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