In the late 19th century it was found that the rotation of the Earth (i.e. the length of the day) was both irregular on short time scales, and was slowing down on longer time scales. In fact, observing the position of the Moon, Sun and planets and comparing this with their ephemerides was a better way to determine the time.
atomic (cesium) clockss had become available. Comparing the clock rates with astronomical observations allowed to match atomic clocks with ephemeris time, and a new definition of the second based on cesium clocks was accepted in 1972.
The difference between ET and UT is called Delta-T; it increases irregularly with about half a second per year. International Atomic Time (TAI) was set equal to UT2 at 1 January 1958 0:00:00 . At that time, Delta-T was already about 32.18 seconds. The difference between TT (the successor to ephemeris time) and atomic time was later defined as follows: