The Moon's orbit is more eccentric, 0.0549, than most planets. I can understand that planets get eccentric by disturbing each other like under the late heavy bombardment. And likewise for multiple moon systems. But shouldn't the debris after the collision with Theia have been circulirized? Is its orbit being changed by the planets, at every opposition with Venus?

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    $\begingroup$ I expect it has something to do with the asymmetry of the earth's geoid: principles.ou.edu/earth_figure_gravity/geoid It's hard to maintain a perfectly symmetric orbit when you're not orbiting a perfect sphere. $\endgroup$ – Wayfaring Stranger Aug 14 '15 at 1:27
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    $\begingroup$ I don't know how much a factor that would be. The earth, while lumpy, still has a center of gravity that the moon orbits. I think a much larger factor is gravitational perturbations in n-body systems. The moon in particular is tugged on by the sun and in fact, if the moon was twice as far from the earth as it is, it would likely get a highly irregular orbit prior to escaping from the earth. Earth's True region of stability (see link) isn't that much further than the moon is currently. en.wikipedia.org/wiki/Hill_sphere#True_region_of_stability $\endgroup$ – userLTK Aug 14 '15 at 11:16
  • $\begingroup$ @userLTK Charon has 0.00 eccentricity, I think that supports the idea that it is interaction with n-bodies causes Luna's eccentricity because the nearly single moon Pluto system never gets close to any third gravity well. So early on when the Moon was much nearer, it might have been more circular? Could there be any tendency to orbital resonance of the Moon's orbit, or its orbit's precession, with other planets? $\endgroup$ – LocalFluff Aug 14 '15 at 11:39
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    $\begingroup$ Charon-Pluto or Pluto-Charon is an interesting system cause they have 4 much smaller moons orbiting around them which don't seem to affect the eccentricity of Pluto-Charon at all, believed to be the result of a collision. It's an interesting system to me. Not strictly related, but you might enjoy this video and article: latimes.com/science/sciencenow/… Nix football shape clearly affects it's rotation significantly. Not sure what effect that has on it's orbit. $\endgroup$ – userLTK Aug 14 '15 at 12:03
  • $\begingroup$ I got the Hill-Sphere wrong, cause I looked at the chart, didn't read the article. Earth's Hill Sphere is about 1.5 million KM, so the true region of stability, 1/3rd to 1/2 of that is 500 to 750,000 KM, the moon, gets no further than about 405,000 KM, so it's safely stable. But not so stable as to not be subject to perturbation. $\endgroup$ – userLTK Aug 14 '15 at 12:54

First, your claim that the moon is more eccentric isn't entirely true (lets ignore Pluto):


Mercury .205
Mars .094
Saturn .057
Moon .055
Jupiter .049
Uranus .046
Earth .017
Neptune .011
Venus .007

So, it's really not abnormal, though it might be unusually eccentric for a non captured moon, but that's may be due to it being as close to the sun as it is.

Also, our solar system might not be exactly normal. It's fairly common for solar systems to have a gas giant close to the sun (explained here). And there are some very odd solar systems - same link.

  • $\begingroup$ Luna is in the middle, okay. But Venus is closer to the Sun and still very circular, although tipped over. Mercury and Mars on either side of us, are the most eccentric. It isn't obvious to me how distance to the Sun is correlated with that. Especially since the outer planets are thought to have danced around with each other, but I've never heard of any planet coming close enough to Earth to disturb Luna's eccentricity since its creation. I see no system to this madness. $\endgroup$ – LocalFluff Aug 14 '15 at 13:31
  • $\begingroup$ I don't see a system either, so I agree with you there. Neptune is rather strange too being presumably moved the most out of all the planets, starting out much closer to the sun, but it's orbit is nearly circular. If I was to guess, Neptune, being outside all the other planets, it might not be affected as much, tending towards circular, but that doesn't explain Earth and Venus. It's possible Earth and Venus are more circular than they should be by dumb luck - but that's just a wild guess. Not all perturbations would necessarily create eccentricity, some could undo eccentricity. $\endgroup$ – userLTK Aug 14 '15 at 13:56
  • $\begingroup$ I'd be interesting in knowing how eccentric the moon's orbit is when compared to the earth-moon barycenter instead of the center of earth. $\endgroup$ – BillDOe Aug 19 '15 at 20:07
  • $\begingroup$ I think eccentricity is measured around the barycenter. Source: [en.wikipedia.org/wiki/Barycenter $\endgroup$ – userLTK Aug 19 '15 at 22:00

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