# Second (unit of time)

The unit of time, the **second**, abbreviated **s**, was at one time considered to be the fraction 1/86.400 of the mean solar day. The exact definition of “mean solar day” was left to the astronomers. However, measurements showed that irregularities in the rotation of the Earth made this an unsatisfactory definition. In order to define the unit of time more precisely, the 11th CGPM (1960) adopted a definition given by the International Astronomical Union based on the tropical year 1900.

Experimental work, however, had already shown that an atomic standard of time, based on a transition between two energy levels of an atom or a molecule, could be realized and reproduced much more accurately. Considering that a very precise definition of the unit of time is indispensable for science and technology, the 13th CGPM (1967/68) replaced the definition of the second by the following:

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 ground state of the cesium 133 atom.

It follows that the hyperfine splitting in the ground state of the cesium 133 atom is exactly 9.192.631.770 hertz, \(\nu (^{133}\textrm{Cs})_{\textrm{hfs}}\) = 9.192.631.770 Hz. The symbol \(\nu (^{133}\textrm{Cs})_{\textrm{hfs}}\) is used to denote the frequency of the hyperfine transition in the ground state of the cesium atom.

At its 1997 meeting, the CIPM affirmed that: This definition refers to a cesium atom at rest at a temperature of 0 K.

This note was intended to make it clear that the definition of the SI second is based on a cesium atom unperturbed by black body radiation, that is, in an environment whose thermodynamic temperature is 0 K. The frequencies of all primary frequency standards should, therefore, be corrected for the shift due to ambient radiation, as stated at the meeting of the Consultative Committee for Time and Frequency in 1999.