Barycentric Coordinate Time

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 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 was defined in 1991 by the International Astronomical Union, in Recommendation III of the XXIst General Assembly.^[1] It was intended as one of the replacements for the ill-defined 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.

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). 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 an ongoing 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 Dates and the Gregorian calendar are used. For continuity with its predecessor Ephemeris 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.

References

↑ IAU(1991) Recommendation III

Time measurement and standards

Chronometry Orders of magnitude Metrology

International standards	UTC UTC offset UT ΔT DUT1 IERS ISO 31-1 ISO 8601 TAI 12-hour clock 24-hour clock Barycentric Coordinate Time Barycentric Dynamical Time Civil time Daylight saving time Geocentric Coordinate Time International Date Line Leap second New Earth Time Solar time Terrestrial Time Time zone

Obsolete standards	Ephemeris time Greenwich Mean Time Prime meridian

Time in physics	Absolute time and space Spacetime Chronon Continuous time Coordinate time Cosmological decade Discrete time Planck epoch Planck time Proper time Theory of relativity Time dilation Gravitational time dilation Time domain T-symmetry

Horology	Clock Astrarium Atomic clock Complication History of timekeeping devices Hourglass Marine chronometer Marine sandglass Radio clock Watch Water clock Sundial Dialing scales Equation of time History of sundials Sundial markup schema

Calendar	Astronomical Dominical letter Epact Equinox Gregorian Hebrew Hindu Intercalation Islamic Julian Leap year Lunar Lunisolar Solar Solstice Tropical year Weekday determination Weekday names

Archaeology and geology	Dating methodologies Geologic time scale International Commission on Stratigraphy

Astronomical chronology	Galactic year Nuclear timescale Precession Sidereal time

Units of time	Century Day Decade Fortnight Hour Jiffy Lustrum Millennium Minute Moment Month Paksha Saeculum Second Shake Tide Week Year

Related topics	Chronology Duration Mental chronometry Metric time System time Time value of money Timekeeper

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Barycentric Coordinate Time

See also

References