Greenwich Means Time to the World

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What is the SI unit of time?

Official definition of a second

In 1967 a new SI definition of a second was created based on the radiation from the caesium-133 atom.

It is correctly defined as "the duration of 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the caesium-133 atom."


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