![]() Seen from a vantage point in space, the points of maximum bulge extension are displaced from the axis oriented toward A. 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. The material of B exerts resistance to this periodic reshaping caused by the tidal force. Smaller bodies also experience distortion, but this distortion is less regular. an axially symmetric ellipsoid that is elongated along its major axis. For large astronomical bodies that are nearly spherical due to self-gravitation, the tidal distortion produces a slightly prolate spheroid, i.e. ) When B is not yet tidally locked, the bulges travel over its surface due to orbital motions, with one of the two "high" tidal bulges traveling close to the point where body A is overhead. (For the solid Earth, these bulges can reach displacements of up to around 0.4 m or 1 ft 4 in. The elongated distortions are known as tidal bulges. The body of object B will become elongated along the axis oriented toward A, and conversely, slightly reduced in dimension in directions orthogonal to this axis. This creates a gravitational gradient across object B that will distort its equilibrium shape slightly. The gravitational force from object A upon B will vary with distance, being greatest at the nearest surface to A and least at the most distant. The change in rotation rate necessary to tidally lock body B to the larger body A is caused by the torque applied by A's gravity on bulges it has induced on B by tidal forces. If the tidal bulges on a body (green) are misaligned with the major axis (red), the tidal forces (blue) exert a net torque on that body that twists the body toward the direction of realignment.Ĭonsider a pair of co-orbiting objects, A and B. There can be some shifting due to variations in the locked body's orbital velocity and the inclination of its rotation axis. However, in this case the exact same portion of the body does not always face the partner on all orbits. ![]() 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. ![]() With Mercury, for example, this tidally locked planet completes three rotations for every two revolutions around the Sun, a 3:2 spin–orbit resonance. Not every case of tidal locking involves synchronous rotation. The object's orbit may migrate over time so as to undo the tidal lock, for example, if a giant planet perturbs the object. The object tends to stay in this state because leaving it would require adding energy back into the system. 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 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. The effect arises between two bodies when their gravitational interaction slows a body's rotation until it becomes tidally locked. ![]() Alternative names for the tidal locking process are gravitational locking, captured rotation, and spin–orbit locking. However, if both the difference in mass between the two bodies and the distance between them are relatively small, each may be tidally locked to the other this is the case for Pluto and Charon, as well as for Eris and Dysnomia. Usually, only the satellite is tidally locked to the larger body. For example, the same side of the Moon always faces the Earth, although there is some variability because the Moon's orbit is not perfectly circular. In the case where a tidally locked body possesses synchronous rotation, the object takes just as long to rotate around its own axis as it does to revolve around its partner. Tidal locking between a pair of co- orbiting astronomical bodies occurs when one of the objects reaches a state where there is no longer any net change in its rotation rate over the course of a complete orbit. Charon is massive enough that the barycenter of Pluto's system lies outside of Pluto thus, Pluto and Charon are sometimes considered to be a binary system. Pluto and Charon are tidally locked to each other. ![]()
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