# Barycentric Coordinate Time

- Barycentric Coordinate Time
**Barycentric Coordinate Time** (TCB) 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 [*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-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 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 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.

*Wikimedia Foundation.
2010.*

### Look at other dictionaries:

**Barycentric Dynamical Time** — (TDB) was a time standard used to take account of time dilation when calculating orbits of planets, asteroids, comets and interplanetary spacecraft in the Solar system. It was based on a Dynamical time scale but was not well defined and not… … Wikipedia

**Coordinate time** — In the theory of relativity, it is convenient to express results in terms of a spacetime coordinate system relative to an implied observer. In many (but not all) coordinate systems, an event is specified by one time coordinate and three spatial… … Wikipedia

**Geocentric Coordinate Time** — (TCG) is a coordinate time standard intended to be used as the independent variable of time for all calculations pertaining to precession, nutation, the Moon, and artificial satellites of the Earth. It is equivalent to the proper time experienced … Wikipedia

**Barycentric** — can refer to:In astronomy, * Barycentric coordinates (astronomy) are coordinates defined by the common center of mass of two or more bodies * Barycentric Dynamical Time was a time standard in the Solar system * Barycentric Coordinate Time is a… … Wikipedia

**Time from NPL** — Map showing the location of the Anthorn VLF transmitter within Cumbria … Wikipedia

**Time dilation** — This article is about a concept in physics. For the concept in sociology, see time displacement. In the theory of relativity, time dilation is an observed difference of elapsed time between two events as measured by observers either moving… … Wikipedia

**Time standard** — A time standard is any officially recognized specification for measuring time: either the rate at which time passes; or points in time; or both. For example, the standard for civil time specifies both time intervals and time of day. A time scale… … Wikipedia

**Time** — This article is about the measurement. For the magazine, see Time (magazine). For other uses, see Time (disambiguation). The flow of sand in an hourglass can be used to keep track of elapsed time. It also concretely represents the present as… … Wikipedia

**Barycentric coordinates (astronomy)** — In astronomy, barycentric coordinates are non rotating coordinates with origin at the center of mass of two or more bodies.Within classical mechanics, this definition simplifies calculations and introduces no known problems. In the General Theory … Wikipedia

**Time zone** — Timezone and TimeZone redirect here. For other uses, see Time zone (disambiguation). Local time redirects here. For the mathematical concept, see Local time (mathematics). This article is about time zones in general. For a list of time zones by… … Wikipedia