- 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. - 21 Mar '11 11:31

The moon is both*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?**

- 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 15:16

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