# Terrestrial Time

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Terrestrial Time (TT) is a modern astronomical time standard defined by the International Astronomical Union, primarily for time-measurements of astronomical observations made from the surface of the Earth.[1] For example, the Astronomical Almanac uses TT for its tables of positions (ephemerides) of the Sun, Moon and planets as seen from the Earth. In this role, TT continues Terrestrial Dynamical Time (TDT),[2] which in turn succeeded ephemeris time (ET).

The unit of TT is the SI second, the definition of which is currently based on the cesium atomic clock,[3] but TT is not itself defined by atomic clocks. It is a theoretical ideal, which real clocks can only approximate.

TT is distinct from the time scale often used as a basis for civil purposes, Coordinated Universal Time (UTC). TT indirectly underlies UTC, via International Atomic Time (TAI).

## Contents

### Approximation

Sometimes times described in TT must be handled in situations where TT's detailed theoretical properties are not significant. Where millisecond accuracy is enough (or more than enough), TT can be summarized in the following ways:

• To millisecond accuracy, TT runs parallel to the atomic timescale (International Atomic Time, TAI) maintained by the BIPM. TT is ahead of TAI, and can be approximated as TT ~= TAI + 32.184 seconds.[4] (The offset 32.184 s arises from the history.[5])
• TT also runs in parallel with the GPS time scale, which has a constant difference from atomic time ( (TAI - GPS time) = +19 seconds),[6] so that TT ~= GPS time + 51.184 seconds.
• TT is in effect a continuation of (but is more precisely uniform than) the former Ephemeris Time (ET). It was designed for continuity with ET,[7] and it runs at the rate of the SI second, which was itself derived from a calibration using the second of ET (see, under Ephemeris time, Redefinition of the second and Implementations.) TT shares the original purpose for which ET was designed, to be free of the irregularities of mean solar time.
• TT runs a little ahead of UT1 (a refined measure of mean solar time at Greenwich) by an amount known as deltaT = TT - UT1. DeltaT was measured at +65.7768 seconds (TT ahead of UT1) at 0h UTC on 1 January 2009;[8] and by retrospective calculation, deltaT was close to zero around the year 1900. The difference deltaT, though somewhat unpredictable in fine detail, is expected to continue to increase, with UT1 becoming steadily (but irregularly) further behind TT in the future.

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