1-3 The standard of time We can use any phenomenon that repeats itself as a measure of time One second was defined to be 1/86400 of one day In 1967, the 13th CGPM adopted a new definition The second is the duration of 9. 192, 631.770 vibrations of a(specified) radiation emitted by a (specified) isotope of the cesium atom1-3 The standard of time • We can use any phenomenon that repeats itself as a measure of time. • In 1967, the 13th CGPM adopted a new definition: • One second was defined to be 1/86400 of one day. The second is the duration of 9,192,631,770 vibrations of a (specified) radiation emitted by a (specified) isotope of the cesium atom