UT is a measure of the rotation of the Earth. Atomic clocks and other techniques have revealed that the rotation of the Earth is slightly irregular. Astronomers need several kinds of time scale, including a time scale that is as uniform as possible. Ephemeris Time was the first effort at a uniform time scale; it was the independent variable in the equations of motion of the bodies in the solar system. Later it was replaced by TT. For practical purposes, TT is the same as International Atomic Time (TAI), except there is an offset so the value of TT matches the value of ET on 0 h 0 m 0 s January 1, 1977 TAI (see Resolution A4, Recommendation III in the XXIst General Assembly of the International Astronomical Union (1991).