On the other hand, the thing we call The Moon shouldn't really be considered a moon. The Earth/Moon system should really be classified as two planets in essentially the same orbit around the Sun.<p>As I think was discussed recently in another thread here, and was pointed out by Isaac Asimov in one of his science essays a long time ago, if you were to plot the paths of the Earth and Moon through space, you'd find they are both approximately 12-sided convex polygons around the Sun, out of phase by pi/12. The Moon's path does not look like something a kid would draw with a Spirograph, contrary to popular opinion.<p>When you look at moons of other planets, their paths do look like a Spirograph drawing.<p>If you are having trouble visualizing this, imagine two horses racing around a standard horse racing track. On the first straightaway imagine horse 1 is in the lead. At the first turn, horse 2 gets the inside track and pulls ahead. On the back straightaway, horse 2 leads, but on the second turn, horse 1 gets the inside track and takes the lead back in the turn.<p>It probably wouldn't even occur to you to think of horse 2 as having done an orbit around horse 1, yet if there were a remote control camera mounted on horse 1, and you were controlling it from the stands and you were trying to keep horse 2 in view at all times, you'd find that you have had to rotate the camera around a full circle. So, from horse 1's frame of reference, horse 2 indeed did orbit it once!<p>Now imagine the horses on a modified track that instead of two straightaways and two half-circle turns has four straightaways and four quarter-circle turns. Now horse 1 thinks horse 2 circled it twice.<p>That's essentially what the Earth and Moon are doing, but there are 24 turns in the race course, and the straightaways are not there--as soon as you leave one turn you are starting the next. So, from Earth's point of view it looks like the Moon goes around us 12 times a year. But alien astronomers watching would be like spectators at the horse race--they'd just see two planets orbiting the Sun in nearly the same order, taking turns using the inside track to pull ahead.<p>Another way to look at it is to consider force ratios. For moons such as those of Mars, or Jupiter, or Saturn, and so on, if you look at the force on the moon from the planet, and the force on it from the Sun, you find the ratio of those two is greater than 1. The planet "pulls harder" than the Sun does.<p>For the Earth and Moon, the ratio is less than 1. The Sun is pulling harder on the Moon than the Earth is!