Catherine Richards, of Hampden, asks :-

Does the moon rotate and if not why not?

Frank Andrews, then an astronomer educator at Wellington's Carter Observatory, responded.

All large objects in space have formed as a result of the gravitational collapse of diffuse gas and therefore rotate.

You will be aware that the Moon causes the ocean tides on Earth. It also causes atmospheric and earth tides as well. The Earth also causes tides on the Moon and this, like an earth tide, results in a slight distortion of the Moon's, and Earth's, shape. To do this requires energy, and this has to come from somewhere. In these cases, from the energy of the Earth and Moon's axial rotation.

The Moon's mass is only 0.0123 of the Earth's mass so its rotation rate has slowed much more quickly than the Earth's. Geologists have been able to show that the Earth rotated more rapidly in the distant past than it does now; about 16 hours per revolution. This gradual slowing in rotation rate has now reached the point where it takes the Moon the same time to rotate on its axis as its orbital period of 27.322 earth days. Hence from Earth we always see the same side of the Moon. This has happened to all the moons of the Solar System and is a stable configuration.

The Earth's rotation is still gradually slowing down. This situation will continue until the Earth and Moon always show the same face to each other. This will be in the very distant future; in fact the Sun will probably have become a Red Giant and destroyed the Earth before this can happen.

Another interesting aspect of the Moon's synchronised rotation is that the energy required to slow its rotation is no longer being used for this purpose, but it has to go somewhere! (Law of Conservation of Energy). It is now possible to use laser-ranging techniques to determine the Moon's distance with an accuracy of about 4 cm. As a result it has been possible to show that some of this energy is gradually pushing the Moon away from the Earth at a rate of about one centimetre a year while the rest is being dissipated as heat within the Earth!