Comment Re:Weird choice of measurements (Score 1) 85
I'm not qualified to explain the details, but it comes down to the fact that we had to pick an easily-reproducible definition of the second, and at the time, the ground-state Cs-133 hyperfine transition was the one that made sense.
Today there are more stable clock designs based on other quantum transitions but the caesium standard serves well enough for almost all purposes that it's not worth the trouble to change it.
There is no such thing as "perfect time," or perfect knowledge of just what time it is, but we have a very good wiorking approximation of it, one that has actually outpaced Moore's Law in terms of increasing stability over the past few decades. Time is still the ultimate basis for virtually all other physical measurements, so it pays to define and understand it as precisely as we can.