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Old 01-06-08, 03:52 PM   #1
shapelike
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Gravity.

Four and a halfish million years ago something the size of Mars slammed into the not-quite-finished-cooking-yet Earth and ejected enough debris to eventually form the moon. As my understanding goes, this ejected matter from the impact was drawn together out in space by the force of gravity and was also trapped in orbit around the Earth by its gravity.

Why didn't all that matter just get sucked directly back to the Earth, since its gravity would be so much stronger than the larger moon chunks that were out there at the start, and for that matter why didn't the entire moon get sucked back into the Earth? I don't understand how gravity can cause all that minute debris in space to form together as a new body but not pull that matter into a much larger planet that's right frigging next to it*.

Okay science people, school me.

*Give or take three or four hundred thousand kilometers.
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Old 01-06-08, 04:12 PM   #2
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"why doesn't the moon fall into the Earth?" by Google

http://www.google.com/search?hl=en&q...into+the+earth
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Old 01-06-08, 04:31 PM   #3
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Originally Posted by shapelike View Post
Four and a halfish billion years ago

Okay science people, school me.
fixed that for ya.

oh, and if i recall correctly, the moon is slowly getting farther away from earth; it's going just a little too fast to remain in orbit indefinitely.
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Old 01-06-08, 04:31 PM   #4
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Or Earth falling into the sun. That would suck.
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Old 01-06-08, 04:49 PM   #5
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fixed that for ya.

oh, and if i recall correctly, the moon is slowly getting farther away from earth; it's going just a little too fast to remain in orbit indefinitely.
What's a zero here or there?
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Old 01-06-08, 05:15 PM   #6
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Million, billion... Hell, if you ask crackpot Kent Hovind, he'll tell you the earth formed, or rather,was created, no more than about 4000 years ago...

Good thing he's in a safe place for the next decade...
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Old 01-06-08, 05:38 PM   #7
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Old 01-06-08, 05:51 PM   #8
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Why didn't all that matter just get sucked directly back to the Earth, since its gravity would be so much stronger than the larger moon chunks that were out there at the start, and for that matter why didn't the entire moon get sucked back into the Earth?
The moon's orbit is slowly decaying. Give it some time for chrissakes!
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Old 01-06-08, 05:53 PM   #9
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The moon's orbit is slowly decaying. Give it some time for chrissakes!
Actually, it's moving away at a rate of 2.54 cm/year.
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Old 01-06-08, 05:53 PM   #10
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I don't understand how gravity can cause all that minute debris in space to form together as a new body but not pull that matter into a much larger planet that's right frigging next to it*.
You mean like when the planets formed from matter spinning about the sun rather than being pulled directly into it?
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Old 01-06-08, 05:55 PM   #11
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Actually, it's moving away at a rate of 2.54 cm/year.
I think you're right. I remember hearing something about this as a kid, that the moon would have taken up a huge portion of the sky millions of years ago.
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Old 01-06-08, 06:10 PM   #12
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http://www.teachersdomain.org/resour...bit/index.html
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Old 01-06-08, 09:21 PM   #13
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One cannot catch uncle gravity sleeping and walk up walls. . .

don't ask how I know this.
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Old 01-06-08, 09:38 PM   #14
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Originally Posted by Tom Stormcrowe View Post
Actually, it's moving away at a rate of 2.54 cm/year.
Wow! That's almost an inch a year!!! Maybe if we made the Earth a little heavier...
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Old 01-06-08, 11:45 PM   #15
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Folks the OP didn't ask how does it stay there now but how did all the stuff smashed off of the proto-Earth stay in orbit. Easy friend, most of it didn't. The slow stuff dropped back, the fast stuff went on out, and the stuff with the right range of speeds did go into a whole slew of orbits. Enough stuff likely was in a near circular elliptical orbit to be a focus for some of the rest to condense on. Most of the rest would be slung by near misses out into space or hit the Earth or Moon. The person who can solve the general Three Body problem may get the Nobel Prize for math. A lot of the finer stuff is still out there and can be seen under the right viewing conditions.

The gegenschein accounts for some of it.
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Old 01-07-08, 12:51 AM   #16
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Okay science people, school me.
Gravity - you know, along with getting older, it's a woman's worst enemy, you know...
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Old 01-07-08, 03:15 AM   #17
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The moon's orbit is slowly decaying. Give it some time for chrissakes!
The Moon is slowly moving away from Earth. The additional energy it takes to cause that comes from the Earth's rotation...we're gradually slowing down. It's all due to tides.
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Old 01-07-08, 03:21 AM   #18
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The person who can solve the general Three Body problem may get the Nobel Prize for math.
Alas, mathematicians have been slighted...there is no Nobel for math.

Awarded for Outstanding contributions in Physics, Chemistry, Literature, Peace, and Physiology or Medicine.
The Sveriges Riksbank Prize in Economic Sciences in Memory of Alfred Nobel, commonly "identified with" the Nobel Prize, is awarded for outstanding contributions in Economics.
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Old 01-07-08, 03:01 PM   #19
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Old 01-07-08, 03:06 PM   #20
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LOL @ Psydotek
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Old 01-07-08, 03:10 PM   #21
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I do what i can.
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A girl once asked me to give her twelve inches and make it hurt. I had to make love to her 3 times and then punch her in the nose.
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Old 01-07-08, 03:14 PM   #22
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I do what i can.
Thanks for the after lunch laugh.
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Old 01-07-08, 04:12 PM   #23
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Actually, it's moving away at a rate of 2.54 cm/year.
Is it really 1.00 inch per year? Or was it actually ~1 inch/year, converted with far too much precision?
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Old 01-07-08, 08:52 PM   #24
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This item from NASA says that the increase is about 3.8 cm (1.5 inches) per year: http://sunearth.gsfc.nasa.gov/eclips...olloLaser.html
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