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Moon questions

  1. May 21, 2008 #1


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    Found this in another forum:

    1) Is the moon really orbiting the earth?

    2) What is the effect of the Universe's expansion on the Moon/Earth or Sun/Earth distance?
    http://www.news.cornell.edu/stories/March06/CMB_Timeline300.jpg [Broken]
    From that image, there's been about 10-20% universe expansion during the last 4 billion years.
    Does that translate into any effect on the "small" distances in our Solar System?

    Last edited by a moderator: May 3, 2017
  2. jcsd
  3. May 21, 2008 #2
    1) Yes the moon is orbiting the Earth, but very slowly (it only goes around the Earth about 12 times a year), so it's true to say that the motion about the sun is the dominant factor in the Moon's motion in the solar system. In a similar way, the Sun's motion around the Galactic Centre is greater than the Earth's motion about the Sun, but it's still true to say the Earth orbits the Sun.

    2) The expansion of the universe is negligibly small on the scale of the solar system - the Earth's orbit about the sun should grow something like a few nanometres per hour. The force of gravity is more than strong enough to overwhelm such a minuscule perturbation. Only in the vast open spaces of the universe where gravity is negligibly weak does the expansion manifest.
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