### 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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