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Terrestrial Time (TT) is the modern time scale that is used instead of ephemeris time, which is obsolete.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 relativityGeneral relativity (GR or general relativity theory (GRT is the theory of gravitation published by Albert Einstein in 1915. The conceptual core of general relativity, from which its other consequences largely follow, is the Principle of Equivalence which. At perihelion the earth moves faster and is also deeper in the sunThe Sun (also called Sol is the star in our solar system. Planet Earth orbits the Sun. Other bodies that orbit the Sun include other planets, asteroids, meteoroids, comets and dust. Not all objects passing through the solar system have been orbitally capt's gravitational potential well, and both of these effects slow the rate of clockA clock (from the Latin cloca " bell") is an instrument for measuring time. A clock can be a physical instrument (an especially accurate one is called a chronometer). The clock in its modern form (24 hour clock) has been in use since at least the 15th cens on the earth. At aphelion in July the opposite holds. The international communities of precision timekeeping, astronomyAstronomy which etymologically means " law of the stars," (from Greek: + nomos) is a science involving the observation and explanation of events occurring outside Earth and its atmosphere. It studies the origins, evolution, physical and chemical propertie, and radioFor other uses see: radio (disambiguation Radio is a technology that allows the transmission of signals by modulation of electromagnetic waves with frequencies below those of light. Radio waves Radio waves are a form of electromagnetic radiation, and are broadcasts are preparing to create a new timescale based on observations of an ensemble of pulsarCrab Nebula pulsar, showing surrounding nebular gases stirred by the pulsar's magnetic field and radiation. A pulsar which originally stood for pulsating radio source is a rapidly rotating neutron star, whose electromagnetic radiation is observed in reguls. 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: