The Moon has a dark side

Perhaps it isn't amazing that an album that's sold 45 million copies spawned an astronomy myth.

By Michael E. Bakich

Half the Moon — the side facing away from the Sun — always lies in darkness. Because the Moon spins, however, that half changes constantly.

Photo by David Jenkins

In 1973, British music group Pink Floyd released one of the all-time best-selling albums, The Dark Side of the Moon. Since then, many people have assumed that one-half of the Moon — the side that faces away from Earth, sometimes called the "farside" — permanently remains in darkness. That's not true.

What is true is that, at any given moment, half of the Moon is in darkness. But the dark side is always the side that faces away from the Sun. That's also the case with Earth. Half our planet experiences day while night occurs on the other half. Because the Earth and Moon rotate (spin), some areas are moving into sunlight as different areas move into darkness. Earth rotates once a day, while the Moon takes 27.3 days to spin once.

So, remember, half the of Moon is always dark, but that half is constantly changing.

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