# second

##### SI base unit
Name Symbol Quantity
second s time ### Definition

The second, symbol s, is the SI base unit of time.

The second is defined by taking the fixed numerical value of the caesium frequency ΔνCs, 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−1.

The definition of the second implies the exact relation ΔνCs = 9 192 631 770 Hz. Inverting this relation gives an expression for the unit second in terms of the defining constant ΔνCs : $1 \mspace{4mu} \text{Hz} \mspace{8mu} = \dfrac{\Delta \nu _{Cs}}{9 \mspace{4mu} 192 \mspace{4mu} 631 \mspace{4mu} 770}\\ \\ \\ 1 \mspace{4mu} \text{s} \mspace{20mu} = \dfrac{9 \mspace{4mu} 192 \mspace{4mu} 631 \mspace{4mu} 770} {\Delta \nu _{Cs}}$

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 133Cs atom.

### History

For historical reasons, the second was originally defined as 186 400 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.