地球时
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地球时（英语：Terrestrial Time），简称TT，是现代在地球表面上的时间标准，是位于大地海平面体验的原时。在天文学，则为局限在地球上的观测者显示星历表的时间座标。它与地心座标时（Geocentric Coordinate Time）有直接的关联，是天文的地球系统时间标准。由于广义相对论中的引力时间膨胀，地球时的滴答声是恒定的比例，并且比地心座标时缓慢些。
Terrestrial Time
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Terrestrial Time 地球时
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), 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.
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