18 Mar 11
Originally posted by quater14Is 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.
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?
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21 Mar 11
Originally posted by quater14The moon is both
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?
- 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
21 Mar 11
Originally posted by Shallow Bluehttp://www.wolframalpha.com/input/?i=sin(x)%2Bx/100+from+-5+to+5
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+-100+to+100