Barycentric coordinates (astronomy)

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In astronomy, barycentric coordinates are nonrotating coordinates with origin at the center of mass of two or more bodies.

Within classical (Newtonian) mechanics, this definition simplifies calculations and introduces no known problems. In the General Theory of Relativity, problems arise because, while it is possible, within reasonable approximations, to define the barycentre, the associated coordinate system does not fully reflect the inequality of clock rates at different locations. The text Essential Relativistic Celestial Mechanics by Victor A. Brumberg (Adam Hilger, London, 1991) ISBN 0-7503-0062-0 explains how to set up barycentric coordinates in General Theory of Relativity and official specifications for application in the Solar System may be found at these IAU sites: [1], [2]

The coordinate systems involve a world-time, i.e. a global time coordinate that could be set up by telemetry. Individual clocks of similar construction will not agree with this standard, because they are subject to differing gravitational potentials or move at various velocities, so the world-time must be slaved to some ideal clock; that one is assumed to be very far from the whole self-gravitating system. This time standard is called Barycentric Coordinate Time, abbreviated "TCB."