Thursday, August 04, 2011

Two moons

I've come across a number of fantasy and SF stories set on a planet with two moons. Some of the images you'll come across show a lack of understanding about how the phases of a moon would work, but that's really beside the point here...

Maybe it was those stories that made me take note of this article about a theory that the earth once had two moons. After that presumed early-Earth/Mars-sized-planet collision that likely created the moon, there might have actually been two moons, and there might have remained two moons for ten or a hundred million years before the second moon's orbit destabilized and it (eventually) crashed into the larger moon, creating some of the anomalies mentioned in the article.

It's an interesting parallel to one theory about that initial impact. One possibility is that the Mars-sized planet was at a Lagrange point with Earth and the sun for fifty million years or more until the orbit degraded, leading to the crash. If that crash led to a second moon at a Lagrange point for ten million years until its orbit was disturbed and they crashed...it's like chaos theory's fractals...

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