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second (s)

Document v1 · published by Koskamo · updated 2026-10-05

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The second (symbol s) is the SI base unit of time, per the BIPM SI Reference Point.

  • Name: second
  • Symbol: s
  • Unit type: SI base unit
  • Quantity kind: time
  • Definition (status current, in force since 2019-05-20): The second, symbol $\rm{s}$, is the SI unit of time. It is defined by taking the fixed numerical value of the caesium frequency, $\Delta\nu_{\rm{Cs}}$, the unperturbed ground-state hyperfine transition frequency of the caesium 133 atom, to be ${9\:192\:631\:770}$ when expressed in the unit ${\rm{Hz}}$, which is equal to ${\rm{s}}^{-1}$.
  • Defining constant: hyperfine transition frequency of Cs-133 (fixed numerical value 9 192 631 770)
  • Defining equation: 1\;{\rm{Hz}} = \frac{\Delta\nu_{\rm{Cs}}}{{9\:192\:631\:770}} \quad {\rm{or}} \quad 1\;{\rm{s}} = \frac{{9\:192\:631\:770}}{\Delta\nu_{\rm{Cs}}}
  • Defining resolution: https://si-digital-framework.org/bodies/CGPM#CGPM26-Res1
  • Superseded definitions: second1960 (1960-10-18 to 1967-10-12); second1967 (1967-10-13 to 2019-05-19)

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si-digital-framework.org
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Document v1
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BIPM SI Reference Point (si-digital-framework.org), the SI defining constants, base and derived units, and prefixes; CC BY 3.0 IGO

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  1. Published by Koskamo

    Version 1