because
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.
A day is how long the earth takes to rotate once and that in our time is 24 hours [well, 23 hours 56 mins and 4 seconds] and is fixed.
In internet time [beats] it is 1000 beats exactly. Different units give different answers.
One last example - the distance between say, New York & Miami is fixed but can be expressed in Meters or Yards but neither the meter nor yard were defined by that distance.
I hope that makes some sense.
Why is the second not defined by length of the day?
2 answers
Oh and I don't think this is a Chemistry question.