Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: thedoc on 14/11/2013 12:30:02
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Scott Borthwick asked the Naked Scientists:
Why doesn't Earth and Moon gravitate towards each other like two magnets? Is it centrifugal force keeping these two bodies apart?
Greetings from Cape Town. Awesome show with Redi every Friday.
Cheers,
Scott
What do you think?
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Why doesn't Earth and Moon gravitate towards each other like two magnets? Is it centrifugal force keeping these two bodies apart?
Yes, it's the centrifugal force just with anything in orbit around the earth.
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The moon is moving fast so the edge of the earth is falling away faster than the moon can fall. If you throw a baseball really hard and it moves really fast it will go over the curve of the earth before it hits the earth and it will keep on going in orbit.
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The moon's doing it's best to hit us, but it just keeps missing. In a system of two objects, it turns out that if they don't collide the first time they approach each other, they will either separate and never come back together or else they end up in a stable orbit. [There are a few qualification to that: orbits gradually decay due to the curvature of space around massive objects (if General Relativity is correct, but the decay will still happen even if some other theory replaces it); tidal effects are driving the moon further away from the Earth, to the point that its orbit will become elongated and eventually it will collide with the Earth (though I think that depends on interaction with the Sun as well) though this collision will be prevented if the Sun expands enough to destroy them both first; and a planet going round a star can orbit further out over time as the star loses mass. Other things can affect orbits if there are more than two objects involved, such as the pull of other planets and drag from gas.]