# Earth-Moon center of mass?

1. Dec 16, 2005

### dimensionless

Does the center of mass between the earth and the moon follow an elliptical path around the sun? Does it wobble a little bit?

2. Dec 16, 2005

### pervect

Staff Emeritus
The exact path of the center-of-mass of the Earth-moon system is very slightly perturbed from an ellipse by the gravitational attraction of the other planets. However, it is very close to an ellipse because this perturbing influence is very small.

3. Dec 16, 2005

### Homer Simpson

Wouldnt the CoM between Earth and Moon wobble each month (or moon cycle) as it follows the elliptical path caused by the moon revolving once around the earth??

4. Dec 16, 2005

### dimensionless

I know that the center of mass would be constant if they were both just orbiting each other.

Could the sun perturb this?

Last edited: Dec 16, 2005
5. Dec 16, 2005

### D H

Staff Emeritus
No, because the Earth is also orbiting about the Earth-Moon CoM.

6. Dec 16, 2005

### Homer Simpson

Last edited by a moderator: Apr 21, 2017
7. Dec 16, 2005

### Homer Simpson

Sorry to butt in with my own question, but can you explain this? (Not the actual tides part, but how it causes only one side of the moon to face earth. I always thought this was just a fluke, that the moon revolves once on its own axis for every one rev around the earth. Thinking about it, that would have to be one precise fluke!!)

I would guess that the CoG of the moon is off center, closer to the Earth/Moon CoG due to the bulging effect of the earth on the moon.

Is there any good net sources for general learning on this stuff you would suggest?

Last edited: Dec 16, 2005
8. Dec 16, 2005

### D H

Staff Emeritus
While the Moon does have some rather high non-spherical mass concentrations, it is primarily the second zonal harmonic (the "J2" term) that leads to the Moon's tidally-locked revolution. The second zonal harmonic by itself does not result in an off-center center of mass.