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Time is defined operationally to be that which is measured by clocks. The SI unit of time is the second, which is defined to be "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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What is time measured against? [duplicate]

So I was wondering what is time measured against? I don't know if my question is clear enough, but how do you measure one second? …
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