This article is about terrestrial time; for other meanings of TT, see TT (disambiguation).

Terrestrial Time (TT) is the modern time scale that is used instead of ephemeris time, which is obsolete.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 relativity for an observer on the surface of the Earth. TT is a new name for, and is equivalent to, Terrestrial Dynamical Time (TDT).

TDT was defined in 1976 by the IAU to be the counterpart of Barycentric Dynamical Time (TDB) as measured by clocks ticking SI seconds on the surface of the earth. TDT was intended to be a theoretically ideal representation of International Atomic Time (TAI). Subsequently the IAU decided that the name of TDT was a misnomer because it did not correspond directly to anything dynamical in the theories of motion for bodies in the solar system. In 1991 the IAU renamed TDT to be simply Terrestrial Time (TT).

The distinction between TT and TAI is that the rules for computing TAI have been changed several times; at one point even the rate of TAI was changed in a discontinuous fashion. The rates of TT and TDB are defined such that they deviate only by periodic terms due to the orbital motion of the earth with respect to the solar system barycenter. When the earth is at perihelion in January TDT ticks more slowly than TDB because of the combined effects of special relativity and general relativity. At perihelion the earth moves faster and is also deeper in the sun's gravitational potential well, and both of these effects slow the rate of clocks on the earth. At aphelion in July the opposite holds. 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.

For most practical purposes, TT can be calculated as follows:

TT = TAI + 32.184 s

See also

• Coordinated Universal Time
• Universal Time
• Sidereal Time
• International Atomic Time