Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: pavlovs dog on 13/02/2007 17:39:44
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Hi all,
I was thinking about an eclipse i saw in France a while ago and thought how amazing it is that the moon is the same size as the sun when looking at them both from the Earth. Is this an amazing coincidence or is there an explanation for this, like some sort of link between the moon and the sun?
Seems pretty interesting...
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It is a simple coincidence, though a quite remarkable one. The moon is believed to have formed after a collision with a Mars sized body and the proto-Earth. 10% of the material expelled from the Earth fell into orbit around it and quite rapidly coalesced to form the moon.
At that time the moon was much closer to the Earth - around 20,000 ,miles IIRC. This raised huge tides on the Earth, and in the solid body of the moon. This slowed the Earth's rotation, over billions of years, from around six hours to the twenty four hours we are familiar with.
This reduced the angular momentum of the Earth which was transfered to the moon, which consequently moved further away from the Earth. It continues to move further away in a consistent fashion (just as the Earth's rotation continues to slow.) So, millions of years ago, when the moon was closer it would have totally obscured the sun during an eclipse, so that the corona would not be visible. In millions of years time it will have moved far enough away that it will only be able to obscure part of the sun. Even now, depending upon the precise distance of the Earth from the sun and the moon from the Earth this can happen, so that we get what is called an annular eclipse where the sun is visible all around the eclipsing moon.