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Recently, I started to search more about the orbit of the moon, and noticed there are certain periodic patterns or cycles for which I did not find any satisfactory explanation.

The first cycle can be seen right away by looking at the graph of the distance with time: enter image description here

It can be noticed that the perigee and apogee are not constant, and have a cycle of ~206 days.

Another cycle that can be seen is when one zooms into the graph: enter image description here

The orbit of the moon also consists of many oscillations of period ~24.8 hours. And they are quite significant, having an amplitude of around 2.5% of the average distance of the moon (384,400km).

What are the main causes of these two cycles?

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    $\begingroup$ Where did you get your data for these plots? $\endgroup$
    – cms
    Sep 20, 2021 at 14:12
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    $\begingroup$ Also, I cant see what units the y-axis is in, nor the x-axis at all. A plot is pretty useless without its axis, yes? $\endgroup$
    – cms
    Sep 20, 2021 at 14:14
  • $\begingroup$ I got the data from Stellarium. And the units don't really matter here, since this is a conceptual discussion. But if you are interested, the y-axis is in AU and the entire graph (Fig. 1) has an interval of 2 years (730 days), with 1 hour spacing between each point. $\endgroup$
    – WordP
    Sep 20, 2021 at 14:22
  • $\begingroup$ Ok… we are getting somewhere: the y-axis is a distance in AU. Distance from what-to-what? $\endgroup$
    – cms
    Sep 20, 2021 at 16:22

1 Answer 1

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In your second cycle, the period (close to $24$ hours) and the magnitude (about $1.5$ times the radius of the Earth) both suggest this is a diurnal cycle. Is it possible you are measuring the distance of the Moon from a fixed location on the surface of the Earth, so the measured distance is affected by the Earth’s daily rotation ?

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  • $\begingroup$ With data from Stellarium your last point is quite likely. $\endgroup$ Sep 20, 2021 at 15:49
  • $\begingroup$ I believe you are correct. I checked the distance of the moon at the same time in different locations and it gives different values. $\endgroup$
    – WordP
    Sep 20, 2021 at 16:19

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