Concept
Barycentric Coordinate Time (TCB, from the French Temps-coordonnée barycentrique) is a coordinate time standard intended to be used as the independent variable of time for all calculations pertaining to orbits of planets, asteroids, comets, and interplanetary spacecraft in the Solar System. It is equivalent to the proper time experienced by a clock at rest in a coordinate frame co-moving with the barycenter (center of mass) of the Solar System: that is, a clock that performs exactly the same movements as the Solar System but is outside the system's gravity well. It is therefore not influenced by the gravitational time dilation caused by the Sun and the rest of the system. TCB is the time coordinate for the Barycentric Celestial Reference System (BCRS). TCB was defined in 1991 by the International Astronomical Union, in Recommendation III of the XXIst General Assembly. It was intended as one of the replacements for the problematic 1976 definition of Barycentric 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. The transformation between TCB and Geocentric Coordinate Time (TCG) may be approximated with an uncertainty not larger than in rate as: where and are the barycentric coordinate position and velocity of the geocenter, with the barycentric position of the observer, , is the origin of TCB and TCG defined so that 1977 January 1, 00:00:00 TAI is 1977 January 1, 00:00:32.184 TCG / TCB, is the sum of gravitational potentials for all solar system bodies apart from the Earth evaluated at the geocenter, and is similarly the sum . The approximation discards higher powers of as they have been found to be negligible. 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 1.550505 × 10−8 (about 490 milliseconds per year).