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. 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), which in turn succeeded ephemeris time (ET). TT shares the original purpose for which ET was designed, to be free of the irregularities of mean solar time.
The unit of TT is the SI second, the definition of which is currently based on the caesium atomic clock, 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).
Read more about Terrestrial Time: Approximation, History, Current Definition, Realization, Relativistic Relationships
Famous quotes containing the word time:
“Keep your own secret, and get out other people’s. Keep your own temper, and artfully warm other people’s. Counterwork your rivals with diligence and dexterity, but at the same time with the utmost personal civility to them: and be firm without heat.”
—Philip Dormer Stanhope, 4th Earl Chesterfield (1694–1773)