Caesium standard
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A caesium standard is a primary frequency standard in which electronic transitions between the two hyperfine ground states of caesium-133 atoms are used to control the output frequency.
By the definition of the SI second, the frequency of the radiation in a transition between the two hyperfine ground states corresponds, in the absence of external influences (e.g., the Earth's magnetic field), to a frequency of 9,192,631,770 Hz.
The number 9,192,631,770 was chosen so that the cesium second equaled, to the limit of human measuring ability in 1960 when it was adopted, the existing standard ephemeris second based on the earth's orbit around the sun. Because no other measurement involving time had been as precise, the effect of the change was less than the experimental uncertainty of all existing measurements.
Source: from Federal Standard 1037C and from MIL-STD-188
