The modern definitions of units feel even more arbitrary because they are inextricably tied to more organic origins. Consider the often made fun of fahrenheit scale which was the first to define a reasonably repeatable degree size by using two widely available reference points as the 100 and 0 ends of the scale, human body temperature for the high end and an ammonium chloride ice water mix for the low end.

The definition of a second was a bit jankier. Etymologically the name comes from a second hand added to a watch face to give some kind of indicator that the minutes are passing by. NIST has an excellent writeup on this subject. Over time different repeatable ways to measure a second have been determined all with the goal of having some action a human could use to calibrate their device’s second measurement to so their seconds are as long as everyone else’s.

The point is, we didn’t choose a second to be defined as some number of atomic oscillations. We had an already agreed upon definition of a second that used less precise methods than modern technology demanded and used a natural phenomenon that could be very accurately measured to make a less arbitrary definition.

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