- Barycentric Coordinate Time
**Barycentric Coordinate Time**(TCB) is a coordinatetime standard intended to be used as the independent variable of time for all calculations pertaining to orbits ofplanet s,asteroid s,comet s, and interplanetaryspacecraft in theSolar system . It is equivalent to theproper time experienced by a clock at rest in a coordinate frame co-moving with thebarycenter of the Solar system: that is, a clock that performs exactly the same movements as the Solar system but is outside the system'sgravity well . It is therefore not influenced by thegravitational time dilation caused by theSun and the rest of the system.TCB was defined in 1991 by the

International Astronomical Union , in [*http://www.iers.org/MainDisp.csl?pid=98-133 Recommendation III of the XXIst General Assembly*] . It was intended as one of the replacements for the ill-definedBarycentric Dynamical Time (TDB). Unlike former astronomical time scales, TCB is defined in the context of the general theory of relativity. The relationships between TCB and other relativistic time scales are defined with fully general relativistic metrics.Because the reference frame for TCB is not influenced by the

gravitational potential caused by the Solar system, TCB ticks faster than clocks on the surface of the Earth by about 1.6 × 10^{−8}(about 490 milliseconds per year). Consequently, the values of physical constants to be used with calculations using TCB differ from the traditional values of physical constants. (The traditional values were in a sense wrong, incorporating corrections for the difference in time scales.) Adapting the large body of existing software to change from TDB to TCB is a formidable task, and as of 2002 many calculations continue to use TDB in some form.Time coordinates on the TCB scale are conventionally specified using traditional means of specifying days, carried over from non-uniform time standards based on the rotation of the Earth. Specifically, both

Julian Date s and theGregorian calendar are used. For continuity with its predecessorEphemeris Time , TCB was set to match ET at around Julian Date 2443144.5 (1977-01-01T00Z). More precisely, it was defined that TCB instant 1977-01-01T00:00:32.184 exactly corresponds to the TAI instant 1977-01-01T00:00:00.000 exactly, at the geocenter. This is also the instant at which TAI introduced corrections for gravitational time dilation.

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