Le site de vulgarisation scientifique de l’Université de Liège. ULg, Université de Liège
International atomic time (TAI)

Time scale founded on the frequency of radiation emitted by caesium-133 atoms. A second is  precisely defined as being the duration of 9192631770 periods of the radiation corresponding to the transition between the hyperfine levels F=3 and F=4 of the fundamental state of the caesium-133 atom.

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