Lunartics!

[Popeye (Bert)]

In the 10 or so years that I have been observing I have always considered the moon to be a nuisance when I was chasing DSOs and only DSOs.
Now I think I might turn my attention to our nearest neighbour and make a study of the features on its surface and other aspects of the moon.
Synchronous rotation is where it all starts getting difficult for me. I cannot get my head around the concept that we only see one face of the moon because its rotation is in synch with the planet Earth's rotation. I can see that it would appear to rotate from 'background stars' perspective to use Steve Massey's (The Night Sky) words but it seems to me that its more like being stuck in gravitational pull like a hammer thrower at the busy end of the ball and the ball being the moon.
Is there someone 'out there' who can explain synchronous rotation in a way that an old sailor can understand or if there is a suitable text?
And Steve mentions that the moon is in an eliptical orbit and I understand that and why we get perigee and apogee but I wonder if that's because (now don't fall about laughing!) when the moon gets 'behind' us, so to speak, and Earth rushes forward in its orbit around the sun, it needs to 'catch up' and then rush toward us when its in front of our orbit?
Chee! I'm not explaining this very well! Am I like that?
Cheers!
Popeye

[Quark (Trevor)]

Hi Popeye,

There are many examples of synchronous rotation within our Solar System with the example of our Moon the nearest.

A picture is worth a thousand words and I have a demonstration that I often use with school groups that graphically demonstrates synchronous rotation of our Moon.

I normally ask for two volunteers and make one student the Earth and the other the Moon. This is done indoors, I have my Earth in the middle of the room with my Moon a suitable distance from it.

I ask the rest of the group to note what wall my Moon is facing, then I have my Moon prescribe one quarter of an orbit around the Earth, but always facing the Earth. After one quarter of an orbit, although my Moon is still facing the Earth, it is now facing a different wall of the room, as in completing one quarter of an orbit it has also completed one quarter of a rotation on it's axis.

I then have my Moon do another quarter of its orbit, all the time still facing the Earth. Now, after half of one orbit of the Earth, my Moon is facing the wall in the room that initially was behind it at the start of the orbit. My Moon, in traveling half way round one orbit of the Earth has completed half of one rotation on it's axis.

I then have my Moon complete another quarter of one orbit around the Earth, effectively three quarters of one orbit, my Moon is now facing yet another different wall of the room and has rotated three quarters of one rotation on its axis.

I then have my Moon complete it's orbit. It has been facing the Earth for the entire orbit but has faced all four walls of the room in doing so. Therefore my Moon has rotated ounce on it's axis in preciously the same period that it took to complete one orbit of the Earth.

This is synchronous rotation.

The ultimate example of total synchronous rotation, involves Pluto and it's largest Moon Charon. This is the only example of total synchronous rotation within our Solar System. The time that it takes Charon to rotate ounce on it's axis is the same time that Charon takes to complete one orbit of Pluto which is the same time it takes Pluto to complete one rotation in it's axis. Therefore if you were on the surface of Pluto and could see Charon, Charon would always be in the same position in the sky. If on the other hand, you were on the surface of Pluto, opposite from Charon, then you would never know that Charon existed, Charon would never appear in your local sky.

Hope this helps.
Regards
Trevor

[Popeye (Bert)]

Thanks for that explanation, Trevor. I understand what you are saying yet, to me, the moon is apparently rotating only as seen from 'background stars.' So your students are the 'background stars' and can see a rotation but your Earth student is seeing only one face as though it is locked in a gravitational grip. This is what I puzzle over. And as a qualified Industry Trainer (retired) I understand that adults need to have insight to properly accept a new concept. Quadratic equations were a puzzle to me once but I was able to use them. Then one night at sea I was about to drop off to sleep and I began to think about QE and suddenly I was hit by insight and understood how they worked! Aha!
Perhaps I need to meditate on this problem in order to get the insight that I need. Or a kick up the rear!
Cheers!
Bert

[Quark (Trevor)]

Hi Bert,

Forget the background stars, in this instance they are irrelevant, just consider the two bodies in isolation, don't try to over complicate the simple dynamics of what is happening.

If we look at the Moon in total isolation and watched it rotate ounce on it's axis we would see all faces of the Moon. Consider from my example, what happened to the Moon during one orbit of the Earth. In that example the Moon rotated ounce on it's axis in the same period that it it takes to complete the orbit. But because it's orbital period is the same as it's rotational period, the Moon always presents, more or less, the same face to the Earth.

This is a simple concept that has nothing to do with background stars at all, just the two bodies involved.

Regards
Trevor

[Jay-qu]

This phenomenon is known as tidal locking. Mercury is also tidally locked to the sun.

It happens when a satellite gets close (in astro terms) to its planet/star. This means that the gravitational force on the close side is much greater than the gravitational force on the far side of the satellite, keeping the close side closer permanently.

[jjjnettie (Jeanette)]

Here's another analogy that may either help or hinder.
Put a 20c piece down on the table. Run the fingernail of your index finger (your fingernail represents the side of the Moon facing the Earth) around the edge of the coin. You will see that your finger needs to turn 360 degrees to go completely around the coin= one rotation of the moon.

[Popeye (Bert)]

Hmmm. Well a restless night as I pondered Trevor's answer (and I did the same thing before I put the question but with a cup and its handle in the centre pointing to a golf ball and could see that it turned,) and considered my analogy of a ball at the end of a chain and being swung around before being thrown. Does the ball rotate? Is it facing the person swinging it? Does it appear to rotate to spectators? Hmmm. And I understand tidal locking and know that Mercury is tidally locked to the sun as the moon is to us. Hence my concept in the face of more expert knowledge.
Perhaps Trevor is right. Perhaps I am overcomplicating the issue.
But what about my other question? Does the moon in its orbit around the earth 'lag' as 'we' race around the sun in our orbit, and need to 'catch up' and get in 'front' thus causing the eliptical orbit and therefore perigee and apogee? Is there an in depth text on these matters that I can refer to? I can find nothing in our state library catalogue.
Thanks folks and thank you for your patience.
Bert

[WhiteStarLine (Bill)]

Hi Popeye,

Digging around for an animation that explains this, I came across a good one: It 'paints' a red and a blue crater on the dark and light sides respectively and shows what you would see if the moon was non-synchronous, then shows you what you actually observe due to synchronous rotation. You can jump straight to the movie (4 MB) or read the full explanation.

[jjjnettie (Jeanette)]

Quoted from Virtual Moon Atlas glossary

Libration.
Apparent oscellatin of the Moon for a terrestrial observer. With its effect, it's possible to observe a small part of the Far Side. It exists 3 different librations: Longitude Libration, Latitude Libration and Diurnal Libration.

Longitude Libration.
According to constant rotation speed and to variable revolution speed of the moon, longitude libration allows a terrestrial observer to see East and West limits of the far side of the moon.

Latitude Libration.
Because of the Lunar orbit inclination compared to Earth orbit around the Sun, it's possible to see North and South limit parts of the Far Side of the Moon.

[Quark (Trevor)]

Hi Bert, it is really great that you are taking the time to think this through. Understanding, after all is obviously you goal.

Very good texts that I would recommend are; The New Solar System by Beatty, Petersen & Chalkin 4th ed, and Universe by Freedman & Kaufmann 6th ed.

Both of these books are used as University texts, I believe Universe is now in its 7th edition. Our local library has The New Solar System.

Bert, the Moon has a most interesting orbit. As you have already mentioned it is elliptical. The eccentricity of its orbit being 0.0549.

However the Moon also wobbles slightly on its axis (libration) and nods up and down along its north south axis (nutation). This wobble and nodding allows us, at different times to see slightly more than half of the surface of the Moon, about 59% overall.

The animation posted by Bill should help.
The analogy that you have mentioned relating to a hammer thrower would be an ideal analogy for the total synchronous rotation of Pluto and Charon. From the point of view of the hammer throwers back, you would never see the ball.

Stick with it Bert.
Regards
Trevor

[Popeye (Bert)]

I've already answered this once and my wife called about something and I forgot to click 'Submit Reply.' (She's forgiven!)
Thanks to all who have responded to my question. I did understand libration and have a good working knowledge of Lat and Long. But I am grateful for the time and effort in making sure that I was aware of the phenomenom jjjnettie. And to WhiteStarLine (another seafarer?) I tried the link but all that I got was a blank page so I will try again but thanks for the effort.
I have mulled over the input and my own analogy with the hammer thrower and when Trevor explained the relationship with Pluto and Charon I can see that what I was looking down the barrel at was a type of geostationary satellite. And now the mud is clearing. ('At last!' they gasp.)
I was trying to picture the moon rotating on its axis in time with the earth rotating on its axis something like a roller brush on a sphere. Yes! Some mothers do have 'em!
Thank you for the references, Trevor. When I searched the State Library I looked for subjects on the moon only and could find nothing more that childrens books and novels. I have 'The Idiots Guide to the Sun' (about my level) as I wanted that when my wife bought me the PST. And I was looking for a similar 'text.' No doubt these will be for reference only so I will have to have them sent up from Hobart I suppose and read them in the local library. St Helens is not a huge place. We don't even have a roundabout here - but its really nice and quiet for us retirees!
I donated all of my Uni texts to the WA library and it would be nice if more people did that so that more reference books could be available to the general public.
Cheers!
Bert

[WhiteStarLine (Bill)]

Originally Posted by Popeye

I tried the link but all that I got was a blank page

Hi Bert,

The movie provides a good explanation and is worth watching but may take a few seconds to download. Assuming you are uisng Internet Explorer, try right clicking your mouse on the link and then choose 'Save Target As . . .'. Download to your hard disk and play using Apple Quicktime.

I'm assuming you have broadband as it could take 10 - 15 minutes on dialup!

[Popeye (Bert)]

Thanks, Bill. We've been away for a few days and so I missed your post. I did use some patience and finally I got the animation. Very well explained too, thank you. I passed it on to other members of AST in Launceston in case anyone was interested.
I do have broadband, but this is Tassie and things can be a bit slow - but in a nice way!
I was with the Grey Funnel Line for 20 years.
Cheers!
Bert