Tidal locking

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A separate article treats the phenomenon of tidal resonance in oceanography.

See the article tidal acceleration for a more quantitative description of the Earth-Moon system.

Tidal locking makes one side of an astronomical body always face another, like the Moon facing the Earth. A tidally locked body takes just as long to rotate around its own axis as it does to revolve around its partner. In the case of the Moon, this period is just over 4 weeks, and no matter where you are on the Earth you always see the same face of the Moon. (In fact, we may see about 59% of the moon's total surface with repeated observations, due to apparent oscillations in its movement called librations, but for general purposes the Moon can be considered entirely tidally locked to the Earth.) The entirety of the far side of the Moon was not seen until 1959, when photographs were transmitted from the Soviet spacecraft Luna 3.

Tidal locking occurs in astronomical bodies that orbit each other closely. It results in the orbiting bodies synchronizing their rotation so that one side always faces its partner (or, alternately, places them in orbital resonance). Tidal locking can potentially occur in any orbiting object, not just planetary ones. Typically the smaller body becomes locked first (e.g. the Earth's moon).

Gravitational attraction between two bodies produces a tidal force on each of them, stretching each body along the axis oriented towards its partner and compressing it along the other two perpendicular axes. This will distort the orbiting bodies' shapes slightly. Larger astronomical bodies which are near-spherical due to self-gravitation, become slightly prolate (ovoid).

If either of the two orbiting bodies is rotating relative to the other, this prolate shape is not stable. The rotation of the body will cause its long axis to move out of alignment with the other object, and the tidal force will have to reshape it to restore the situation. In a sense, the tidal bulges "move" around the body as it rotates to stay in alignment with the other object. This is most clearly seen on Earth by how the ocean tides rise and fall with the rising and setting of the Moon.

Since it takes a small but nonzero amount of time for the bulge to shift position, it is always located slightly away from the nearest point to the other object. For the case of a rotation period shorter than the orbital period, this bulge is located in the direction of the rotation. This misalignment causes a small but steady and significant force acting to slow the rotation of the body. This is because the bulge is pulled on by the other object's gravity, resulting in a slight force pulling the surface of the body in the opposite direction of its rotation. The rotation of the body slowly decreases, with its orbital momentum being boosted in the process. In the opposite case of a rotation period longer than the orbital period, the rate of rotation is increased at the expense of orbital momentum instead.

Almost all moons in the Solar System are tidally locked with their primaries, since they orbit very closely and tidal force increases rapidly (as a cubic) with decreasing distance. Pluto and its moon Charon are an extreme example of a tidal lock. Charon is the biggest moon in the Solar System in comparison to its planet and also has a very close orbit. This has made Pluto also tidally locked to Charon. In effect, these two celestial bodies revolve around each other (their mass center lies outside of Pluto) as if joined with a rod connecting two opposite points on their surfaces.

Until radar observations in 1965 proved otherwise, it was thought that Mercury was tidally locked with the Sun. Instead, it turned out that Mercury has a 3:2 spin-orbit resonance, rotating three times for every two revolutions around the Sun; the eccentricity of Mercury's orbit makes this resonance stable. The original reason astronomers thought it was tidally locked was because whenever Mercury was best placed for observation, it was always at the same point in its 3:2 resonance, so showing the same face, which would be also the case if it were totally locked. More subtly, the planet Venus may be tidally locked with the planet Earth: whenever the two are at their closest approach to each other in their orbits, Venus always has the same face towards Earth. (The tidal forces involved in this lock are extremely small and it may be primarily a result of coincidence; see the article on Venus for more detail.) In general, any object that orbits another massive object closely for long periods is likely to be tidally locked to it.

Close binary stars throughout the universe are expected to be tidally locked with each other, and extrasolar planets that have been found to orbit their primaries extremely closely are also thought to be tidally locked to them. One example, confirmed by MOST, is Tau Boötis, but strangely in this case it is a star tidally locked by a planet.de:Gebundene Rotation fr:Rotation synchrone it:Rotazione sincrona ja:自転と公転の同期