Why do we always see the same side of the Moon from Earth?
The Moon always shows us the same face because Earth's gravity has slowed down the Moon's rotational speed. The Moon takes as much time to rotate once on its axis as it takes to complete one orbit of Earth. (Both are about 27.3 Earth days.) In other words, the Moon rotates enough each day to compensate for the angle it sweeps out in its orbit around Earth.
Gravitational forces between Earth and the Moon drain the pair of their rotational energy. We see the effect of the Moon in the ocean tides. Likewise, Earth's gravity creates a detectable bulge -- a 60-foot land tide -- on the Moon. Eons from now, the same sides of Earth and Moon may forever face each other, as if dancing hand in hand, though the Sun may balloon into a red giant, destroying Earth and the Moon, before this happens.
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