Barycentric Coordinate Time

Barycentric Coordinate Time (TCB) was defined in 1991 by the International Astronomical Union as one of the replacements for the ill-defined Barycentric Dynamical Time (TDB). TCB is the timescale which would be produced by a clock ticking SI seconds at rest in a coordinate frame co-moving with the barycenter of the solar system. The relationship between TCB and Terrestrial Time (TT) is defined with a fully general relativistic metric. As such, TCB is intended to be used as the modern replacement for Ephemeris Time (ET).

Because TCB is defined for clocks which are at rest and not in the gravitational potential of the solar system, TCB ticks faster than TDB (and TT and ET) by about 1.5 parts in 100 million, or about half a second per year. Consequently, the values of physical constants to be used with calculations using TCB differ from the traditional values of physical constants. Adapting the large body of existing software to change from TDB to TCB is a formidable task, and as of 2002 the ephemerides continue to use TDB.