Why is the Moon's center of mass off center? Because of the tidal lock with the Earth?

Answer 1

Yes. The Moon was formed from the Giant Impact after Earth had already been in existence for 40-50 million years. Gravity from the Earth caused the newly formed Moon's mass to be off center slightly towards the direction of Earth. It is 1.2 miles off center. This is further confirmed by the Moon's crust being thinner on the side toward Earth. Heavier elements such as those in the core would be pulled closer towards Earth than the lighter crust elements.

--for morningfox-- here is your url

http://www.nineplanets.org/luna.html...

Answer 2

Probably by the way it formed in Earth's orbit after Nemisis hit Earth, kicking off the material that formed the moon in the first place.

It always keeps one face to Earth *because* of tidal lock, but I couldn't say if it's tidal lock that made the moon's center of gravity not in the center of the moon.

Answer 3

An article published in "Nature", an international Journal of science, confirms that the Moon's centre of mass is displaced eastward (as seen from Earth).

This means that it is not just a tidal effect: a bulge caused by tidal effect should lie on the line that joins Earth's centre and Moon's centre. Thus, the tidal effect could be used to explain why the Moon's centre of mass would be displaced towards us (or away from us) compared to its geometrical centre.

But sideways?

The paper itself has more to do with a kind of shellfish (that lives on Earth) where changes in appearance in the cross-section of the shell are directly linked to the tides and, therefore, to the effect of the Moon on Earth's oceans.

If you find fossils that cover a long enough period, you can find very accurate information about the evolution of the lunar orbit.

Seems these guys have achieved a level of accuracy that allows them to compare the actual tides (a result of the real position of the mass of the Moon) with the tides that can be calculated if the Moon's mass had the same centre as the geometrical Moon.

You can see the reference to the article here:
http://www.nature.com/nature/journal/v275/n5681/abs/275606a0.html

But I have not found the entire text yet.

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I found another one:

Convection in the Moon and the Existence of a Lunar Core
by S. K. Runcorn
Proceedings of the Royal Society of London. Series A, Mathematical and Physical Sciences, Vol. 296, No. 1446, A Discussion on the Physics of the Moon and Its Environment (Feb. 7, 1967), pp. 270-284
This article consists of 15 pages.

Again, it is an article that I cannot read for free (well, not yet). You can see the first page and the abstract at
http://links.jstor.org/sici?sici=0080-4630(19670207)296%3A1446%3C270%3ACITMAT%3E2.0.CO%3B2-F

(or, go to www.scholar.google.com and search for
Moon "centre of mass"

In the abstract, the author writes
"The ellipticity of the earthward bulge, determined statistically from the geometrical librations, is over twice that determined dynamically from Cassini's laws."

No, I'm not sure either what it really means. But it confirms:
- there is an Eastward bulge,
- the bulge can be measured based on statistics of librations -- the libration is the difference between the rate at which the Moon rotates (very constant rate) and the rate at which the Moon orbits Earth (not so constant); there are times where we see "over the eastern edge" and times when we see over the western edge. There is also north-south libration.
- Cassini has already thought about this kind of thing in 1693.
See Cassini's Laws in wikipedia.

His laws appear to be based on the fact that the Moon's centre of mass is located at the Moon's geometric centre. So I guess this means that if you observe any differences (as identified by Runcorn), then there must be a difference.

Answer 4

Well, good question.
The off center of mass is probably due to heavier elements located off center....but I don't believe anyone knows for sure.

I doubt if it has anything to do with tidal lock w/ earth.....Although it is possible since there is an off center tidal torque due to the moon's orbital inclination with respect to the earth's equatorial bulge.....thus the earth's tidal effect on the moon 'may' have contributed to it.

Answer 5

Where did you get the idea that the Moon's center of mass is "off center"? Please give a source. I never heard of such a thing.

And please don't say "I heard it on some TV show". Tell us WHICH TV show.

Answer 6

It's a gravitational lock.