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YujiroDemonBackHanma t1_j1q8l7r wrote

Reply to comment by sciencedthatshit in What is the currently-accepted theory for how the moon was formed? by CopperGenie

Question... if something external hit the planet, ejecting debris that eventually formed into the moon, why is the moon so far away? Or, shouldn't the ejected pieces eventually fall to the planet and merge with earth again?

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Game_Minds t1_j1qh57r wrote

The moon is actually really, really close in cosmic terms for a moon of its size relative to earth. Saturn and Jupiter's moons are much smaller AND much farther away. and it is in fact slowing down veeeeeeeeery marginally over time, so if the sun didn't explode eventually it might make it back to earth

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sciencedthatshit t1_j1r3hsr wrote

The moon is slowing migrating away from Earth, not towards it. And, to be pedantic, stars like the sun don't explode. They swell, puff off outer layers and then shrink into a white dwarf.

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Game_Minds t1_j1r5o47 wrote

Ahhh, okay. I remembered the 1.5 inches per year but in the wrong direction-- I think i mixed up decaying orbits / Lagrange points for closer satellites that are impacted by atmospheric friction. I had forgotten that the weird phenomena of rotation and tidal gravitational forces combine to speed up the moon's orbit, functionally the same thing as escaping. And yes, explode was a convenient fill in word for "expand into a red giant, assimilating most of the inner bodies in the solar system, then shed the outer layers in a not-quite-explosion, leaving a white dwarf core", but you're right, that's incomplete

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zekromNLR t1_j1v2qkw wrote

A recent simulation suggests one possible way is that the impact debris initially coalesced into two bodies. One of those did indeed fall back into Earth, but the other one, due to momentum exchange with the first one, got enough momentum to get a stable orbit. And from there, tidal interactions slowly transferred momentum from the Earth's rotation to the Moon's orbit, widening its orbit and slowing down the rotation.

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