- published: 06 Oct 2015
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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 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).
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).
Sometimes times described in TT must be handled in situations where TT's detailed theoretical properties are not significant. Where millisecond accuracy is enough (or more than enough), TT can be summarized in the following ways:
"Terrestrial" Time Lapse Doodle
AstroSynth - Terrestrial Time
Extra Terrestrial time lapse sculpting
[Samsung SMART CAMERA NX30] Terrestrial Wonders Time-Lapse
The Time I Met An Extra-Terrestrial
E.T. the Extra-Terrestrial | Screaming Down The House: Michael meets E.T. for the first time
Astronauts try extra-terrestrial lettuce for the first time
Terrestrial Time
The Spider's Lair - Terrestrial Enclosure Time Lapse
3D time laps displacement by Terrestrial SAR Interferometry
E.T. The Extra-Terrestrial OST Give Them Time
How to Carve E.T. the Extra-Terrestrial Pumpkin - Halloween Time Lapse
Seth Shostak on SETI (Search for Extra Terrestrial Intelligence)
Time Dilation - "Terrestrial at Heart"