Geocentric Coordinate Time

Geocentric Coordinate Time (TCG) was defined1991 byInternational Astronomical Union as one ofreplacements forill-defined Barycentric Dynamical Time (TDB). TCG istimescale which would be produced byclock ticking SI seconds at rest incoordinate frame co-moving withcenter ofearth. The relationship between TCGTerrestrial Time (TT)defined withfully general relativistic metric. TCGintendedbe used asindependent variabletimeall calculations pertainingprecession, nutation,moon,artificial satellites ofearth.

Because TCGdefinedclocks whichnot rotating withsurface ofearthnot ingravitational potential ofearth, TCG ticks faster than TDB (and TTET) by about 7 parts10 billion, or about 20 milliseconds per year. Consequently,valuesphysical constantsbe usedcalculations using TCG differ fromtraditional valuesphysical constants. Adaptinglarge bodyexisting softwarechange from TDBTCG isformidable task,as2002 many calculations continueuse TDB.