**second**

## SI base unit |
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Name | Symbol | Quantity | |

second | s | time | |

## DefinitionThe second, symbol s, is the SI base unit of time. The second is defined by taking the fixed numerical value of the caesium frequency Δ |

The definition of the second implies the exact relation Δ*ν*_{Cs} = 9 192 631 770 Hz. Inverting this relation gives an expression for the unit second in terms of the defining constant Δ*ν*_{Cs} :

The effect of this definition is that the second is equal to the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the unperturbed ground state of the ^{133}Cs atom.

### History

For historical reasons, the second was originally defined as ^{1}⁄_{86 400} of a day. One day being the time taken for the Earth to complete one rotation about its axis.

A day can be divided into 24 hours, each hour being subdivided into 60 minutes, and each minute being further subdivided into 60 seconds, giving a total of 86 400 seconds.