<P> Consider a pair of co-orbiting objects, A and B . 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 . </P> <P> 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 . This creates a gravitational gradient across object B that will distort its equilibrium shape slightly . 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 . The elongated distortions are known as tidal bulges . (For the solid Earth, these bulges can reach displacements of up to around 0.4 metres (1.3 ft).) 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 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 . </P> <P> 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 . Seen from a vantage point in space, the points of maximum bulge extension are displaced from the axis oriented toward A. If B's rotation period is shorter than its orbital period, the bulges are carried forward of the axis oriented toward A in the direction of rotation, whereas if B's rotation period is longer, the bulges instead lag behind . </P> <P> Because the bulges are now displaced from the A--B axis, A's gravitational pull on the mass in them exerts a torque on B . The torque on the A-facing bulge acts to bring B's rotation in line with its orbital period, whereas the "back" bulge, which faces away from A, acts in the opposite sense . However, the bulge on the A-facing side is closer to A than the back bulge by a distance of approximately B's diameter, and so experiences a slightly stronger gravitational force and torque . The net resulting torque from both bulges, then, is always in the direction that acts to synchronize B's rotation with its orbital period, leading eventually to tidal locking . </P>

How did the moon achieve its synchronous rotation