# Geocentric Coordinate Time

**Geocentric Coordinate Time** (

**TCG**) was defined in

1991 by the

International Astronomical Union as one of the replacements for the ill-defined

Barycentric Dynamical Time (TDB). TCG is the timescale which would be produced by a clock ticking

SI seconds at rest in a coordinate frame co-moving with the center of the earth. The relationship between TCG and

Terrestrial Time (TT) is defined with a fully general

relativistic metric. TCG is 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.

Because TCG is defined for clocks which are not rotating with the surface of the earth and not in the gravitational potential of the earth, TCG ticks faster than TDB (and TT and ET) by about 7 parts in 10 billion, or about 20 milliseconds per year. Consequently, the values of physical constants to be used with calculations using TCG differ from the traditional values of physical constants. Adapting the large body of existing software to change from TDB to TCG is a formidable task, and as of 2002 many calculations continue to use TDB.