Terrestrial Time | Wikipedia audio article
Channel: wikipedia tts
Date: 2018-12-14

This is an audio version of the Wikipedia Article:
Terrestrial Time


00:01:16 1 History
00:03:02 2 Current definition
00:05:58 3 Realization
00:06:14 4 Approximation
00:08:16 5 Relativistic relationships
00:09:42 6 See also
00:10:13 7 References
00:11:51 8 External links



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SUMMARY
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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 Earth.
For example, the Astronomical Almanac uses TT for its tables of positions (ephemerides) of the Sun, Moon and planets as seen from Earth. In this role, TT continues Terrestrial Dynamical Time (TDT or TD), which in turn succeeded ephemeris time (ET). TT shares the original purpose for which ET was designed, to be free of the irregularities in the rotation of Earth.
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, 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 indirectly underlies UTC, via International Atomic Time (TAI).  Because of the historical difference between TAI and ET when TT was introduced, TT is approximately 32.184 s ahead of TAI.