Joshua Dyal said:As to whether or not the Moon is, in fact, a moon, saying that it isn't is a bit forward, since what a moon is is defined by the Moon...Personally, I don't think there's much to be gained from saying the Moon isn't a moon because we're a double planet system, since saying so doesn't really imply anything very different about our situation other than the fact that our own is quite massive relative to it's planet than most of the other such systems in our solar system.
But we are different from all the other planets in this system. All other satellites in the system have a gravitational center in the middle of the body they orbit. Luna does not- it and Earth revolve around a point within Earth's outer core. If you look at Earth's orbit around the sun, it looks like a sine wave because it does (weakly) revolve around Luna. That is why Luna is not a moon but rather a small planet.