Nights) Are Getting Longer|url=http://www.scientificamerican.com/article.cfm?id=earth-rotation-summer-solstice|magazine=[[Scientific American]]|access-date=5 December 2011}}</ref> 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".

The Earth's rate of rotation is slowing down mainly because of tidal interactions with the Moon and the Sun. Since the solid parts of the Earth are [[ductile]], the Earth's equatorial bulge has been decreasing in step with the decrease in the rate of rotation.