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The standard for the second was once based on a fraction of a solar day. Why...

The standard for the second was once based on a fraction of a solar day. Why is this is not a good standard?

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The standard for the second is not good because irregularities in rotation of earth could not be taken into account by astronomical theory and it also does not allow required accuracy to be achieved. Therefore, to define unit of time more precisely, the previous definition of a second had to be changed and now its:-

Second is the duration of 9 192 631 770 periods of radiation which correspond to the transition between two hyperfine levels of the ground state of cesium 133 atom.

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