**Quote from guyy on 02/09/08 at 23:26:04:**But this is really just a center-of-mass problem in disguise, so it's pretty simple to calculate yourself (which might be better, since this sounds like a homework question

) using COM = [(Planet mass)*(Planet position) + (Moon mass)*(Moon position)] / (Total mass of system). So in this case, putting the planet at the origin, the center of mass is [(0.61)*(0) + (0.094)*(213,160 km)] / (0.61 + 0.094) kilometers from the planet's center; if that's bigger than 11480, it's above the surface. (You can add the "Earth Mass" part if you want to, but it just cancels out, so the answer is the same.)

So it would simplify to: [ 0 + 20,037.04 ] / [ 0.704 ] = 28,461.705 km (rounded to thousandth)

Assuming that I got that number correct, I would then subtract the radii of the Moon and Planet to get the planet's distance from the center of the system right? (11,480 / 2)+(4840 / 2) = 8160 km so I presume 28,461.705 - 8160 = 20,301.705 km

So the system data would be, assuming all these calculations are correct:

surface-to-surface distance: 205,000 km

center-to-center distance: 213, 160 km

planet surface to barycenter: 20,301.705 km

moon surface to barycenter: 184,698.295 km

Only element I've neglected to add is orbital period, but I do have equations of that which I'll go over (only because I am unsure if I misplace a zero in the moon's mass before), but previously I got 13.204 Days for the orbital period.

Let me know if I got something incorrect in the barycenter/COM equations. I had a feeling, when I realized the planet was only ~6.5 times heavier than its moon (compared to the Earth that's roughly 81 some-times heavier than our moon) that there was a solid chance it'd be a technical double-planet.