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:
““A young man in the dark am I
But a wild old man in the light
That can make a cat laugh, or
Can touch by mother wit
Things hid in their marrow bones
From time long passed away....””
—William Butler Yeats (1865–1939)