Terrestrial Time (
TT) is the modern
time scale that is used instead of
ephemeris time, which is obsolete. TT was introduced by the
International Astronomical Union (IAU) in
1991 as the coordinate
time scale consistent with the theory of
general relativity for an observer on the
surface of the
Earth. For all practical purposes, it is equal to:
TT = TAI +32.184 s