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Tidal locking

What is tidal locking?

The Moon keeps the same face toward Earth because it turns once in the same time it goes around us. That match is tidal locking — not a dark side that never sees the Sun.

A mark on the Moon stays aimed at Earth all the way around. The Sun still lights the left half of the ball. The far side has plenty of daytime.

Far side is not dark side

The far side is the hemisphere we do not see from Earth. The dark side is whatever hemisphere is in night right now. They coincide only at full moon, when we are looking at the lit face.

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How the match was forced

Earth’s tides in the Moon’s solid body lagged the bulge, torqued the spin, and bled rotational energy until the periods matched. The same kind of torque is slowly pushing the Moon outward and lengthening our day.

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On the hub: Sun–Earth–Moon simulator · Moon

Related questions

What is libration? Libration is a small wobble in the Moon’s facing. We still have one near side, but over time we see about 59% of the surface, not 50%. The extra is a peek around the edges.

Why does the Moon change shape? The Moon makes no light of its own. One half is always lit by the Sun. Phases are your changing view of that lit half as the Moon goes around Earth — about 29.5 days from full moon to full moon.

What causes tides? The Moon’s gravity pulls harder on the water nearer to it than on the Earth’s centre, and harder on the centre than on the far-side water. The ocean stretches into two bulges. Earth then turns under those bulges, so most coasts see two high tides a day.

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