Editing Terrestrial Time (section)

{{Short description|Time standard for astronomical observations from the Earth}}
{{Use dmy dates|date=July 2021}}
'''Terrestrial Time''' ('''TT''') is a modern astronomical [[time standard]] defined by the [[IAU|International Astronomical Union]], primarily for time-measurements of astronomical observations made from the surface of Earth.<ref>The 1991 definition refers to the scale agreeing with the SI second "on the geoid", i.e. close to mean sea level on Earth's surface, see [http://iau.org/static/resolutions/IAU1991_French.pdf IAU 1991 XXIst General Assembly (Buenos Aires) Resolutions], Resolution A.4 (Recommendation IV).  A redefinition by [http://syrte.obspm.fr/IAU_resolutions/Resol-UAI.htm resolution of the IAU 2000 24th General Assembly (Manchester)], at Resolution B1.9, is in different terms intended for continuity and to come very close to the same standard.</ref>
For example, the [[Astronomical Almanac]] uses TT for its tables of positions ([[ephemeris|ephemerides]]) of the Sun, Moon and planets as seen from Earth. In this role, TT continues '''Terrestrial Dynamical Time''' (TDT or TD),<ref name=iau1991-4-n4>TT is equivalent to TDT, see IAU conference 1991, Resolution A4, recommendation IV, note 4.</ref> which succeeded [[Ephemeris time|ephemeris time (ET)]]. TT shares the original purpose for which ET was designed, to be free of the [[ΔT (timekeeping)|irregularities in the rotation of Earth]].

The unit of TT is the [[SI second]], the definition of which is based currently on the caesium [[atomic clock]],<ref name=iau1991-4-2>IAU conference 1991, Resolution A4, recommendation IV, part 2 states that the unit for TT is to agree with the SI second 'on the geoid'.</ref> but TT is not itself defined by atomic clocks.  It is a theoretical ideal, and real clocks can only approximate it.

TT is distinct from the time scale often used as a basis for civil purposes, [[Coordinated Universal Time]] (UTC).  TT is indirectly the basis of UTC, via [[International Atomic Time]] (TAI).  Because of the historical difference between TAI and ET when TT was introduced, TT is 32.184 s ahead of TAI.