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Moons orbit

  1. Aug 19, 2007 #1
    can somebody plz help me examine the truth of the statement

    "the moon moves in a near-circular orbit around earth. because the earth is so much more massive, it own motion is not appreciably affected by the moons presence"
  2. jcsd
  3. Aug 19, 2007 #2


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    Gravity affects both objects equally (equal and opposite reaction). The result is that both objects orbit their combined center of mass.

    Mass of the Moon: 7.349 x 10^22 kg
    Mass of the Earth: 5.9736 x 10^24 kg
    Average distance between the two: 384,400 km

    Find the combined center of mass and you'll find it lies inside the Earth. I guess "appreciably" is a subjective term, since the combined center of mass is still quite a ways from the Earth's center of mass.
  4. Aug 19, 2007 #3


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    It's about 400 miles from centre is it not?
  5. Aug 19, 2007 #4


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    According to this article - http://en.wikipedia.org/wiki/Center_of_mass

    the barycenter of the earth-moon system is 4670 km from the center of the earth. For comparison, the earth's radius is approximately 6380 km. The barycenter is about 1000 miles below the earth's surface on a line between the center of the moon and center of earth.

    Perhaps one is thinking of the barycenter of the sun-earth system.
    http://www.applet-magic.com/centermass.htm - This also mentions the barycenter of the earth-moon system.
    Last edited: Aug 19, 2007
  6. Aug 19, 2007 #5


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    Here's a page I made with an animation showing the Earth and Moon orbiting their common barycenter. The view is from slightly above the Moon's orbital plane. The animation file is an .avi, so your browser must be set to display .avi files.


    Here is the same type of animation for Pluto:
  7. Aug 20, 2007 #6
    hey thanks guys this info helped alot.
    tony that animation was pretty good thanks
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