[time-nuts] Why 9,192,631,770 ??

[Don Lewis]

I've gotten myself confused ...easily done when you're not a physicist (just
a lowly EE.)   :-)

I am studying cesium clock design and trying to learn how these complex
instruments actually work. I seem to be getting the understanding on the
'workings' of the clock ...the tube, the counters, disciplining, etc.

But I have a higher level question I need help understanding. (Remember
...not a physicist here.)

Who decided 9,192,631,770 cycles of 'light' constitute one second?  I don't
mean who the person was, or which company or institution or when. 

I ask, why not ...771 cycles ...or ...669 cycles????

What I need is the understanding of what a 'second' is defined to be without
having the definition of a 'second' to begin with.

This seems all so recursive to me.

Thank you.  (remember - - you guys said there were no dumb questions - - or
at least, we should ask them anyway.)