Editing Rotation (section)

== {{anchor|astronomy}}Astronomy ==
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[[File:Star trails over the Paranal Residencia, Chile.jpg|thumb|[[Star trail]]s caused by the [[Earth's rotation]] during the [[Long-exposure photography|camera's long exposure]] time<ref>{{cite news|title=An Oasis, or a Secret Lair?|url=http://www.eso.org/public/images/potw1340a/|access-date=8 October 2013|newspaper=ESO Picture of the Week|url-status=live|archive-url=https://web.archive.org/web/20131011042106/http://www.eso.org/public/images/potw1340a/|archive-date=11 October 2013}}</ref> ]]
{{Further|Rotation period|Earth's rotation}}

In [[astronomy]], rotation is a commonly observed phenomenon; it includes both spin (auto-rotation) and orbital revolution.

=== 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]].)

=== Revolution ===
{{Main|Orbital revolution}}

While ''revolution'' is often used as a synonym for ''rotation'', in many fields, particularly astronomy and related fields, ''revolution'', often referred to as ''orbital revolution'' for clarity, is used when one body moves around another while ''rotation'' is used to mean the movement around an axis. Moons revolve around their planets, planets revolve about their stars (such as the Earth around the Sun); and stars slowly revolve about their [[galaxial center]]s. The motion of the components of [[galaxy|galaxies]] is complex, but it usually includes a rotation component.

=== Retrograde rotation ===
{{Main|Retrograde motion}}

Most [[planet]]s in the [[Solar System]], including [[Earth]], spin in the same direction as they orbit the [[Sun]]. The exceptions are [[Venus]] and [[Uranus]]. Venus may be thought of as rotating slowly backward (or being "upside down"). Uranus rotates nearly on its side relative to its orbit. Current speculation is that Uranus started off with a typical prograde orientation and was knocked on its side by a large impact early in its history. The [[dwarf planet]] [[Pluto]] (formerly considered a planet) is anomalous in several ways, including that it also rotates on its side.