This discovery was made independently by German astronomers and the Observatory’s Nicolas Stoyko (1894-1976), thanks to the recently invented, extremely stable quartz clocks. The irregularities in the rotation of the Earth are due to the motion of large masses of air and water, and are thus unpredictable. The Earth’s rotation had been used to define time, but due to these irregularities it was not a good timekeeper. This led to the introduction in 1964 of  ephemeris time, based on the motion of the Earth around the Sun, and, in 1967, the still more accurate atomic time, based on the frequency of a standard caesium atom.

caption : Nicolas Stoyko - credits : Observatoire de Paris