Question

how do we know a second is 9,192,631,770 cycles?

Answer 1

The current SI Definition of a second

```
The second is the duration of 9192631770 periods of the radiation
corresponding to the transition between the two hyperfine levels
of the ground state of the caesium 133 atom.
```
This definition refers to a caesium atom at rest at a temperature of 0 K.

Answer 2

In my attempt at brevity, I think I may have been too vague. I do understand the current measurement of a "second".

My question is, how do we know how many cycles a second is measured in, unless we already know the duration of a second?

In looking around I am getting closer to the answer I am looking for, but input is still welcome... and needed.