Quote from: GrimJesta;244166Hrmm. Well played. You win this round. I was so proud of my Lugosi-inspired hiss and everything.

I thought the moon was moving away? Or is that Roche thing further out?

-=Grim=-

It is moving out - but that costs energy (taken from Earth's rotation, if I remember) and eventually there's no more available. At that point it stops spiraling out, and sometime thereafter it starts to spiral back in. Sometime after that there's a Roche moment. It should be really cool to watch happening, except I don't think anyone will be living on this rock when it happens, which is just as well, because some of that lunar mass will be coming down hard.

It's all coming back to me. Wow. The moon raises tides in the ocean. But that water is rotating, like everything else on the surface of the earth, so even as it heaps up, it's heading east. That is, slight tidal bulge is spun a little off-center to the east, with respect to the earth moon axis. That shifts the earth's apparent center of gravity very slightly up and east, from the moon's perspective. Different center of gravity, different orbit: that allows the moon's orbit to creep just a little wider, in effect dragged into a faster, higher orbit by the ocean's slightly off-center mass.

The energy doesn't come from nowhere. The earth loses a little rotational speed from all this. Basically, the earth is powering the moon's outward spiral.

Eventually, that rotational speed loss slows the earth's rotation to the point where it is turning once a month, not once a day. At that point, the earth's rotation is locked to the moon's orbit, and ocean's tidal bulges stay nicely under the moon, so they don't haul on the moon to go faster any more. But you still have solar tides (about once a month now) and the usual ocean friction that tides always cause, so the earth keeps slowing. Now you get an offcenter tidal bulge again - lagging to the west this time, pulling against the moon's orbit. Now the moon starts to drift in instead of out.

Eventually, the earth's gravity pulls the moon apart, simply by pulling harder on the near bits than it does on the far bits, since gravity works that way. When the difference in pull mounts up enough, rip. Pieces of moon tear free, grind, and spread out - some forming a pretty ring, some forming an utterly lethal hail of vast moon rocks. I think the earth's surface probably ends up molten from the energy of the bombardment; at any rate the oceans should do a nice boil.

Wait long enough, and the Earth/sun system should play the same game, except I suspect the earth vaporizes from the heat before it shatters. Unless the sun goes out first, I don't remember. We're toast either way by then.

Have a nice day.

Awesome dude. That rocked.

-=Grim=-