Before revision: Solar System 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.The 1991 definition refers to the scale agreeing with the SI second "on the geoid", i.e.
After revision: 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.The 1991 definition refers to the scale agreeing with the SI second "on the geoid", i.e.