Tidal locking occurs when an orbiting astronomical body always has the same face toward the object it is orbiting. The effect arises between two bodies when their gravitational interaction slows a body’s rotation until it becomes Tidally locked. With Mercury, this tidally Locked planet completes three rotations for every two revolutions around the Sun, a 3: 2 spin–orbit resonance.
About Tidal locking in brief
Tidal locking occurs when an orbiting astronomical body always has the same face toward the object it is orbiting. This is known as synchronous rotation: the tidally locked body takes just as long to rotate around its own axis as it does to revolve around its partner. In the special case where an orbit is nearly circular and the body’s rotation axis is not significantly tilted, such as the Moon, tidal locking results in the same hemisphere of the revolving object constantly facing its partner on all orbits. If both the difference in mass between the two bodies and the distance between them are relatively small, each may be tidally lock to the other; this is the case for Pluto and Charon. The effect arises between two bodies when their gravitational interaction slows a body’s rotation until it becomes Tidally locked. Over many millions of years, the interaction forces changes to their orbits and rotation rates as a result of energy exchange and heat dissipation. When one of the bodies reaches a state where there is no longer any net change in its rotation rate over the course of a complete orbit, it is said to be tidly locked. The object tends to stay in this state when leaving it would require adding energy back into the system, for example, if a giant planet perturbs the object. Not every case of tidal locking involves synchronous rotate. With Mercury, this tidally Locked planet completes three rotations for every two revolutions around the Sun, a 3: 2 spin–orbit resonance. For large astronomical bodies that are nearly 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. 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. When B is not yet tidally locks, the bulges travel over its surface due to orbital motions, with one of two tidal bulges traveling close to the point where body A is overhead. This results in a tandem orbit in which B’s angular momentum is boosted in tandem with A’s orbital momentum, leading to tidal locking. The whole system is conserved in this process, so that when B slows down, it loses rotational momentum and loses its rotational angular momentum, its orbital momentum. The resulting resulting orbit is called a tandem B–A–B–B system. The tidal locking process is conserving in this system, so when B loses its rotation momentum, it experiences a stronger gravitational and torque and experiences a slightly stronger gravitational force and so experiences a tidal lock. The net torque from both bulges, then, is always in the direction that acts to synchronize B’s orbital rotation with its orbital period.