Tidal locking, also known as gravitational locking, captured rotation and spin–orbit locking is a phenomenon occurs when a planet of an astronomical object that always faces one side in it's orbit and is most common on moons. This is known when a body in space orbits another body in a way that the body’s year and day are equal in length. The body spins around its own axis once for each time it orbits around another, specific body in space. By having equal years and days, this orbiting body shows the same side of its face to those looking at it from the other body, or the body that being rotated around. Usually, only the satellite/moon is tidal locked to the larger body. Like Earth and the moon, the same side of the moon always faces Earth although the orbit is not perfectly circular.
Most moons are tidally locked are tidally locked with their primaries, because they orbit very closely and tidal force increases decreasing distance. Pluto and Charon is a special example of a tidal lock where Charon is a large moon compared to its primary and has a relatively close orbit. Pluto's other moons, are chaotically tidally locked due to the effect of Charon. Earth's 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. When the Earth is observed from the Moon, the Earth doesn't appear to seen across the sky but appears to remain in the same place, rotating on its own axis.
Asteroid moons tidal locking is currently largely unknown.
Mercury has a 3:2 spin–orbit , rotating three times for every two revolutions around the Sun, which results in the same positioning at those observation points. Mercury has been in the 3:2 rotation state early after its formation, approximately 10-20 million years.
Venus and Earth has an extreme close orbit, unknown if a tidal locking result.
Exoplanet Proxima Centauri b orbits it's parent star is tidally locked, expressing both a synchronized rotation and a 3:2 spin–orbit resemblance like Mercury
Close binary stars are found to be tidal locked, before being formed into 1 star. Tau Boötis, an unusual example, a star might be tidal locked by it's planet Tau Boötis b. Since stars are gaseous bodies that rotate with different rates at different latitudes, the happening of the tidal lock is by Tau Boötis's magnetic field.
Solar system planets
|Parent body||Tidally locked satellites|
|Sun||Mercury (3:2 rotation)|
|Jupiter||Metis, Andrastea, Callisto, Ganymede, Europa, Io, Thebe, Amalthea|
|Saturn||Pan, Atlas, Prometheus, Pandora, Epimetheus, Janus, Mimas|
|Unranus||Miranda, Ariel, Umbriel, Titania, Oberon|