The calculation of a second has been updated in 2019
The second is defined by taking the fixed numerical value of the cesium
frequency āĪ½, the unperturbed ground-state hyperfine transition frequency of
the cesium 133 atom, to be 9 192 631 770 when expressed in the unit Hz, which
is equal to sā1.
The conference where this was made official was quite a beautiful, upbeat demonstration of people from around the world coming together to agree upon a change:
