The Classical Cesium Standard

We will now take up the type of atomic clock that has been elevated to the status of the primary standard of time, displacing the historical role of astronomical observations in the definition of the unit of time, the second. In 1967 the 13th General Conference on Weights and Measures, attended by delegates from about 40 countries, signatories of the Treaty of the Meter, adopted a new definition of the international unit of time. At that conference there was overwhelming support to the idea that the time had come to replace the existing definition, based on the earth's orbital motion around the sun, by an atomic definition. The wording of the new definition is as follows: “The second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the fundamental state of the atom of cesium-133.” The ten-digit number assigned in the definition was chosen to agree with the then existing definition of the second, known as the “ephemeris second,” which had been adopted in 1956. This latter definition was based on the length of the so-called tropical year, that is, the length of time for the earth to complete its orbit around the sun and return to a point where its axis again makes the same angle with respect to the earth-sun direction; it is the repetition period of the seasons. The obvious drawback to this definition is the practical one of not being available except through the intermediary of stable clocks that must be checked after the fact.