[DManA]

The definition of a second has nothing to do with the rotation of the Earth:

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 cesium 133 atom.

[roadcat]

During my lifetime, the definition of a second had everything to do with the rotation of the Earth, as it did for millenia. Scientists redefined it in 1967. I’m a traditionalist.

[SolidRedState]

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The definition of a second has nothing to do with the rotation of the Earth:
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 cesium 133 atom.

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So we have only been measuring seconds since we were able to figure out the transition in cesium 133 atoms?

I did not know that.

[cynwoody]

The definition of a second has nothing to do with the rotation of the Earth:

This is true.

But an important use of seconds is to tell human local time. As in when does the sun rise, when is it noon, when is sunset?

Those times all depend on the earth's rotation and its orbital period around the sun. Neither of those take orders from the Cesium atom.

Hence, the leap second, a hack to allow the real Cesium second to be used for measurements that actually need its precision, while still allowing high noon to occur precisely when it should.