R E Smart of Whanganui asks :-

Why does the Earth rotate and orbit but the Moon only orbits and doesn't rotate?

Alan Gilmore, an astronomer at the University of Canterbury's Mt John Observatory, responded.

Seen from anywhere but the Earth, the moon does rotate. It rotates once per orbit thus keeping the same face toward us.

The moon almost certainly did rotate faster in the past. The best current theory for the Moon's origin is that it formed from debris resulting from a collision between Earth and a Mars-sized planet a long time ago. The young moon orbited much closer to the earth than now.

The moon pulls on the Earth and makes the sea tides that we all know. Similarly the Earth makes tides on the Moon. This makes it slightly Rugby-ball shaped with a pointy end toward us. If the moon was rotating, seen from Earth, then this tidal bending would drag on its spin. That's what stopped the moon spinning. The effect was greater in the past when the moon was closer. We say the moon is now 'tidally locked' to the Earth.

The Moon's tides on Earth are slowing down Earth's spin too, but the effect is tiny. This is the because the moon is only 1/80th of the mass of Earth. Our day is lengthened by around 2 milliseconds (0.002 second) per 100 years. The spin energy (angular momentum) lost by the Earth makes the Moon's orbit bigger by 38 mm per year.

The moon doesn't keep exactly the same face to us. Its orbit around Earth is elliptical or egg-shaped. It moves faster when closer. The Moon's spin is steady. So the spin gets a little ahead and a little behind as the Moon circles the Earth. This causes the Moon to seem to wobble slowly over a month. Astronomer call this 'libration'. It has allowed mapping of about 5/8ths of the Moon's surface from Earth.