### barycenter

**(center of mass, CM)**
(center of mass of two orbiting bodies)

**Barycenter** is the **center of mass** (i.e., **CM**)
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 this point to that body's position sum to a zero-length
vector.
For a system of just two bodies, this simplifies to:
mass × distance from the barycenter is the same for each.
For some purposes, such as from a long distance away from the
orbiting bodies, all of them together can be treated as a single
mass at the barycenter, a simplification in calculations.
This is the classical-mechanics calculation of the barycenter,
but relativity affects its location, and under extreme
circumstances, a classical calculation can be significantly off.

A system's barycenter may fall within one of the bodies,
which happens when one body is far more massive than the other(s)
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.

While a barycenter is a **center of mass**, the latter term also applies
to a single object: the Earth has a center of mass, undoubtedly
very close to a point half-way between the poles, but due to surface
topology and density differences within the Earth, is not exactly
at that point.

The adjective **barycentric** means "having to do with the barycenter".
In astronomy, **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. (Outside astronomy,
the same phrase, **barycentric coordinates** has a different meaning.)

The related term, **center of gravity** (**CG**) is sometimes used
for the *center of mass*, but more accurately refers to the effect
of gravity on an object, i.e., if the object is suspended at
that point, the surrounding gravitational field exerts no torgue
on the object (force to turn it). If the field is not uniform, an
object's center of gravity may not match its center of mass: this
happens to an insignificantly-small degree virtually everywhere and
can be significant in extreme circumstances.

(*physics,orbits,mass,astronomy*)
**Further reading:**

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

**Referenced by pages:**

apsis

International Celestial Reference System (ICRS)

J_{2}

Julian date (JD)

orbital speed

primary

reduced mass

stellar mass determination

tidal force

time standard

terrestrial time (TT)

Index