Astronomy
How the Moon's Tidal Lock Affects Its Rotation
Quick fact
The Moon's rotation period is about 27.3 days, exactly the same as its orbital period around Earth.
Why this is interesting
You've probably noticed the Moon always looks the same from Earth. But why does it never show its other side?
Read the full explanation
Understanding How the Moon's Tidal Lock Affects Its Rotation
Imagine spinning a ball on a string around your head. If you make the ball spin at just the right speed, one side will always face you. That's what happens with the Moon and Earth. The Moon rotates on its axis, but its rotation period matches the time it takes to complete one orbit around Earth. This is called synchronous rotation. Because of this, from Earth we always see the same hemisphere of the Moon. The other side, often called the far side, is never visible from Earth.
A deeper explanation
The reason for this synchronous rotation is tidal locking. Earth's gravity exerts a stronger pull on the side of the Moon facing Earth than on the far side. This differential gravitational force creates tidal bulges in the Moon's solid body (since it's rocky) and earlier in its history, when the Moon was molten, these bulges were significant. As the Moon rotated, these bulges would be dragged out of alignment with the Earth-Moon line, and the constant flexing and friction dissipated rotational energy. Over millions of years, this slowed the Moon's rotation until the bulges aligned with the Earth-Moon axis. At this point, the rotation period exactly matches the orbital period, and one side permanently faces Earth. The locking affects the Moon's rotation by making it identical to its revolution, which is why we never see the far side from Earth. This also means the length of a lunar day (from one noon to the next) is about 29.5 Earth days, because the Moon has to rotate slightly more than 360° to bring the sun back to the same point in the sky.