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Time and frequency: SI unit of time (second)
SI unit of time (second)
SI Brochure, Section

    The unit of time, the second, 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, Resolution 9) 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, Resolution 1) 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 caesium 133 atom.

    It follows that the hyperfine splitting in the ground state of the caesium 133 atom is exactly 9 192 631 770 hertz, nu(hfs Cs) = 9 192 631 770 Hz.

    At its 1997 meeting the CIPM affirmed that:

      This definition refers to a caesium 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 caesium 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.

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Decisions relating to the base units of the SI
SI Brochure, from Appendix 1


    CIPM, 1956 definition of the second as a fraction of the tropical year 1900
    11th CGPM, 1960 ratifies the CIPM 1956 definition of the second
    CIPM, 1964 declares the caesium 133 hyperfine transition to be the recommended standard
    12th CGPM, 1964 empowers the CIPM to investigate atomic and molecular frequency standards
    13th CGPM, 1967/68 defines the second in terms of the caesium transition
    CCDS, 1970 defines International Atomic Time, TAI
    14th CGPM, 1971 requests the CIPM to define and establish International Atomic Time, TAI
    15th CGPM, 1975 endorses the use of Coordinated Universal Time, UTC
    CIPM, 2006 secondary representations of the second
    23rd CGPM, 2007 on the revision of the mise en pratique of the definition of the metre and the development of new optical frequency standards
    CIPM, 2009 updates to the list of standard frequencies
    24th CGPM, 2011 on the possible future revision of the International System of Units, the SI
    24th CGPM, 2011 on the revision of the mise en pratique of the metre and the development of new optical frequency standards
    CIPM, 2013 updates to the list of standard frequencies
    CIPM, 2015 updates to the list of standard frequencies

    [updated 2016]
Time and frequency
SI unit of time (second)
Committee structure
Reference time scales
What time is it?
Strategic plan
International equivalence
Calibration and measurement capabilities (CMCs)
International comparisons
At the BIPM
BIPM technical work
BIPM calibrations for time transfer
BIPM Annual Report on Time Activities
Recent BIPM publications in the scientific literature
FTP server of the BIPM Time Department
BIPM Time Department Database
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IERS Conventions Center
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