Tidal locking

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. This causes only one constant side or hemisphere to face the partner body. The most common situation in our solar system is for a satellite to face its planet (like our Moon). However, if the difference in mass between the two bodies and their separation is small, both may become tidally locked to each other. The best known example of this are Pluto and Charon.

Earth's Moon

In the case of the Moon, both its rotation and orbital period are just over 4 weeks. The effect is that no matter where you are on the Earth you always see the same face of the Moon. The entirety of the far side of the Moon was not seen until 1959, when photographs were transmitted from the Soviet spacecraft Luna 3.

(More precisely, despite the Moon's rotational and orbital periods being exactly locked, we may actually see about 59% of the moon's total surface with repeated observations due to the phenomenon of librations. These are primarily caused by the Moon's varying orbital speed due to the eccentricity of its orbit.)

Mechanism

Consider the case of a significantly large body — a moon — orbiting a much larger planet. Assume the body to be massive enough that its structural integrity gives way beneath its gravity, pliably collapsing into a sphere. The moon will eventually become tidally locked to the planet via the following process:

1. The moon orbits the planet with some orbital period (4 weeks for our moon).
2. However, when they start out, their rotational periods are different from each other and from the orbital period.
3. Now, if there was no planet, the shape of the moon's gravity well would be spherical, and so would the moon. (More precisely, in a rotating body there is some polar flattening because centrifugal force reduces the gravity felt by matter at the equator. However, let us ignore this for simplicity, as it does not impact the tidal locking mechanism.)
4. There is a planet, however, which slightly changes the shape of the gravitational well in which the moon sits. The gravitational well becomes elongated like a rugby football with the long axis pointing towards the planet. The Earth's Moon is slightly elongated in the Earth direction in just this manner. By always pointing towards the planet, this long axis makes one revolution around the moon's center every orbit.
5. Well, that's the gravitational well, but if the moon starts out rotating faster (say), then the impetus of its planet facing bulge keeps carrying it forward of where the gravitational well would like it to be. The result of these competing factors (gravtational well wanting to make the bulge point to the planet, and its impetus (inertia) wantng to carry the bulge forward) is that the bulge ends up always facing mostly towards the planet, but also bit forward towards the direction of rotation. Note: there is always also an identical bulge facing in the opposite direction, approximately away from the planet. As the moon rotates, these "bulges" make one trip around its solid surface in one rotation, and are equivalent to tides. The retardation of the bulges in the Earth's oceans due to the Moon's gravity are what causes high tide to occur about an hour after the moon has passed overhead.
6. Rock (ice, gas, water, whatever) is not elastic, and when it is continually being reshaped by two bulges passing by during every rotation, friction is created. The planet is always pulling the bulge somewhat backwards, so the effect is that the moon is being gradually slowed down by this friction. Over time the rotational speed slows down. The rotational energy of the moon is being lost into space as heat.
7. Finally, as the rotation time comes close to matching the orbit time, the lag of the bulge becomes less and less, the friction becomes smaller, and the moon approaches the equilibrium situation when its rotation matches the orbital time. It has become tidally locked.
8. The time it takes for this to happen depends on how much friction is created, and how much energy has to be lost by the moon. Certainly the closer the moon is to the planet, or the larger the planet, the faster tidal locking occurs. The moon's composition also has an effect. Given infinite time, all orbiting bodies would become tidally locked to each other.

For a moon rotating slower than its orbit, a similar effect occurs, except the moon's inertia makes the bulge lag behind the gravitational well. Then, the planet is always pulling the bulge forward, which acts to speed up the moons rotation, until it once again matches the orbit time. Of course, energy is also lost due to the friction, and the moon ends up orbiting closer to the planet than originally.

The tidal locking effect is also experienced by the "planet", but at a vastly slower rate. For example, the Earth's rotation is gradually slowing down because of the Moon, by an amount that becomes noticeable over geological time in some fossils. For similar sized bodies the effect may be of comparable size for both, and both may become tidally locked to each other. Pluto and Charon are good examples of this - you can only see Charon from one hemisphere of Pluto!

Tidal locking is a purely gravitational effect, and does not require the bodies to be planets and moons. For example, it is thought that many binary stars are mutually tidally locked.

The effect carries over largely unchanged for small and non-spheroidal bodies. The main difference is that the tidal bulges travel over the irregular surface of the objects, rather than over a sphere.

Finally, in some cases where the orbit is 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 - locked to its orbit around the Sun in a 3:2 resonance.

Occurrence

Moons

Most significant 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. Notable exceptions are the irregular satellites of the gas giant planets, which orbit much further away than the large well-known moons.

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.

The tidal locking situation for asteroid moons is largely unknown, but closely-orbiting binaries are expected to be tidally locked, as well as, obviously, contact binaries.

Planets

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 enough periods is likely to be tidally locked to it.

Stars

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.