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Is the moon closer or further away to us than predicted by Newton

  1. Aug 7, 2009 #1
    I saw on t.v. that the moon's distance to earth as measured by lasers reflecting off of the mirrors left on the moon by Apollo 13 was a bit off from Newton's Fg = Gm1m2 / r^2 formula. I don't recall if the moon was closer or further away from Earth than what was predicted by Newton's formula.
     
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  3. Aug 7, 2009 #2

    Vanadium 50

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    I don't think this question makes sense. One can have an orbit at any radius, so there is no radius "predicted by Newton". There are differences between GR and Newton, but radius is not one of them.
     
  4. Aug 7, 2009 #3

    George Jones

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    Are you sure that Apollo 13 left mirrors on the Moon? :biggrin:

    The average distance between the Earth and the Moon is increasing at the rate of almost two inches per year due to the tidal gravitational interaction of the Earth and the Moon, but, as Vanadium 50 posted, this is predicted by Newtonian gravity.
     
  5. Aug 7, 2009 #4

    Vanadium 50

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    Tossed 'em out the window perhaps?
     
  6. Aug 7, 2009 #5
  7. Aug 7, 2009 #6

    negitron

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    Yeah, we know. Appolo 13 didn't leave anything on the Moon.
     
  8. Aug 8, 2009 #7
    This is a very good question. Newton could have used his knowledge of the Earth's gravitational force in combination with the moon's sidereal period (Kepler's third law) to calculate the distance to the moon. Laser ranging could verify these relations. These laser measurements are now being used to look for GR effects. See
    http://www.physics.ucsd.edu/~tmurphy/apollo/basics.html
    In addition, click on "track our progress" at the bottom of this URL to see details of the laser ranging measurements.
     
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