# Lunar perigee frequency

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## Summary:

How many days are there between each lunar perigee, and is it constant?

## Main Question or Discussion Point

Hello-
I was looking at a table of the dates of lunar perigees for 2020, and I noticed that most of the perigees were between 27 and 29 days apart. So the perigees do not appear at a constant frequency, which I did not expect.

There are even a pair, between June 30 and July 25, which are obviously only 25 days apart. Why the inconsistency?

Does it have to do with the sun's gravitational pull or something? I recall reading somewhere that the Moon doesn't simply orbit the Earth, but the two orbit the Sun together in a kind of dance.

The Sun's pull on the Moon is actually stronger than the Earth's pull on the Moon. Maybe this is why perigees do not occur at a constant frequency? Maybe a pro astronomer could shed some light on this, no pun intended.
-Scott

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The Sun's pull on the Moon is actually stronger than the Earth's pull on the Moon. Maybe this is why perigees do not occur at a constant frequency? Maybe a pro astronomer could shed some light on this, no pun intended.
-Scott
Yes, the effect of Sun on Moons orbit is significant. Perigee distance for Moon can vary as much as 25 thousands km within half a year. Therefore, the deviations from average 27.55 days long anomalistic month are noticeable.
Good resource to to visualize Moon's perigee variability is below:
https://www.lunarplanner.com/LunarPerigee/

OK, thanks. I was thinking that the best time to look at the Moon through a telescope would be when it is at perigee; pretty obvious, eh?

Janus
Staff Emeritus
Gold Member
Here's a site that shows the visual difference between Lunar apogee and perigee.
As to looking at the Moon in a telescope, you will get better views during the quarter moon phases than during a full moon. During a full Moon you are looking at the Moon from pretty much the same direction as light is striking it, you won't get much contrast and things will look pretty flat. During the quarter Moon, the light is coming from the side and the features cast visible shadows which causes them to "pop out" more.
Here's an example of a sphere with a bit of a bumpy surface as it appears with straight on lighting vs. side lighting.

The surface texture is more noticeable in the right image.

Motore
So I guess a perigee quarter-Moon would be a kind of astronomers' super Moon!

phyzguy
The moon's orbit is pretty circular. It's only a few percent closer than the average at perigee. You really won't notice any difference unless you make careful measurements.

I figure lunar features, through a telescope, look just a bit larger at perigee. I understand that comparing perigee to apogee is like comparing quarter to a nickel. It's not a huge difference. I'm not going to take my telescope out every night, so I like to choose the best nights.

Janus
Staff Emeritus
Gold Member
The moon's orbit is pretty circular. It's only a few percent closer than the average at perigee. You really won't notice any difference unless you make careful measurements.
Silly me. I forgot t include the link to the page I mentioned:
https://www.fourmilab.ch/earthview/moon_ap_per.html

Bandersnatch
Yes, thanks for the link. It is also interesting that, although the "face" of the Moon we see is always the same, due to the tidal locking of the Moon relative to the Earth, there are small changes in the look of the Moon's face due to librations.

I was also looking at an article about the fate of the Moon, not in terms of human exploration, but in terms of the distant future and the Moon's drift away from the Earth. The article said that there will eventually be an equilibrium situation when the Earth's spin is tidally locked with regard to the Moon, at which point the Moon will be about 35 percent farther away than it is now. This of course will take billions of years. https://www.physlink.com/Education/AskExperts/ae429.cfm

Bandersnatch
The article said that there will eventually be an equilibrium situation when the Earth's spin is tidally locked with regard to the Moon, at which point the Moon will be about 35 percent farther away than it is now. This of course will take billions of years. https://www.physlink.com/Education/AskExperts/ae429.cfm
If we ignore that the Sun will die sometime during the process, possibly evaporating our subject matter, and instead we imagine the tidal interactions continue as usual forever, then there's no equilibrium. There would be, if the Earth and the Moon were the only two bodies in the solar system.

However, just as the two are getting tidally locked to each other, the same interactions happen between the Earth and the Sun, where the star is 'attempting' to match Earth's rotation with its revolution.
The current strength of those tidal forces is about half as much as those raised by the Moon. But while the latter diminishes as the satellite recedes and the length of a day extends - reaching zero when a (lunar) month equals a day - the former will keep acting for as long as a month doesn't equal a year.

So while currently the Moon is receding at the expense of Earth's rotational angular momentum towards the equilibrium distance, at some point the tidal interactions from the Sun will take over. Those will cause our planet's rotation to keep slowing down even beyond the E-M tidal lock equilibrium. As a result, the Moon will find itself revolving faster than the Earth is rotating, which will reverse the E-M interaction, and begin to bring it down until the two finally collide.

Klystron
Gold Member
Or exceeds the Roche Limit for two orbiting bodies.

Ah, interesting points, by "Sun die" I assume the Sun will still be there, it'll just be much different. It will be a red giant and it may envelop both the Earth and the Moon and they would be absorbed into the Sun, and the Moon wouldn't have a chance to get to the Roche Limit.

Or the surface of the Sun might not reach the Earth/Moon, nor evaporate the Earth/Moon, in which case eventually the Moon would get so close the Earth it would be torn apart. Then I suppose the Earth would have a ring? (Assuming those particles don't get evaporated by the red-giant Sun.)

Bandersnatch