Explanations Regarding the Moons Motion, Please

[Andy ES]

Hi,

I'm pretty happy regarding what causes liberation of the the Moon, I think this is due to the Moons elliptical orbit and differences of distance from the Earth and different orbital speeds but constant rotational speed allowing more or less of the surface to be turned towards us?

And why the moon appears to nod due to its inclined orbit relative to the Earth allowing us to view above and below it?

However the motion that still confuses is the rocking side to side. I think it’s got something to do with the Earths 23.5 degree tilt.

Any simple explanation or link to a resource will be gratefully received.

(Please also feel free to put me right on my other assumptions 😁)

Thanks.

[vlaiv]

Have you read this?

https://en.wikipedia.org/wiki/Libration

Maybe there is an answer to your question there.

[PhotoGav]

There are three kinds of Lunar libration: libration in longitude, libration in latitude and diurnal libration.

You describe libration in latitude above with the cause of the Moon ‘nodding’ being down to the tilt of the Moon, 5.1° to the ecliptic and 1.5° on its axis; meaning that sometimes it is above the ecliptic and we see more of its southern region and sometimes it is below the ecliptic so we see more of its northern region.

You also describe libration in longitude, the ‘shaking’ of the Moon from side to side, resulting in us seeing around the eastern and western edges. This is due to the elliptical nature of the Moon’s orbit. When it is closer to the Earth (around perigee) it is traveling faster, so it has orbited a bit more than usual, meaning that we see around more of its eastern limb (right side), and when it is further away (at apogee) it is traveling slower, so has orbited a bit less, meaning we see around its western limb (left side). (I think I have my Lunar East and West the right way round! Always confusing!).

Finally there is diurnal libration. This is a form of libration in longitude, i.e. ‘shaking’. At moonrise, we can see a bit further around its eastern limb and later that night, at moonset, we can see a bit further around its western limb.

All this libration means that despite the Moon being tidally locked and always showing the same face to Earth, we can see a total of about 59% of the lunar surface. The first time we saw the far side of the Moon was in 1959 when the Russian spacecraft, Luna 3, photographed the Moon whilst in orbit and radioed back the images to Earth.

I hope that helps and makes some sense. 

[CentaurZ]

The explanations have been great, so I can add very little. There are also miniscule true librations of only a few minutes in lunar latitude and longitude. Often writers refer to the Moon's elliptical orbit as the cause of certain things. Actually, they are due to the Moon's eccentric (Earth off-center) orbit. The Moon's orbit is close to being circular; the ellipticity is hardly noticeable in a diagram, but the eccentricity is obvious. The two "e" terms are related non-linearly mathematically, but I won't get into that since it's not the point. here.

Below is my rendition of the Moon's upcoming librations beginning each night at 00 Hr CDT (UT-5). It does not consider the 1˚ diurnal librations which would be different for each location on Earth.

Below that is my rendition of the lunar orbit.

Edited September 24, 2023 by CentaurZ