The same side of the Moon always faces Earth because the Moon is tidally locked to Earth. This means that the Moon's rotation period on its axis is exactly the same as the time it takes to orbit Earth once, about 27.3 days. As a result, the Moon rotates exactly once for every orbit, keeping the same hemisphere pointed towards Earth at all times.
Tidal Locking Explanation
Tidal locking is caused by Earth's gravitational forces exerting a torque on the Moon, creating an elongated shape (a bulge) along the axis facing Earth. Over billions of years, Earth's gravity has slowed the Moon's spin so that this bulge remains aligned with Earth. If the Moon were to rotate faster or slower, the bulge would shift, which is energetically unfavorable, so the current synchronous rotation is stable.
Observations from Earth
Because of tidal locking, one side of the Moon is always visible from Earth (called the near side), while the other side (the far side or "dark side") is mostly hidden. However, due to a slight wobble called lunar libration, observers on Earth can see about 59% of the Moon's surface over time, not just 50%.
In summary, the synchronous rotation caused by tidal locking ensures the Moon's same side faces Earth continuously. This phenomenon is common among many moons in the solar system and is a result of prolonged gravitational interactions between celestial bodies.
