Atomic second

For a long time, the second was defined as 1/86,400 part of the mean solar day whose irregularities, caused by factors including the slowing rotation of the Earth, were demonstrated by the atomic clock. At the 13th General Conference on Weights and Measures in 1967 in Paris, the second was redefined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom.