[extropy-chat] Re: SPACE: new planet?

[Don Dartfield]

On Sat, 21 Feb 2004, Mike Lorrey wrote:

> Perhaps we have a problem with terms here. What would you call the point
> which both earth and the moon orbit around?

I've usually heard it called "center of gravity" or "center of mass" or
"barycenter."  That point is the one I was referring to -- 4900km from
the center of the earth.  Well, 4877km, apparently:

http://en.wikipedia.org/wiki/Center_of_mass

Obviously that is still inside the earth, but Pluto and Charon, and
Jupiter and the Sun, have barycenters outside of the larger body, so they
really do look like they orbit a common point rather than just having the
larger body appear to wobble while the smaller body orbits it.