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Miketmbt
2006-Nov-15, 01:27 AM
Sorry if this has come up many times before.
My question is about the distance between the earth and the moon after it was formed.
Assuming that the rate the moon moving away from the earth has always been somthing like 3.8cm pre year. (I know its not good to assume but I have no idea how to figure out if it was faster or slower a billion years ago.) Any way. I came up with a distance of 261,928km 4.7 billion years ago. That is still a huge distance. So does that mean that as the earth was forming, the impact that made the moon flung out the material 261,928km into space? Did I do the math wrong? Wouldnt the rate at which the moon moves away from the earth have been slower when it was closer because of stronger gravitational forces?

01101001
2006-Nov-15, 01:33 AM
There's no reason to apply the current separation rate to the past, or the future.

It is not easy to estimate how far away from the Earth the Moon was when it formed, but simulations suggest is was about 3-5 times the radius of the Earth, or about 19-30 thousand km. (The Moon is currently about 84,000 km [correction: about 384,000 km] away from Earth or 3-4 thousand times further away than this.) The Moon probably couldn't have formed closer than 3 Earth radii because tidal forces from the Earth would just pull it apart again, and it is unlikely that the impact could have ejected material further than 5 Earth radii. It's not a totally easy questions to answer though as it depends a lot on the (unknown) details of the impact and how the hot material behaved in space.

The exact rate of the Moon's movement away from Earth has varied a lot over time. It depends both on the distance between the Earth and the Moon, and the exact shape of the Earth. The details of continents and oceans moving around on Earth actually change the rate, which make it a very hard thing to estimate. The rate is currently slowing down slightly, and it is estimated that in about 15 billion years the Moon's orbit will stop increasing in size.

RocketGator
2006-Nov-15, 01:39 AM
Can you imagine seeing the Moon that huge in the sky? Damn.

Jeff Root
2006-Nov-15, 03:16 AM
Zero One,

It looks like the Astronomer lost the most significant digit of
the Moon's distance in the passage you quoted. 384,000 km.

-- Jeff, in Minneapolis

Miketmbt
2006-Nov-15, 03:30 AM
Ok. I thought that the rate was increasing with time and distance not decreasing. SO if that is the case 4 earth radii seems plausible i guess. Thats over 300,000km in 4.5bln years Thank you

BISMARCK
2006-Nov-15, 03:43 AM
Like the quotation pointed out, the Moon is currently receding, and I remember reading that in the distant future, the recession will stop and the Moon will start spiraling in, and, if the Earth is still around at this point, the Moon will eventually break into a ring system when it gets to the Roche limit.

01101001
2006-Nov-15, 04:04 AM
It looks like the Astronomer lost the most significant digit of
the Moon's distance in the passage you quoted. 384,000 km.

Good catch. Reported. And, I'll edit my quote so it makes better sense.

hhEb09'1
2006-Nov-15, 04:20 AM
Good catch. Reported. And, I'll edit my quote so it makes better sense.And what did they mean by "3-4 thousand times further away than this"? It looks to me that the "thousand" is extraneous, and shouldn't be there, based upon the previous error in distance being accepted. O, and Gillianren will probably back me on this, it should be "farther".

Tim Thompson
2006-Nov-15, 04:44 AM
See if this helps (from the Talk.Origins Archive (http://www.talkorigins.org/)):

The Recession of the Moon and the Age of the Earth-Moon System (http://www.talkorigins.org/faqs/moonrec.html)