Why is a second equal to the duration of 9,192,631,770 periods of radiation corresponding to the transition between two hyperfine levels of the ground state of the caesium-133 atom?
Why is the number of periods so complicated?
It could be any simple number, why is it exactly 9,192,631,770?
Answer
That number, 9192631770, was chosen to make the new definition of the second as close as possible to the less precise old second definition. This meant that except for very precise measurements instruments calibrated before the new second was defined would not have to be recalibrated.
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