Editing Rotation (section)

=== Spin ===
[[Star]]s, [[planet]]s and similar bodies may spin around on their axes. The rotation rate of planets in the [[Solar System]] was first measured by tracking visual features. [[Stellar rotation]] is measured through [[Doppler shift]] or by tracking active surface features. An example is [[sunspots]], which rotate around the Sun at the same velocity as the [[photosphere|outer gases]] that make up the Sun.

Under some circumstances orbiting bodies may lock their spin rotation to their orbital rotation around a larger body. This effect is called [[tidal locking]]; the Moon is tidal-locked to the Earth.

This rotation induces a [[Centrifugal force (fictitious)|centrifugal acceleration]] in the reference frame of the Earth which slightly counteracts the effect of [[gravitation]] the closer one is to the [[equator]]. [[Earth's gravity]] combines both mass effects such that an object weighs slightly less at the equator than at the poles. Another is that over time the Earth is slightly deformed into an [[oblate spheroid]]; a similar [[equatorial bulge]] develops for other planets.

Another consequence of the rotation of a planet are the phenomena of [[precession]] and [[nutation]]. Like a [[gyroscope]], the overall effect is a slight "wobble" in the movement of the axis of a planet. Currently the tilt of the [[Earth]]'s axis to its orbital plane ([[obliquity of the ecliptic]]) is 23.44 degrees, but this angle changes slowly (over thousands of years). (See also [[Precession of the equinoxes]] and [[Pole star|Pole Star]].)