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Science Forum

  1. 18 Mar '11 13:29
    Why, when the moon is moving away from the earth every year, is it closest to earth on this Perigee? , closest to earth today for 18 years?
  2. 18 Mar '11 14:07
    Originally posted by quater14
    Why, when the moon is moving away from the earth every year, is it closest to earth on this Perigee? , closest to earth today for 18 years?
    Is the moon moving away from the earth every year? Do you have a reference for that? 18 years is approximately the period of the lunar nodal cycle.
  3. 18 Mar '11 23:29
    Yes I read it somewhere and if you google it, there are lot of article about the moon drifting away an inch every year.
  4. 21 Mar '11 11:31
    Originally posted by quater14
    Why, when the moon is moving away from the earth every year, is it closest to earth on this Perigee? , closest to earth today for 18 years?
    The moon is both
    - drifting away from the Earth slowly, centimetres a year, and
    - wobbling in and out in its orbit, over several years, by much larger amounts.

    Over the small period of a century, the latter is much more noticable.
    Over the geological ages, the former starts adding up, because it's not cyclical.

    Get a plotting calculator (there used to be one in the PowerPack for MS Windows, don't know if there still is) to plot the graph of sin(x)+x/100. View this graph first from x=-5 to x=5, and then from x=-100 to x=100. You'll get the idea.

    Richard
  5. Standard member Palynka
    Upward Spiral
    21 Mar '11 15:16
    Originally posted by Shallow Blue
    The moon is both
    - drifting away from the Earth slowly, centimetres a year, and
    - wobbling in and out in its orbit, over several years, by much larger amounts.

    Over the small period of a century, the latter is much more noticable.
    Over the geological ages, the former starts adding up, because it's not cyclical.

    Get a plotting calculator (there ...[text shortened]... graph first from x=-5 to x=5, and then from x=-100 to x=100. You'll get the idea.

    Richard
    http://www.wolframalpha.com/input/?i=sin(x)%2Bx/100+from+-5+to+5

    http://www.wolframalpha.com/input/?i=sin(x)%2Bx/100+from+-100+to+100
  6. Standard member mikelom
    Ajarn
    22 Mar '11 15:40
    Originally posted by quater14
    Yes I read it somewhere and if you google it, there are lot of article about the moon drifting away an inch every year.
    Yeah, but Japan just moved 2.4m closer to the moon - what's an inch?