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Author Topic: How close has the moon been as it's moving away?  (Read 4166 times)

Offline mike2niner4

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How close has the moon been as it's moving away?
« on: 24/02/2009 16:51:21 »
Hey! I'm curious as we know the moon is moving away 3cm a year or whatever the figure is, but how close has it been before it started moving away? Would we have seen a bigger moon (im thinking when there was early complex life on earth)in the sky??

Interesting to know!!

Thanks

Mike ;D


 

Offline dentstudent

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How close has the moon been as it's moving away?
« Reply #1 on: 24/02/2009 17:36:27 »
Since the formation of the moon was due to the collision between a mars sized planet and an inchoate earth, you could state that the distance between the newly formed moon and the "new" earth was very small indeed!
« Last Edit: 24/02/2009 17:41:15 by dentstudent »
 

Offline LeeE

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How close has the moon been as it's moving away?
« Reply #2 on: 24/02/2009 19:46:05 »
It's thought, I believe, that the collision between the Mars-sized Theia (the name given to the planetesimal) and the proto-Earth completely destroyed Theia and that the Moon subsequently formed by accretion from the debris thrown up by the collision in to orbit about the proto-Earth.

As to how far out the proto-Moon formed isn't certain but it's mean distance from Earth is now 384400 km.  If the recession from the Earth was a constant 3 cm/year, and taking the age of the Moon as 4.5 billion years ago then it's moved out at least 135000 km.  However, as it would have been traveling faster when it was closer it's rate of recession would have been higher, meaning it's receded a lot more than that.

Taking in to account the slowing of the Earth's rotation, as well as the current recession rate at it's current distance as part of the process, it shouldn't be too hard for a maths wiz to get a rough figure - does anyone fancy doing the calculus to work it out?
 

Offline DoctorBeaver

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How close has the moon been as it's moving away?
« Reply #3 on: 24/02/2009 20:13:15 »
I did read an article about this a while ago but all I can remember is that I was amazed how close it was. I'll see if I can find it again
 

Offline DoctorBeaver

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How close has the moon been as it's moving away?
« Reply #4 on: 24/02/2009 20:17:09 »
Here you go - 3-5 times the Earth's radius!

from http://curious.astro.cornell.edu/question.php?number=699

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 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.
 

Offline LeeE

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How close has the moon been as it's moving away?
« Reply #5 on: 24/02/2009 20:22:01 »
Good answer.  I'd love to have been able to see that.  I seem to recall hearing that the moon would have actually caused tides in the surface of the Earth at that point, with the (semi-solid) ground rising and falling as the Moon passed.
 

Offline Vern

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How close has the moon been as it's moving away?
« Reply #6 on: 24/02/2009 22:00:36 »
Can you imagine what the tides would be like with the moon that close. They would probably wash the whole planet.
 

Offline DoctorBeaver

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How close has the moon been as it's moving away?
« Reply #7 on: 24/02/2009 22:01:38 »
It sure must have been a pretty hairy ride.
 

Offline LeeE

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How close has the moon been as it's moving away?
« Reply #8 on: 25/02/2009 11:41:44 »
Can you imagine what the tides would be like with the moon that close. They would probably wash the whole planet.

Just to clarify Vern, I don't think there was any liquid water on the planet at that point; it's the ground that would have been moving up and down.  The Earth was spinning a lot faster at that point too, so the Moon would appear to pass every few hours (I can't remember exactly how fast but around every 4-6 hours rings a bell)
 

Offline Vern

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How close has the moon been as it's moving away?
« Reply #9 on: 25/02/2009 11:49:38 »
I suspect its true that the earth would have been liquid; the oceans would have been in the atmosphere. Tidal effects would contribute to the temperature and slow the cooling.
 

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How close has the moon been as it's moving away?
« Reply #9 on: 25/02/2009 11:49:38 »

 

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