Nataleigh Le Bas, of Waimea College, asks :-

Is it a coincidence that we see only one side of the moon?

Frank Andrews, an astronomer with the Carter Observatory in Wellington, responded.

It is certainly no coincidence that the moon rotates on its axis in exactly the same time that it takes to orbit the Earth. (That is 27.322 Earth days.)

Living close to the sea you will be used to seeing the tides rise and fall twice a day. This is caused by the moon's gravity acting on our oceans. What you probably wont have noticed is that the Earth's surface also heaves up and down just like the sea. These movements are called "Earth Tides" and represent about 30cm of up and down motion twice a day.

These tidal movements need energy and most of this energy comes from slowing down the Earth's daily rotation by a very small amount. Geologists have found evidence that a very long time ago, early in the Earth's history, our planet seems to have taken about sixteen hours to rotate on its axis instead of the twenty-four hours it takes today.

The answer to your question lies in the fact that the earth is much heavier than the Moon and used to raise "Moon Tides" on our satellite which were much bigger than those the Moon raises on Earth. The energy for these came from the Moon's rotation. Because the Moon is only 1.23 per cent as heavy as the Earth its rotation period has been slowed down much more quickly than ours. For this reason the Moon's rotation period has now become exactly the same as its orbital period round the Earth. The same is true for the moons orbiting the other planets.

Hence from Earth we always see the same side of the Moon.