Science Puzzle
Why Do We Always See the Same Side of the Moon?
Wherever you live on Earth, the Moon shows you the same face: the same dark patches, the same bright craters, month after month. Some people say this proves the Moon doesn't spin at all.
What is the Moon really doing?
The Answer
The Moon does spin: exactly once for every trip round the Earth, about every 27.3 days.
Try it with a chair. Walk all the way round it while always facing it. By the time you're back where you started, you've faced every wall of the room in turn, so you've turned round once while going round once. The Moon does the same. Look at Fig. 1 again: from day 0 to day 14 the crater swings from pointing left to pointing right. If the Moon really didn't spin, that crater would keep pointing the same way in space the whole time, and over a month we'd see every side of it.
The perfect match isn't luck. Long ago the Moon spun faster. Earth's gravity stretched it very slightly into an egg shape, and as the Moon turned, Earth kept tugging on those bulges, acting like a gentle brake. Over millions of years the spin slowed until it matched the orbit, with the long side pointing at Earth for good. This is called tidal locking, and most of the large moons in the solar system are locked to their planets in the same way.
One myth to put away: the far side isn't the dark side. It gets just as much sunlight as the side we see. During a new Moon, when our side is dark, the far side is in full daylight.
The principle: Tidal locking. Tidal forces slowed the Moon's spin until it turned exactly once per orbit, so the same face always points towards Earth.