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Tilt of the earth's axis. Why does it change?

  1. Apr 8, 2012 #1
    I recently read that the earth's axial tilt fluctuates between 22 degrees and 24 degrees. Can someone explain what causes the tilt to change? If I assume the earth is perfectly spherical, gravitational forces cannot change the tilt.

    So, is that the reason i.e. that the earth is not perfectly round?

    Or is there a different explanation for the tilt angle change?

    Thank you
  2. jcsd
  3. Apr 8, 2012 #2


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  4. Apr 8, 2012 #3
  5. Apr 9, 2012 #4
    The idea of ice build up near the poles seems plausible. But I have found several sources that claim that other planets have an effect: For instance, the moon is said to stabilize the tilt change. Can someone explain how another body can exert a gravitational force that changes the tilt of the earth?

    Thank you.
  6. Apr 9, 2012 #5

    D H

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    The Earth is not a perfect sphere. The largest deviation from spherical is the equatorial bulge caused by the Earth's daily rotation. Because gravity is a 1/r2 force, this equatorial bulge gives the Moon and Sun a handle that their gravitational fields can grab on to. Technically it's called gravity gradient torque. It is this gravity gradient torque that causes the Earth's rotation axis to precess and nutate.
  7. Apr 9, 2012 #6
    Thank you!
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