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How much closer was the Moon say 200,000 years ago?

  1. Mar 24, 2010 #1
    much difference?
     
  2. jcsd
  3. Mar 24, 2010 #2
    (Wikipedia)
     
  4. Mar 24, 2010 #3
    so what would dat be?
     
  5. Mar 24, 2010 #4

    russ_watters

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    Staff: Mentor

    You can't calculate it yourself? Make an effort. This isn't a place to have things spoon-fed to you.
     
  6. Mar 25, 2010 #5
    4.7 miles

    you think that would make the moon appear much closer? i mean if suddenly the moon appeared 5 miles closer would you notice?
     
  7. Mar 25, 2010 #6
    how far is the moon away from the earth?

    384,403 kilometers is the number i got for an average. 4.7 miles is about 7 kilometers.

    would you notice a change of 7 kilometers if you were almost 400 000 kilometers away?
     
  8. Mar 25, 2010 #7
    Is there any way to prove it has been moving away at this exact speed for that long? 38 millimeters a year is barely anything, so wouldn't some of the millions of impacts along the surface influence its direction?
     
  9. Mar 25, 2010 #8
    yes.
     
  10. Mar 25, 2010 #9
    you have fantastic depth perception then.
     
  11. Mar 25, 2010 #10

    russ_watters

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    Staff: Mentor

    The effect of the impacts is miniscule becaues they make up a tiny fraction of the mass of the moon. 200,000 years is a pretty short time, so it is unlikely that it has changed much in that time.
     
  12. Mar 25, 2010 #11
    Interesting question, even though, I think, it's based on a misconception. The recession is not an inertial but a friction effect, namely the result of a constant conversion of lunar orbital momentum into tidal effects on Earth. So, counteracting this effect by bombarding the moon with impactors would be not like changing the direction of a rolling car by throwing balls at it, which is tedious but easily doable given enough balls, but like trying to keep the car from eventually stopping due to energy loss, which is much harder in the long run.

    Quantitatively, the figure means that the Moon's distance from Earth increases by one part in 10^10 per year, which roughly corresponds to a change in orbital speed by the same fraction, so the annual change in momentum is

    delta p ~ lunar mass * change in lunar orbital speed ~ 10^23 kg * 10^3 m/s / 10^10 ~ 10^16 kg m/s

    The typical speed of an impactor is on the order of tens of kilometres per second, so counteracting this would require an asymmetric bombardment on the order of 10^12 kg per year. I wasn't able to find a cumulative figure anywhere, but several publications gave total impact masses for certain impactor size ranges, and all of those were in the range of 10^3 to 10^6 kg per year per mass decade (i.e. impactor mass between 10^n and 10^(n+1) kg), so I'm guessing the grand total won't exceed 10^9 kg. And most of that is symmetric, so the momentum contributions will cancel each other out.

    So, the answer seems to be that the effect of impacts is negligible in comparison with that of tidal losses, unless I made a major mistake somewhere.
     
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