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| [[File:Tidal locking of the Moon with the Earth.gif|thumb|300px|Tidal locking results in the Moon rotating about its axis in about the same time it takes to orbit Earth. Except for [[libration]] effects, this results in the Moon keeping the same face turned towards Earth, as seen in the figure on the left. (The Moon is shown in polar view, and is not drawn to scale.) If the Moon were not spinning at all, it would alternately show its near and far sides to Earth, while moving around Earth in orbit, as shown in the figure on the right.]]
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| '''Tidal locking''' (or '''captured rotation''') occurs when the [[gravitational gradient]] makes one side of an [[astronomical body]] always face another, an effect known as '''synchronous rotation'''. For example, the same side of the [[Moon]] always faces the [[Earth]]. A tidally locked body takes just as long to rotate around its own axis as it does to revolve around its partner. This causes one hemisphere constantly to face the partner body. Usually, at any given time only the [[satellite]] is tidally locked around the larger body, but if the difference in mass between the two bodies and their physical separation is small, ''each'' may be tidally locked to the other, as is the case between [[Pluto]] and [[Charon (moon)|Charon]]. This effect is employed to [[Gravity-gradient stabilization|stabilize]] some artificial satellites.
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| ==Mechanism==
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| The change in [[Rotation period|rotation rate]] necessary to tidally lock a body B to a larger body A is caused by the [[torque]] applied by A's [[gravity]] on bulges it has induced on B by [[tidal force]]s.
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| ===Tidal bulges===
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| A's gravity produces a tidal force on B that distorts its gravitational [[Mechanical equilibrium|equilibrium]] shape slightly so that it becomes elongated along the axis oriented toward A, and conversely, is slightly reduced in dimension in directions [[perpendicular]] to this axis. These distortions are known as tidal bulges. When B is not yet tidally locked, the bulges travel over its surface, with one of the two "high" tidal bulges traveling close to the point where body A is overhead. For large astronomical bodies that are near-[[Sphericity|spherical]] due to self-gravitation, the tidal distortion produces a slightly [[prolate spheroid]] - i.e., an axially symmetric [[ellipsoid]] that is elongated along its major axis. Smaller bodies also experience distortion, but this distortion is less regular.
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| ===Bulge dragging===
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| The material of B exerts resistance to this periodic reshaping caused by the tidal force. In effect, some time is required to reshape B to the gravitational equilibrium shape, by which time the forming bulges have already been carried some distance away from the A–B axis by B's rotation. Seen from a vantage point in space, the points of maximum bulge extension are displaced from the axis oriented towards A. If B's rotation period is shorter than its orbital period, the bulges are carried forward of the axis oriented towards A in the direction of rotation, whereas if B's rotation period is longer the bulges lag behind instead.
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| ===Resulting torque===
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| [[File:Tidal torque.png|thumbnail|If the tidal bulges of a body are misaligned with the major axis, the tidal forces exert a net torque on that body that twists the body towards the direction of realignment.]]
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| Because the bulges are now displaced from the A–B axis, A's gravitational pull on the mass in them exerts a torque on B. The torque on the A-facing bulge acts to bring B's rotation in line with its orbital period, whereas the "back" bulge, which faces away from A, acts in the opposite sense. However, the bulge on the A-facing side is closer to A than the back bulge by a distance of approximately B's diameter, and so experiences a slightly stronger gravitational force and torque. The net resulting torque from both bulges, then, is always in the direction that acts to synchronize B's rotation with its orbital period, leading eventually to tidal locking.
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| ===Orbital changes===
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| [[File:MoonTorque.jpg|thumb|alt=Tidal Locking|If rotational frequency is larger than orbital frequency, a small torque counteracting the rotation arises, eventually locking the frequencies (situation depicted in green)]]
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| The [[angular momentum]] of the whole A–B system is conserved in this process, so that when B slows down and loses rotational angular momentum, its ''orbital'' angular momentum is boosted by a similar amount (there are also some smaller effects on A's rotation). This results in a raising of B's orbit about A in tandem with its rotational slowdown. For the other case where B starts off rotating too slowly, tidal locking both speeds up its rotation, and ''lowers'' its orbit.
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| ===Locking of the larger body===
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| The tidal locking effect is also experienced by the larger body A, but at a slower rate because B's gravitational effect is weaker due to B's smaller size. For example, Earth's rotation is gradually slowing down because of the Moon, by an amount that becomes noticeable over geological time in some fossils.<ref>{{cite book
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| | first=Imke | last=de Pater
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| | year=2001
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| | title=Planetary Sciences
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| | publisher=Cambridge| isbn=0521482194
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| | page=34}}</ref> For bodies of similar size the effect may be of comparable size for both, and both may become tidally locked to each other. The [[dwarf planet]] [[Pluto]] and its satellite [[Charon (moon)|Charon]] are good examples of this—Charon is only visible from one hemisphere of Pluto and vice versa.
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| ===Rotation–orbit resonance===
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| Finally, in some cases where the orbit is [[eccentricity (orbit)|eccentric]] and the tidal effect is relatively weak, the smaller body may end up in an [[orbital resonance]], rather than tidally locked. Here the ratio of rotation period to orbital period is some well-defined fraction different from 1:1. A well known case is the rotation of [[Mercury (planet)|Mercury]]—locked to its orbit around the Sun in a 3:2 resonance.
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| Many exoplanets (especially the close-in ones) are expected to be in spin–orbit resonances higher than 1:1.
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| For example, the super-Earth Gliese 581 d would most probably be in a spin–orbit resonance of 2:1, rotating twice for each orbit of its host star. <ref name="rotation"/>
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| ==Occurrence==
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| ===Moons===
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| [[File:Synchronous rotation.svg|thumb|Due to tidal locking, the inhabitants of the central body will never be able to see the satellite's green area.]]
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| Most significant moons in the [[Solar System]] are tidally locked with their primaries, because they orbit very closely and tidal force increases rapidly (as a [[Cubic function|cubic]]) with decreasing distance. Notable exceptions are the irregular outer satellites of the [[gas giant]]s, which orbit much farther away than the large well-known moons.
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| [[Pluto]] and [[Charon (moon)|Charon]] are an extreme example of a tidal lock. Charon is a relatively large moon in comparison to its primary and also has a very close [[orbit]]. This has made Pluto also tidally locked to Charon. In effect, these two [[celestial body|celestial bodies]] revolve around each other (their [[Barycentric coordinates (astronomy)|barycenter]] lies outside of Pluto) as if joined with a rod connecting two opposite points on their surfaces.
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| The tidal locking situation for [[asteroid moon]]s is largely unknown, but closely orbiting binaries are expected to be tidally locked, as well as [[Contact binary (asteroid)|contact binaries]].
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| ====The Moon====
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| [[File:FullMoon2010.jpg|thumb|Because the [[Moon]] is 1:1 tidally locked, only [[near side of the Moon|one side]] is visible from [[Earth]].]]
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| The Moon's rotation and orbital periods are tidally locked with each other, so no matter when the Moon is observed from Earth the same hemisphere of the Moon is always seen. The [[Far side (Moon)|far side of the Moon]] was not seen in its entirety until 1959, when photographs were transmitted from the [[Soviet Union|Soviet]] spacecraft [[Luna 3]].
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| Despite the Moon's rotational and orbital periods being exactly locked, about 59% of the Moon's total surface may be seen with repeated observations from Earth due to the phenomena of [[libration]] and [[parallax]]. Librations are primarily caused by the Moon's varying orbital speed due to the [[eccentricity (orbit)|eccentricity]] of its orbit: this allows up to about 6° more along its perimeter to be seen from Earth. Parallax is a geometric effect: at the surface of Earth we are offset from the line through the centers of Earth and Moon, and because of this we can observe a bit (about 1°) more around the side of the Moon when it is on our local horizon.
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| ===Planets===
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| It was thought for some time that [[Mercury (planet)|Mercury]] was tidally locked with the Sun. This was because whenever Mercury was best placed for observation, the same side faced inward. Radar observations in 1965 demonstrated instead that Mercury has a 3:2 spin–orbit resonance, rotating three times for every two revolutions around the Sun, which results in the same positioning at those observation points. The eccentricity of Mercury's orbit makes this 3:2 resonance stable.
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| [[Venus]]'s 583.92-day interval between successive close approaches to Earth is equal to 5.001444 Venusian solar days, making approximately the same face visible from Earth at each close approach. Whether this relationship arose by chance or is the result of some kind of tidal locking with Earth is unknown.<ref>Gold T., Soter S. (1969), ''Atmospheric tides and the resonant rotation of Venus'', Icarus, v. 11, p 356-366</ref>
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| ===Stars===
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| Close [[binary star]]s throughout the universe are expected to be tidally locked with each other, and [[extrasolar planet]]s that have been found to orbit their primaries extremely closely are also thought to be tidally locked to them. An unusual example, confirmed by [[Microvariability and Oscillations of STars telescope|MOST]], is [[Tau Boötis]], a star tidally locked by a planet. The tidal locking is almost certainly mutual.<ref name="space.com">[http://www.space.com/scienceastronomy/050523_star_tide.html SPACE.com - Role Reversal: Planet Controls a Star<!-- Bot generated title -->]</ref>
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| ==Timescale==
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| An estimate of the time for a body to become tidally locked can be obtained using the following formula:<ref name="">{{cite journal | author= B. Gladman et al.| title= ''Synchronous Locking of Tidally Evolving Satellites''| journal= Icarus| year= 1996| volume= 122| pages= 166 | doi = 10.1006/icar.1996.0117| bibcode=1996Icar..122..166G}} (See pages 169-170 of this article. Formula (9) is quoted here, which comes from S.J. Peale, ''Rotation histories of the natural satellites'', in {{cite book | editor= J.A. Burns | title= ''Planetary Satellites''| year= 1977| publisher= University of Arizona Press |pages= 87–112| location= Tucson}})</ref>
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| :::<math>
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| t_{\textrm{lock}} \approx \frac{w a^6 I Q}{3 G m_p^2 k_2 R^5}
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| </math>
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| where
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| * <math>w\,</math> is the initial spin rate ([[radian]]s [[Radian per second|per second]])
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| * <math>a\,</math> is the [[semi-major axis]] of the motion of the satellite around the planet (given by average of [[perigee]] and [[apogee]] distances)
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| * <math>I\,</math> <math>\approx 0.4 m_s R^2</math> is the [[moment of inertia]] of the satellite.
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| * <math>Q\,</math> is the dissipation function of the satellite.
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| * <math>G\,</math> is the [[gravitational constant]]
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| * <math>m_p\,</math> is the mass of the planet
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| * <math>m_s\,</math> is the mass of the satellite
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| * <math>k_2\,</math> is the tidal [[Love number]] of the satellite
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| * <math>R\,</math> is the mean radius of the satellite.
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| <math>Q</math> and <math>k_2</math> are generally very poorly known except for the Moon, which has <math>k_2/Q=0.0011</math>. For a really rough estimate it is common to take <math>Q</math>≈100 (perhaps conservatively, giving overestimated locking times), and
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| :::<math>
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| k_2 \approx \frac{1.5}{1+\frac{19\mu}{2\rho g R}},
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| </math>
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| where
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| * <math>\rho\,</math> is the density of the satellite
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| * <math>g\approx Gm_s/R^2</math> is the surface gravity of the satellite
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| * <math>\mu\,</math> is rigidity of the satellite. This can be roughly taken as 3{{e|10}} Nm<sup>−2</sup> for rocky objects and 4{{e|9}} Nm<sup>−2</sup> for icy ones.
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| As can be seen, even knowing the size and density of the satellite leaves many parameters that must be estimated (especially ''w'', ''Q'', and <math>\mu\,</math>), so that any calculated locking times obtained are expected to be inaccurate, to even factors of ten. Further, during the tidal locking phase the orbital radius ''a'' may have been significantly different from that observed nowadays due to subsequent [[tidal acceleration]], and the locking time is extremely sensitive to this value.
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| Because the uncertainty is so high, the above formulas can be simplified to give a somewhat less cumbersome one. By assuming that the satellite is spherical, <math>k_2\ll1\,</math>, ''Q'' = 100, and it is sensible to guess one revolution every 12 hours in the initial non-locked state (most asteroids have rotational periods between about 2 hours and about 2 days)
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| :::<math>
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| t_{\textrm{lock}}\quad \approx\quad 6\ \frac{a^6R\mu}{m_sm_p^2}\quad \times 10^{10}\ \textrm{ years},
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| </math>{{citation needed|date=October 2012}}
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| with masses in kg, distances in meters, and μ in Nm<sup>−2</sup>. μ can be roughly taken as 3{{e|10}} Nm<sup>−2</sup> for rocky objects and 4{{e|9}} Nm<sup>−2</sup> for icy ones. | |
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| Note the extremely strong dependence on orbital radius ''a''.
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| For the locking of a primary body to its satellite as in the case of Pluto, the satellite and primary body parameters can be interchanged.
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| One conclusion is that ''other things being equal'' (such as Q and μ), a large moon will lock faster than a smaller moon at the same orbital radius from the planet because <math>m_s\,</math> grows as the cube of the satellite radius,<math>R</math>.{{contradict-inline|contradicts the non-simplified formula, see talk|date=October 2012}} A possible example of this is in the Saturn system, where [[Hyperion (moon)|Hyperion]] is not tidally locked, whereas the larger [[Iapetus (moon)|Iapetus]], which orbits at a greater distance, is. However, this is not clear cut because Hyperion also experiences strong driving from the nearby [[Titan (moon)|Titan]], which forces its rotation to be chaotic.
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| The above formulae for the timescale of locking may be off by orders of magnitude, because they ignore the frequency dependence of <math>k_2/Q</math>.
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| ==List of known tidally locked bodies==
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| ===Solar System===
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| '''Locked to the [[Earth]]'''
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| * [[Moon]]
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| '''Locked to [[Mars]]'''
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| * [[Phobos (moon)|Phobos]]
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| * [[Deimos (moon)|Deimos]]
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| '''Locked to [[Jupiter]]'''
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| <div style="-moz-column-count:4; column-count:4;">
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| * [[Metis (moon)|Metis]]
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| * [[Adrastea (moon)|Adrastea]]
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| * [[Amalthea (moon)|Amalthea]]
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| * [[Thebe (moon)|Thebe]]
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| * [[Io (moon)|Io]]
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| * [[Europa (moon)|Europa]]
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| * [[Ganymede (moon)|Ganymede]]
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| * [[Callisto (moon)|Callisto]]
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| </div>
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| '''Locked to [[Saturn]]'''
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| <div style="-moz-column-count:4; column-count:4;">
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| * [[Pan (moon)|Pan]]
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| * [[Atlas (moon)|Atlas]]
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| * [[Prometheus (moon)|Prometheus]]
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| * [[Pandora (moon)|Pandora]]
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| * [[Epimetheus (moon)|Epimetheus]]
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| * [[Janus (moon)|Janus]]
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| * [[Mimas (moon)|Mimas]]
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| * [[Enceladus (moon)|Enceladus]]
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| * [[Telesto (moon)|Telesto]]
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| * [[Tethys (moon)|Tethys]]
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| * [[Calypso (moon)|Calypso]]
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| * [[Dione (moon)|Dione]]
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| * [[Rhea (moon)|Rhea]]
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| * [[Titan (moon)|Titan]]
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| * [[Iapetus (moon)|Iapetus]]
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| </div>
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| '''Locked to [[Uranus]]'''
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| * [[Miranda (moon)|Miranda]]
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| * [[Ariel (moon)|Ariel]]
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| * [[Umbriel (moon)|Umbriel]]
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| * [[Titania (moon)|Titania]]
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| * [[Oberon (moon)|Oberon]]
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| '''Locked to [[Neptune]]'''
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| * [[Proteus (moon)|Proteus]]
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| * [[Triton (moon)|Triton]]
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| '''Locked to [[Pluto]]'''
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| * [[Charon (moon)|Charon]] (Pluto is itself locked to Charon)
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| ===Extra-solar===
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| * [[Tau Boötis]] is known to be locked to the close-orbiting [[giant planet]] [[Tau Boötis b]].<ref name="space.com"/>
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| ==Bodies likely to be locked==
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| ===Solar System===
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| Based on comparison between the likely time needed to lock a body to its primary, and the time it has been in its present orbit (comparable with the age of the Solar System for most planetary moons), a number of moons are thought to be locked. However their rotations are not known or not known enough. These are:
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| '''Probably locked to [[Saturn]]'''
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| <div style="-moz-column-count:4; column-count:4;"> | |
| * [[Daphnis (moon)|Daphnis]]
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| * [[Methone (moon)|Methone]]
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| * [[Pallene (moon)|Pallene]]
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| * [[Helene (moon)|Helene]]
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| * [[Polydeuces (moon)|Polydeuces]]
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| </div> | |
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| '''Probably locked to [[Uranus]]'''
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| <div style="-moz-column-count:4; column-count:4;">
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| * [[Cordelia (moon)|Cordelia]]
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| * [[Ophelia (moon)|Ophelia]]
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| * [[Bianca (moon)|Bianca]]
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| * [[Cressida (moon)|Cressida]]
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| * [[Desdemona (moon)|Desdemona]]
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| * [[Juliet (moon)|Juliet]]
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| * [[Portia (moon)|Portia]]
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| * [[Rosalind (moon)|Rosalind]]
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| * [[Cupid (moon)|Cupid]]
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| * [[Belinda (moon)|Belinda]]
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| * [[Perdita (moon)|Perdita]]
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| * [[Puck (moon)|Puck]]
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| * [[Mab (moon)|Mab]]
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| * [[Oberon (moon)|Oberon]]
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| </div>
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| '''Probably locked to [[Neptune]]'''
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| <div style="-moz-column-count:4; column-count:2;">
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| * [[Naiad (moon)|Naiad]]
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| * [[Thalassa (moon)|Thalassa]]
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| * [[Despina (moon)|Despina]]
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| * [[Galatea (moon)|Galatea]]
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| * [[Larissa (moon)|Larissa]]
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| </div>
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| ===Extrasolar===
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| * [[Alpha Centauri Bb]] may be tidally locked to its parent star [[Alpha Centauri]].
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| * [[Gliese 581 c]],<ref>{{cite news| url=http://www.usatoday.com/printedition/news/20070425/1a_bottomstrip25_dom.art.htm | work=USA Today | title=Out of our world: Earthlike planet | first=Dan | last=Vergano | date=2007-04-25 | accessdate=2010-05-25}}</ref> [[Gliese 581 g]],<ref>{{Cite web|url=http://news.sciencemag.org/sciencenow/2010/09/astronomers-find-most-earth-like.html|title=Astronomers Find Most Earth-like Planet to Date|publisher=[[Science (journal)|Science]], USA|date=September 29, 2010|accessdate=September 30, 2010}}</ref><ref>{{Cite web|url=http://www.telegraph.co.uk/science/space/8033124/Gliese-581g-the-most-Earth-like-planet-yet-discovered.html|title=Gliese 581g the most Earth like planet yet discovered|publisher=[[The Daily Telegraph]], UK|date=September 30, 2010|accessdate=September 30, 2010}}</ref> [[Gliese 581 b]],{{citation needed|date=August 2010}} and [[Gliese 581 e]]{{citation needed|date=August 2010}} may be tidally locked to their parent star [[Gliese 581]]. [[Gliese 581 d]] is almost certainly locked either into the 2:1 or the 3:2 spin–orbit resonance with the same star.<ref>{{Cite web|url=http://adsabs.harvard.edu/abs/2012ApJ...761...83M |author=Makarov, V. V.; Berghea, C.; and Efroimsky, M. 2012. |title=Dynamical Evolution and Spin–Orbit Resonances of Potentially Habitable Exoplanets: The Case of GJ 581d." The Astrophysical Journal, Volume 761, Issue 2, article id. 83, 14 pp. (2012). }}</ref>
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| ==See also==
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| * [[Tidal acceleration]]
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| * [[Gravity-gradient stabilization]]
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| * [[Orbital resonance]]
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| ==References==
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| {{reflist|30em|refs=
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| <ref name="rotation">{{cite journal |last1=Makarov |first1=Valeri V. |last2=Berghea |first2=Ciprian |last3=Efroimsky |first3=Michael |title=Dynamical evolution and spin-orbit resonances of potentially habitable exoplanets. The case of GJ 581d |doi=10.1088/0004-637X/761/2/83 |bibcode=2012arXiv1208.0814M |year=2012 |journal=[[The Astrophysical Journal]] |volume=761 |arxiv=1208.0814 |display-authors=1 }}</ref>
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| }}
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| {{DEFAULTSORT:Tidal Locking}}
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| [[Category:Celestial mechanics]]
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| [[Category:Orbits]]
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