Distance or time for example. I look at my watch and I can see how long 1 second takes. I can look at a ruler and see how long 1 centimeter is. But how do they make rulers and watches? How do you define what a centimeter or a second is without just saying “1/10 of a decimeter” or “1/60 of a minute” or just pointing at another ruler/watch?
I guess time is easier since you can just reference recurring events (like moon phases for example) and then go down in scale from there until you get hours, minutes, seconds. But distance just seems completely arbitrary.
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The exact definition of what a “second” is has been refined over and over throughout history.
Right now, our definition of a second is measured using the time that elapses between 9.192631770 x 109 cycles of radiation produced by the transition between two levels of Caesium-133
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