"What in blazes is our moon doing way out there?
It's too far out to be a true satellite of Earth, it's too big to have been captured by the earth. The chances of such a capture having been effected and the moon then having taken up a nearly circular orbit about the earth are too small to make such an eventuality credible.
But, then, if the moon is neither a true satellite of the earth nor a captured one, what is it?"
– Isaac Asimov, "Asimov on Astronomy," Doubleday, 1974; Mercury Press 1963.