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Hubble Expansion

  1. Feb 7, 2008 #1
    A couple of questions:
    The distance to the moon is calculated as increasing by 38 millimetres per year by the laser ranging work of NASA following Apollo 11.

    When I apply the Hubble expansion coeficient to the centre to centre distance from Earth to the Moon, I get just over 28 millimetres per year relative to todays measured distance.

    Firstly, is it valid to apply the Hubble constant to relatively close orbitals in this manner?

    Secondly, if it is valid, is there sufficient error in the centre to cenre distance being stated as an average of 385,000Km to compensate for the difference, (38 to 28)?

    I am of course checking my maths.

    Thanks for any help

  2. jcsd
  3. Feb 7, 2008 #2


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    No, it isn't valid to apply the Hubble expansion on these scales, it's not that the expansion term is small but because the Hubble result applies to an idealized homogeneous Universe as therefore does not work for bound structures that are thousands of times more dense than the mean density.

    I remember learning once why the Moon is moving away, I think it's to do with some orbital dynamics such as tides, but I'm hazy. Hopefully someone else can help you on that part.
  4. Feb 7, 2008 #3
    Thanks Wallace.
  5. Feb 7, 2008 #4
    I had a search around the net and found some info confirming what you said Wallace, it is tides, thanks for the clue.
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