Does the Moon Orbit the Earth, or the Sun?
The Moon that went up a Hill but came down a planet over at Bad Astronomy
[Y]ou have to look at something called the Hill sphere. Basically, it’s the volume of space around an object where the gravity of that object dominates over the gravity of a more massive but distant object around which the first object orbits.
OK, in English — and more pertinent to this issue — it’s the volume of space around the Earth where the Earth’s gravity is more important than the Sun’s. If something is orbiting the Earth inside Earth’s Hill’s sphere, it’ll be a satellite of the Earth and not the Sun.
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Comment from Uncle Al
Time: September 30, 2008, 11:37 am
http://en.wikipedia.org/wiki/Nordtvedt_effect
The moon behaves as the Earth and the moon mutally orbit a common barycenter offset from the Earth’s center of gravity.