Moon's orbit

[DaveC]

My apologies if this topic has been dealt with before, but:

The moon has an elliptical orbit and its rotation is tidally locked with the earth. I understood from something I saw some time back that the near side of the moon has a higher density than the far side - which is why that's the side facing the earth. I think there are things called mascons that make the near side more dense.

Here's ny question: Given that the elliptical orbit causes the moon to show a slightly different face to the earth as it orbits (that is it appears to rotate slightly clockwise and counterclockwise during a complete orbit), will the orbit eventually become circular as a result of the mascons and the effect of earth's gravity?

My question is based on the assumption that it takes some energy to repeatedly pull the mascons away from the gravitational force vector between the earth and the moon and the only source of that energy is the shape and size of the moons's orbit. Am I missing something?

[pvtpylot]

On 2002-06-21 09:36, DaveC wrote:
My apologies if this topic has been dealt with before, but:

The moon has an elliptical orbit and its rotation is tidally locked with the earth. I understood from something I saw some time back that the near side of the moon has a higher density than the far side - which is why that's the side facing the earth. I think there are things called mascons that make the near side more dense.

Here's ny question: Given that the elliptical orbit causes the moon to show a slightly different face to the earth as it orbits (that is it appears to rotate slightly clockwise and counterclockwise during a complete orbit), will the orbit eventually become circular as a result of the mascons and the effect of earth's gravity?

My question is based on the assumption that it takes some energy to repeatedly pull the mascons away from the gravitational force vector between the earth and the moon and the only source of that energy is the shape and size of the moons's orbit. Am I missing something?

I think that in a perfect model of just the moon and earth that would be the case, but the sun also influences the lunar orbit and I suspect that the sun's perturbations will keep the orbit elliptical.

[DaveC]

Thanks for that, pvtpylot. I don't know much about orbital mechanics - as you can tell - and I hadn't even considered that the sun's gravitational field would shepherd the moon into an eliptical orbit. The sun's gravity could be the source of energy to keep those mascons oscillating back and forth across the earth-moon gravitational force vector.

[roidspop]

Someone on this board posted a link to a site that showed a time-lapse movie of the moon's motion over the course of a lunar month...awesome. The moon's libration is vividly evident and you can see the change in the moon's apparent size between apogee and perigee. Worth a look!

http://www-astronomy.mps.ohio-state....2/lunation.gif

[pvtpylot]

I read once (a long time ago in a youth far, far away) that given enough time the sun will actually succeed in capturing the moon from the earth. Does anyone have any info on this notion?

[roidspop]

Asimov dealt with this and demonstrated that the sun's gravity dominates the moon's motion, so he regarded the moon as a co-orbital planet in its own right. At a recession rate of about 4 cm/yr (not constant, of course), in the five billion years or so remaining before the sun becomes a red giant, the moon will have moved off to a great extent and it's ties to the earth would be extremely weak. I wonder if orbital resonance with Venus might be possible, and if that could precipitate large orbital changes, possibly a collision. It would be nice to be able to simulate this chain of events and see what is likely to happen.

[David Hall]

There's a slightly larger version of the libration animation at the APOD.

http://antwrp.gsfc.nasa.gov/apod/ap991108.html

I've had a thought about the libration myself recently. I've been trying to visualize it, but I just can't get my brain around it. The Moon's orbit is an elipse, albiet a small one. I'm sure the orbit must be subject to precession to some extent. Since the libration is due to the eccentricity of the orbit, would precession alter our view in any way? Would we eventually start to see more of one side or the other? Or will we always see the same sides only.

At first I thought the former, but I'm more inclined to think that the orientation of the elipse is immaterial to our view. I hope someone can help me out here.

[DaveC]

On 2002-06-21 12:52, David Hall wrote:
The Moon's orbit is an elipse, albiet a small one. I'm sure the orbit must be subject to precession to some extent. Since the libration is due to the eccentricity of the orbit, would precession alter our view in any way? Would we eventually start to see more of one side or the other? Or will we always see the same sides only.

I think the precession cycle is something like 18 years - which is the reason we have periodic eclipses of the sun. All precession does is change the axis of the elliptical orbit relative to the line between earth and sun. The libration you obseve in the links provided here are due to three factors that don't relate to the orientation of the ellipse:
- the 5 degree inclination of the moon's orbit
- its rotation lags behind or surges ahead of its orbital speed depending on where it is in the ellipse
- parallax when the moon is viewed from opposite edges of the earth

so- I'd guess your second instinct was correct. Precession won't change what we see.

[traztx]

On 2002-06-21 12:52, David Hall wrote:
I've had a thought about the libration myself recently. I've been trying to visualize it, but I just can't get my brain around it...

This might help. Imagine if the Earth were rotating at the same rate as the moon.

Imagine you are standing on the place where the moon is directly above you during perigee.

During perigee, the moon is closer and moving faster, swinging eastward from your point of view. During apogee, the moon is farther and moving slower, swinging westward from your point of view. It appears as if it is moving in circles in the sky.

Now imagine a big parking lot. On row 1, you are parked facing row 100. On row 100 there are 2 cars. One is parked a few spaces to the left. The other is parked a few spaces to the right. You are all facing the same direction, but you can see the left door of one car and the right door of the other. This is similar to what you see as the moon swings east and west.

I hope that helped.
--Tommy

[Kaptain K]

...I wonder if orbital resonance with Venus might be possible...

I don't know ir it qualifies as a "resonance", but the orbital periods of Venus and Earth are very nearly 5:8. [img]/phpBB/images/smiles/icon_smile.gif[/img]

[GrapesOfWrath]

On 2002-06-22 06:36, Kaptain K wrote:
I don't know ir it qualifies as a "resonance", but the orbital periods of Venus and Earth are very nearly 5:8.

It's more like 8:13--still Fibonacci numbers, though. [img]/phpBB/images/smiles/icon_smile.gif[/img]