Frederik's Friday Fact

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Friday 19 Sep 2014: Definition of the second
Since 1967, the second is no longer officially defined as a fraction of a year or a mean solar day. In that year the SI definition of the second was changed to 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 caesium-133 atom.

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