Tidal locking is a fascinating astronomical phenomenon where a satellite's rotation period becomes synchronized with its orbital period. This means the same side of the satellite always faces the primary body it orbits. The most familiar example is our Moon, which is tidally locked to Earth, showing us the same face throughout its orbit.
Tidal forces arise from the gravitational field gradient of the primary body. The satellite experiences stronger gravitational pull on its near side and weaker pull on its far side. This differential force creates two bulges: one on the near side where stronger gravity pulls material toward the primary, and another on the far side where weaker gravity allows centrifugal force to push material outward.
When the satellite's rotation is not synchronized with its orbit, the tidal bulges become misaligned with the line connecting the two bodies. This misalignment creates a gravitational torque that acts to change the satellite's rotation rate. If the satellite rotates faster than it orbits, the torque slows down the rotation. This process gradually transfers angular momentum between the rotational and orbital motions over geological timescales.
After millions of years of gradual synchronization, the satellite's rotation period finally equals its orbital period. At this point, the tidal bulges align perfectly with the line connecting the two bodies, and the net torque averages to zero over each orbit. The satellite has achieved tidal lock equilibrium, where the same hemisphere always faces the primary body. This is the stable end state that explains why we always see the same face of the Moon from Earth.
To summarize what we have learned about tidal locking: This phenomenon occurs when a satellite's rotation period becomes synchronized with its orbital period through gravitational forces. Tidal gradients create bulges that generate corrective torques over geological timescales. The Earth-Moon system perfectly demonstrates this process, showing us the same lunar face throughout history.