Terrestrial Time (TT) is the modern time scale that is used instead of ephemeris time, which is obsolete.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. TT is a new name for, and is equivalent to, Terrestrial Dynamical Time (TDT) which had been defined by the IAU in 1976. For most practical purposes, TT can be calculated as follows:
The IAU recognizes that TAI is a statistical time scale which has changed the rules for its construction several times during its history. The international communities of precision timekeeping, astronomy, and radio broadcasts are preparing to create a new timescale based on observations of an ensemble of pulsars. This new pulsar timescale will serve as an independent means of computing TT, and it may eventually be useful to identify defects in TAI.
