Why we only ever see one side of the Moon

The Moon always shows us the same face, so it's easy to assume it doesn't spin. It does. It turns exactly once for every orbit, about every 27.3 days, so the same side stays pointed at us.

That perfect match isn't luck. Earth's gravity raised tidal bulges in the young Moon and used them as a brake, slowing its spin until it fell into step with its orbit. The same tides are now slowing Earth down and pushing the Moon away, 3.8 centimetres a year.