Quote:
Originally Posted by Magnum PEI
I was told that the moon technically is a planet, and doesn't actually orbit the earth. The two snake around a common centre of mass (which is fairly close to earth) and are actually dual planets.
This was from my physics 20 teacher, who was a bit of a loon.
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Every orbital system orbits around a common center of mass, that's how it works.
It's just that for most of the systems most of us are familliar with, one body is so much more massive that for all intents and purposes, that center of mass is basically the center of one of the bodies. The sun and earth, or the sun and moon, the center of mass is well within the larger of the two bodies.
That's actualy one method of discovering planets, they look for the wobble of a star caused by the orbiting planet.
Pluto and it's largest moon actually orbit around a point that isn't within Pluto itself.
But, rest assured, the moon is not technically a planet, it is a moon.
Edit: just a little checking, the barycenter (center of mass of the earth-moon system) is about 1700 km below the surface of the earth. Earth has a radius of about 6400 km, just for reference.