What phenomenon causes the same side of the Moon to always face Earth?

Tidal locking causes the same side of the Moon to always face Earth. The Moon rotates once on its axis in about 27.3 days, exactly matching the time it takes to orbit Earth. Because these two periods are synchronized, the same lunar hemisphere remains pointed toward us as the Moon travels around its orbit.

This arrangement developed through tidal friction. Earth’s gravity raised subtle tidal bulges in the young Moon, gradually slowing its original rotation until the rotation period matched the orbital period. Once synchronized, the gravitational interaction no longer produces a net change in the Moon’s spin under normal conditions.

A common mix-up is confusing tidal locking with the Moon not rotating. It does rotate; if it did not, different sides would face Earth during its orbit. Another nuance is libration: the Moon’s slightly elliptical orbit and tilted axis let observers see a little beyond its central near side over time. Including libration and parallax, about 59% of the lunar surface can eventually be viewed from Earth, although the far side remains mostly hidden.