High Quality Content by WIKIPEDIA articles! 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).