Explain why the second is not defined by the length of the day.

The day's length is not a settled unit of time.

The earth itself is jiggling around space, going on an amazingly slight bended way and the unrest is dialing down record of tidal movement and the moon moving possibly away.
The day's length changes amid the time and in addition about predictably or something to that effect, and extra hop second is added to the timetable.
The second is portrayed using different unrests of the cesium 133 particle at 0 Kelvin in light of the fact that isolated from quantum mechanics, it isn't at risk to coincidental segments.

Ais the correct answer

It was ernest rutherford