Anyone that has spent any amount of time looking at the moon has got to ask themselves that question. I know it is not that simplistic. The moon is simply too damn big to be captured by the Earth. More than likely it was formed early in Earth's history when Earth was struck by a Mars sized objects.
It is clear that the Moon has been with the Earth for much of Earth's history, but that doesn't go anywhere near far enough to explain the phenomenon that is Earth's largest satellite.
First the Moon is huge, a full quarter the size of Earth....nearly a planet in it's own right. Matter of fact, the Moon is so large that it in truth doesn't orbit the Earth as much a travel along with Earth, in a sinusoidal pattern that sometimes has it slightly ahead and sometimes slightly behind the Earth and one which if you plotted one complete "orbit" you would find that it does not center on the core of Earth but some 3 thousand miles from the center. This means of course that both bodies "orbit" this commonality.
The pair would seem to then have a co orbit something like this, but due to the Moon size, the Sun in reality has more influence on the Moon than the Earth, so the orbit would appear more like this, with the orbit more centered around the sun with the Earth playing tug of war....
But as attractive as these orbits are, in truth the orbit of the Moon around the Sun is almost circular with no where near the large perturbations of these examples. The point being that many of the criteria that have been accepted as rules to go by in determining a planet could apply to the Moon!
Read the IO9 article
here
