Is it something we just made up? Or is it something that was discovered?
In: 1
Originally the second was based on nothing more than the divisions of a 24 hour cycle in base 12.
However, more recently (1967) the second is defined as follows:
>The second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom
Not very eli5, but it states that we now have true objective measurement for the unit, instead of just a division of man made units.
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