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:
“The true reformer does not want time, nor money, nor coöperation, nor advice. What is time but the stuff delay is made of? And depend upon it, our virtue will not live on the interest of our money. He expects no income, but outgoes; so soon as we begin to count the cost, the cost begins. And as for advice, the information floating in the atmosphere of society is as evanescent and unserviceable to him as gossamer for clubs of Hercules.”
—Henry David Thoreau (1817–1862)