Editing Second (section)

===Table===
{| class="wikitable" 
! colspan="3" |Evolution of the Second
|-
! style="width: 40%;" | Decisions of the CIPM
! style="width: 40%;" | Resolution of the CGPM
! style="width: 20%;" | Information
|-style="vertical-align: top;"
|That according to the decisions of the 8th General Assembly of the International Astronomical Union (Rome, 1952), the second of ephemeris time (ET) is the fraction

<math>\frac{12960276813}{408986496}\times10^{-9}</math> of the tropical year for 1900 January 0 at 12 h ET.  
|The second is the fraction <math>\frac{1}{31556925.9747}</math> of the tropical year for 1900 January 0 at 12 hours ephemeris time.
|1956 CIPM
11th CGPM 1960 Resolution 9
|-style="vertical-align: top;"
|The standard to be employed is the transition between the hyperfine levels F=4, M=0 and F=3, M=0 of the ground state <math>^2S_{1/2}</math> of the caesium 133 atom, unperturbed by external fields, and that the frequency of this transition is assigned the value 9192631770 hertz.
|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
|13th CGPM Resolution 1
CIPM 1967
|-style="vertical-align: top;"
|This definition implies that the caesium atom is at rest and unperturbed. In consequence, in its practical realization, measurements must be corrected for velocity of the atoms with respect to the clock reference frame, for magnetic and electric fields including ambient black-body radiation, for spin-exchange effects and for other possible perturbations.
|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 Cs atom unperturbed by black-body radiation, that is, in an environment whose temperature is 0 K, and that the frequencies of primary frequency standards should therefore be corrected for the shift due to ambient radiation, as stated at the meeting of the CCTF in 1999.
|footnote added by the 14th meeting of the Consultative Committee for Time and Frequency in 1999
the footnote was added at the 86th (1997) meeting of the CIPM
GCPM 1998 7th Edition SI Brochure
|-style="vertical-align: top;"
|The definition of a unit refers to an idealized situation that can be reached in the practical realization with some uncertainty only. In this spirit, the definition of the second has to be understood as referring to atoms free of any perturbation, at rest and in the absence of electric and magnetic fields.
A future re-definition of the second would be justified if these idealized conditions can be achieved much easier than with the current definition.

The definition of the second should be understood as the definition of the unit of proper time: it applies in a small spatial domain that shares the motion of the caesium atom used to realize the definition.

In a laboratory sufficiently small to allow the effects of the non-uniformity of the gravitational field to be neglected when compared to the uncertainties of the realization of the second, the proper second is obtained after application of the special relativistic correction for the velocity of the atom in the laboratory. It is wrong to correct for the local gravitational field.
|'''The second, symbol s, is the SI unit of time. It is defined by taking the fixed numerical value of the caesium frequency, Δ''ν''<sub>Cs</sub>, the unperturbed ground-state hyperfine transition frequency of the caesium 133 atom, to be 9 192 631 770 when expressed in the unit Hz, which is equal to s<sup>−1</sup>.'''

The reference to an unperturbed atom is intended to make it clear that the definition of the SI second is based on an isolated caesium atom that is unperturbed by any external field, such as ambient black-body radiation.

The second, so defined, is the unit of proper time in the sense of the general theory of relativity. To allow the provision of a coordinated time scale, the signals of different primary clocks in different locations are combined, which have to be corrected for relativistic caesium frequency shifts (see section 2.3.6).

The CIPM has adopted various secondary representations of the second, based on a selected number of spectral lines of atoms, ions or molecules. The unperturbed frequencies of these lines can be determined with a relative uncertainty not lower than that of the realization of the second based on the <sup>133</sup>Cs hyperfine transition frequency, but some can be reproduced with superior stability.
|'''Current Definition''' resolved in 2018 effective after the 26th GCPM approved the redefinition May 20, 2019.
SI Brochure 9
|}