[Daithi]

I say it is zero. The moons of all the planets are in a tug of war with the sun over their moons. In the case of all the rest of the planets the sun is always losing that tug of war. However, in the case of the earth and moon, the sun is sometimes winning the tug of war, so therefore the earth and moon are really a double-planet. The earth doesn't have a moon.

Edit: Now that I have answered the question I went and used Google -- I've now changed my mind, but since I've Googled I can't tell you why.