Chris Dixon, of Kings High School, asks :-
Why do we have a moon?
Alan Gilmore, an astronomer at the University of Canterbury's Mt John Observatory, responded.
Earth has a moon because some event made the moon.
Any theory of the moon's origin must explain the things we know about the moon. It must explain why the moon is composed of relatively light rock, similar to rocks in the outer layers of the earth. Why the moon has no iron core, or only a tiny core. It must explain how the rocks of the moon were heated to very high temperatures, cooking water and volatile elements out of the them. And why the moon is proportionately large when compared to other moons in the solar system.
The best theory to explain these features is the `giant impact' hypothesis. This theory has the earth hit by a Mars-size planet about 4.5 billion years ago. The off-centre impact vapourized a large amount of both the earth's crust and the impactor's crust and threw the white-hot cloud into orbit around the earth. The impactor's iron core blended with earth's core. Much of the vapourized crustal rock fell back to earth but some gathered together in orbit, making the moon.
Initially the young moon was very close to the earth. The earth was spinning rapidly after the glacing impact. Strong tides made by the nearby moon slowed the earth's spin and pushed the moon into a larger orbit.
This theory doesn't account for every detail but it explains most of the main features.