Final answer:
The Moon's axial rotation period matches its orbital period around Earth—a state known as synchronous rotation—due to tidal forces exerting torques which lead to tidal locking. This state minimizes energy and keeps the same side of the Moon always facing Earth.
Step-by-step explanation:
The phenomenon where the Moon's orbital period around the Earth matches its axial rotation period is known as synchronous rotation. This results in the same side of the Moon always facing Earth. This synchronization occurs due to tidal forces over time, which exert torques and lead to tidal locking. The gravitational pull from Earth causes the elongation of the Moon and this elongation acts like a lever arm for Earth's gravitational torques. Over time, these torques adjusted the Moon's rotation rate so that it matched its orbit, stabilizing when the Moon's rotation period and orbital period were synchronized, minimizing the torque and the Moon's energy state.
The sidereal period of the Moon, which is the time it takes to orbit Earth with respect to the stars, is 27.321662 days. However, the synodic period, which is the time it takes for the Moon to complete its cycle of phases because it depends on the position of the Sun, is longer, approximately 29.530589 days. This is due to the relative motion of the Moon and the Sun as observed from Earth.
In essence, the synchronous rotation of the Moon is the end result of gravitational interaction and the principle that systems tend to evolve toward a minimum energy state. The Moon has settled into a stable orientation with Earth that minimizes energy and prevents significant rotation relative to its orbit.