Mechanics of the Moon

Hello

Here are two shots showing longitudinal libration and the variation of the moon's diameter.

The photos were taken through a 80mm, f/5 achromat refractor on an EQ 3-2 (tracking makes alignment under Registrax much easier). The setup can be acquired for 500 dollars or so, maybe 1000 in Europe (you know how much we hate you guys in the States for having access to prices waaay lower than here?)

The writing may be too small to read on the photos, so here's the info: for the variation in diameter, the moon on the left was photographed on June 23, 2008 and measures 30,46' and is 9,(09)% smaller than the one on the right. The latter was photographed on February 2, 2009 and measures 33,48'.

The libration is less obvious, thus the red squares - I won't bore you with the numbers.

What's funny is that I already had a fairly large collection of decent moon photos when I decided to hunt for the libration /diameter variation. All I had to do is spend an afternoon going through them and selecting the most obvious examples.

[Ignore the noise / harsh contrast that appears for some reason when I rescale, as well as the strange tongues the titles are written in.. sorry.]

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Nicely shown. Now you're going to have to explain it.

Steve.

So it wobbles side to side and back and forth?

Cool.

You pretty much resumed it

The wobble makes us see some 59%, so the dark side is smaller than the lit one. (see - proof Niok users are inferior. in numbers.)

:P

Excuse me for a moment but the back of my head just blew off and is lying in the corner...

Good Lord the knowledge on POTN is amazing.

Euh.. sorry for the back of yer head... I have yet to figure if it's brutal sarcasm

Will return with detailed, boring astro-mechanics and precise math.

I WANT to be a geek. I TRY.

ecce_lex wrote in post #9330256
Euh.. sorry for the back of yer head... I have yet to figure if it's brutal sarcasm

Will return with detailed, boring astro-mechanics and precise math.

I WANT to be a geek. I TRY.

No, No, No...

I meant it as a sincere compliment. I'm impressed with your understanding of said Moon Mechanics but it's a subject that is so far beyond my limited scope of understanding that it (figuratively) makes my head explode.

I'll just shut up and watch from the corner now...

No worries

This sort of physics is much less complex than the 7D AF system lol. A kind soul explained it to me this morning on this forum, so I'm more than happy to give a bit back in return.

Numbers are the refuge of talent-less people. Me in any case :P

ecce_lex wrote in post #9330332
No worries

This sort of physics is much less complex than the 7D AF system lol. A kind soul explained it to me this morning on this forum, so I'm more than happy to give a bit back in return.

Numbers are the refuge of talent-less people. Me in any case :P

53.7% of us agree...

Since you're going to explain, I understand why it wobbles back and forth but why does it wibble side to side?

Sorry for the delay in answering.

The moon's orbit around Earth is not a circle, but rather an ellipse. This not only brings it closer (perigee) and pushes it further (apogee) triggering the apparent change in diameter, but also makes it move faster when closest to Earth. When it moves faster, the angle at which we see it changes, and so, over time, more than half is revealed.

These are two of Kepler's laws - (1) all planets / satellites have elliptical orbits and (3) the closer the orbit, the faster the movement.

i will make a beautiful diagram in paint tomorrow if I get bored at work

Great post and images.

Libration is very noticeable if you photograph the Moon regularly over a month's time. Over the course of one month the Moon rotates at a fairly constant speed, but its speed of orbit is not constant. The Moon's orbit is elliptical not circular. Any orbiting object moves faster the closer it is to the center of what's driving its orbit. Sometimes the regular speed of its rotation leads it as it revolves and other times it falls behind by a bit, this irregular movement causes the Moon to wobble a few degrees.

EDIT: oops I see I walked away from my computer too long before posting and you beat me to it.

Again, great post. Thanks

Thanks for posting the image. I had read that it does this but the image really brings it home.

ecce_lex wrote in post #9331575
The moon's orbit around Earth is not a circle, but rather an ellipse. This not only brings it closer (perigee) and pushes it further (apogee) triggering the apparent change in diameter, but also makes it move faster when closest to Earth. When it moves faster, the angle at which we see it changes, and so, over time, more than half is revealed.

That's the biggest contribution.

Also, the Moon's center of mass isn't quite in the same place as its center of volume and, since it rotates around its center of mass, that gives it a little side-to-side wiggle.

Finally, its orbit around the Earth is tilted about 5 degrees from our equator, so you get a little up-and-down apparent wiggle depending of if it's at the high or low point in its orbital tilt.

OK, class dismissed. Test on Friday.

By the way, great pictures and annotation!

ecce_lex wrote in post #9329846
Hello

Here are two shots showing longitudinal libration and the variation of the moon's diameter.

The photos were taken through a 80mm, f/5 achromat refractor on an EQ 3-2 (tracking makes alignment under Registrax much easier). The setup can be acquired for 500 dollars or so, maybe 1000 in Europe (you know how much we hate you guys in the States for having access to prices waaay lower than here?)

The writing may be too small to read on the photos, so here's the info: for the variation in diameter, the moon on the left was photographed on June 23, 2008 and measures 30°46' and is 9,(09)% smaller than the one on the right. The latter was photographed on February 2, 2009 and measures 33°48' (some three degrees difference, or about the thickness of two fingers at arm's length).

The libration is less obvious, thus the red squares - I won't bore you with the numbers.

What's funny is that I already had a fairly large collection of decent moon photos when I decided to hunt for the libration /diameter variation. All I had to do is spend an afternoon going through them and selecting the most obvious examples.

[Ignore the noise / harsh contrast that appears for some reason when I rescale, as well as the strange tongues the titles are written in.. sorry.]

you just knocked the dust off of 7 year old Astronomy/Astrophysics info that i haven't used since college! and i felt like i should know more, so i wikipedia'd it!