Why has second been defined in terms of periods

unitsandmeasurement
cbse

#1

Why has second been defined in terms of periods of radiations from cesium-133?


#2

Second has been defined in terms of periods of radiation, because
i) this period is accurately defined.
(ii) this, period is not affected by change of physical conditions like temperature, pressure and volume etc.
(iii) the unit is easily reproducible in any good laboratory.