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Moon's Orbit

  1. Dec 14, 2008 #1
    It was reported that the Dec 12th moon was the closest it has been to Earth in 15 years. I've always heard that the moon is actually moving away from the Earth. Is the moon in a specific orbit? Or does its orbit change and if so, does it change in a specific way?
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  3. Dec 14, 2008 #2

    Vanadium 50

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    The moon's orbit is elliptical. I believe the statement is that this is the closest it has been in 15 years when it is full. It's hard to tell - the popular press has it kind of mangled.
  4. Dec 14, 2008 #3


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    Yeah, I seem to recall hearing that it was a full moon coinciding with perigee (or something like that)...
  5. Dec 14, 2008 #4


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    Welcome to PF!

    Hi ADunn! Welcome to PF! :smile:

    The Moon is on an approximately 18-year cycle.

    (That's why eclipses repeat approximately every 18 years … but not visible at the same longitude on the Earth :wink:)

    It was probably the same distance 33 years ago, and 51 years ago, and so on.

    The Moon isn't moving away fast enough for anyone to notice. :smile:
  6. Dec 14, 2008 #5

    D H

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    It's an 18.61 year cycle, to be a bit more precise. The Moon is receding from the Earth at 3.8 centimeters/year on average. The huge variation in the perigee distance, 13,000 miles, completely overwhelms this tiny drift rate.

    The timing of perigee and the perigee distance depends in part on the phase of the Moon. The perigee distance exhibits local minima at full and new Moon.
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