# Defining SI of Time – The Second

The Second denoted by the symbol s is the SI unit of time. It is defined by taking the fixed numerical value of the caesium frequency ΔνCs 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.

This definition implies the exact relation ΔνCs = 9 192 631 770 Hz. Inverting this relation gives an expression for the unit second in terms of the value of the defining constant ΔνCs.

Or in terms of The Second above equation can be written as follows –

The Second is thus equal to the duration of 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the unperturbed ground state of 133Cs atom.