Terrestrial Time
where TT and TCG are linear counts of SI seconds in Terrestrial Time and Geocentric Coordinate Time respectively, formula_2 is the constant difference in the rates of the two time scales, and formula_3 is a constant to resolve the epochs (see below). formula_2 is defined as exactly (In 1991 when TT was first defined, formula_2 was to be determined by experiment, and the best available estimate was ).