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| Space on Jan 18, 2010

Does the dark side of the moon really exist?

Light does shine on the so-called “dark side” of the moon. It’s just that, from our Earthly perspective, we can’t see the illumination.

No, not really. The moon always keeps one face toward Earth, and to do that the moon has to rotate once on its axis every month. If you put a chair in the middle of the room, and you walk around the chair always facing it, then you’ll find that you have to turn your body once with respect to the rest of the room before you can make one complete circuit around the chair.

The moon does something similar. It always keeps one face toward the Earth – but to do that it has to spin on its axis once a month, at the same time that it’s moving in its orbit around Earth. The moon’s spin on its axis is what makes it possible for the moon to have a near side and a far side as seen from Earth.

We never see the far side of the moon – at least not without spacecraft. But it’s not the case that the far side of the moon is always dark. Since the moon spins once a month, a cycle of day and night on the moon lasts that long.

So no matter where you stand on the moon – on the side that faces Earth or the side that doesn’t – you’re going to be standing in sunlight for about two weeks out of every month. The far side of the moon stays the far side from our earthly perspective. But it doesn’t always stay dark.