Re: Large Satellites?
@smartermind - Isaac Asimov, bless his memory, pointed out many years ago that there's something odd about the Earth-Moon system, and that is that the Sun exerts a greater pull on the Moon than the Earth does - a situation which, if memory serves me correctly, exists for no other moon in the solar system.
Also, if you look at a projection of the Moon's motion from a heliocentric view, the moon and the Earth both orbit the Sun in intertwined orbits, and the Moon's orbit about the Sun is always concave.
Given all that, despite the fact that the barycentre of the Earth-Moon system is just about within the Earth's surface, one could make a strong case for arguing that the Moon isn't a moon at all but the other part of a binary planet system along with the Earth.
Alternatively, given the current ruling for what constintutes a planet, as the Earth clearly hasn't cleared its orbit of other sizable bodies (because of the Moon, which isn't actually a moon, one could argue as per the above), then the Earth isn't a planet, unless one counts a binary planet as a planet. (Which then fuels arguments about Pluto/Charon)
And no, I don't think the Earth is not a planet, personally. It's just a MUCH harder thing to define than most people think, when you consider all the varied edge cases that can potentially exist.
And frankly - the universe doesn;t care what we label things. They're out there, they're interesting, they're worthy of investigation, whatever the heck you call them :-}