Why can we only see one side of the Moon? © Getty Images

Why can we only see one side of the Moon?

Because then we wouldn't have had an epic Pink Floyd album, surely...?

Asked by: Greg Ganter, California

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The time taken for the Moon to spin on its axis is almost exactly the same as the time it takes to orbit the Earth. Hence, the Moon always keeps the same side pointing our way. This is not a coincidence. Over billions of years, the Earth’s gravity has forced the Moon to spin synchronously with its orbit.

However, things are a bit more complicated than that. Viewed from Earth, the Moon appears to rock slowly backwards and forwards so that we see a slightly different face throughout the lunar month. There are two main reasons for this. First, the Moon’s orbit around Earth is elliptical not circular so its rotation is sometimes ahead, and sometimes behind, its orbital motion. And second, the Moon’s rotation axis is not at right angles to its orbit around the Earth so we can sometimes see ‘over’ or ‘under’ its poles. Over time this means we actually get to see about 59 per cent of the Moon’s surface.


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