The Moon's Same Face
Why do we always see the same half from Earth?
Summary
The Moon completes its orbit around the Earth and its spin on its own axis in exactly the same time (~27.3 days). That is why it always shows us the same face; its other side can never be seen directly from Earth. In this model you can watch the Earth, Moon and Sun turning and see this synchrony for yourself.
In depth
The Moon's rotation is no coincidence. Over billions of years, the Earth's gravity raised small bulges in the Moon's body; as the Moon spun, energy had to be spent dragging these bulges around, and its spin slowed down. Eventually the Moon's rotation period became equal to its orbital period, a state called "tidal locking". Watch the model closely: as the Moon goes around the Earth, one point on it always faces the Earth. If it did not spin on its axis, we would see all sides of it over the course of an orbit.
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