Space/Science — HabitableZone

Space/Science » in reply to re: The dying Earth

The moon wasn't captured
ER — (archived)

The best evidence today suggests the moon formed from the debris kicked into earth orbit by a collision between our planet and a Mars-sized planetesimal. It was pretty much a shake and bake operation, totally demolishing both worlds, the earth and moon reassembling from the fragments. This happened early in the system's youth, probably about 4 billion years ago. The moon pretty much settled into it's current configuration, with one side facing the earth because it was lopsided, one side was more massiive than the other. Earth gravity pulls the heavy side to face it. But the moon returns the favor.

Since the moon's orbit is sometimes above and sometimes below the earth's equator, it's gravity pulls on earth's equatorial bulge (the earth is fatter at the equator than through the poles) with enough leverage (because earth is not a perfect sphere, otherwise it would make no difference). It's like tapping a spinning top, it's axis of spin slowly rotates, or precesses, every 26000 odd years or so. The big impact feature on the hidden side of the moon probably occured later.

How do we know all this, it's the best fit to the available evidence we have, including the moon rocks the astronauts brought back. Could the story be rewritten? Possibly, more data means we may have to change our minds. The story of how moon and earth formed has changed enormously even during my lifetime, so it would be presumptious to think we know it all now. But you gotta go with the evidence you've got, you just can't make it up as you go along.

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