Second Definition

Definition of the Second

Originally, a second was defined as 1 / 86,400 of a mean solar day.
Irregularities in the rotation of the earth made this general definition problematic, so the following more precise definition was adopted.
In 1960, the second was defined as was defined as 1 / 86,400 of a mean solar day in the year 1900.
Even in 1900, scientists had shown that the frequencies of radiation emitted by transitions of electrons between energy levels within an atom could be measured very precisely.
In 1968, the second was defined as 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. The cesium atom must be at rest at a temperature of absolute zero.
In practice, a temperature of absolute zero is not attainable, so the value of the duration must be extrapolated from the actual temperature.