# Distance of the Moon Cycles

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: 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: 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?

• Where did you get your data for these plots?
– cms
Sep 20, 2021 at 14:12
• 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?
– cms
Sep 20, 2021 at 14:14
• 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. Sep 20, 2021 at 14:22
• Ok… we are getting somewhere: the y-axis is a distance in AU. Distance from what-to-what?
– cms
Sep 20, 2021 at 16:22

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 ?