Question

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

Answer 1

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The length of the day is not a fixed unit of time.

The earth itself is wobbling through space, traveling on a very slight elliptical path and the rotation is slowing down because of tidal action and the moon moving ever so slightly away.

The length of the day not only changes throughout the year but about every 18 years or so, and extra leap second is added to the calendar.

The second is defined using a number of rotations of the cesium 133 atom at 0 Kelvin because apart from quantum mechanics, it isn't subject to extraneous factors.

Answer 2

ty

Answer 3

Anytime :)