barycenter

(center of mass)
(center of mass of two orbiting bodies)

Barycenter is the center of mass of two or more bodies orbiting each other, specifically the one point from which the products of the mass of each body times the vector from the point to the body's position sum to a zero-length vector. For some purposes, all the bodies can be treated as a single mass at the barycenter. It may be within one of the bodies, which happens when one body is massive and the orbital radius is sufficiently small. For example, the barycenter of the Earth and Moon is some distance from the center of the Earth toward the moon, yet within the Earth.

The adjective barycentric means having to do with the center of mass. Barycentric coordinates are coordinates using a barycenter as the origin (i.e., a center of mass frame of reference) and can be useful in analyzing orbits.

Classical mechanics uses a simple ratio of masses to locate the barycenter (mass × distance to the barycenter is the same for each of the bodies), but relativity affects its location, so under extreme circumstances, the classical calculation can be significantly off.

(physics,orbits,mass)
http://en.wikipedia.org/wiki/Barycenter

Referenced by:
International Celestial Reference System (ICRS)
orbital speed
reduced mass
time standard

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