Measurement of Time

The SI unit of time is the second (s). For a long time, the definition of the second was tied to the Earth's own rotation: one mean solar day — the average time interval from one solar noon to the next — was taken as the reference, with a day conventionally divided into 24 hours, each hour into 60 minutes, and each minute into 60 seconds. This gives a mean solar day of $24 \times 60 \times 60 = 86\,400$ seconds, and correspondingly the second was originally defined as $1/86\,400$ of a mean solar day.

This Earth-rotation-based definition, however, turned out not to be perfectly stable: careful observation showed that the length of a solar day gradually changes over long timescales, because the Earth's rotation is itself very slowly slowing down (mainly due to tidal friction from the Moon). A time standard tied to a slowly-changing physical process cannot serve as a truly constant, non-varying reference unit. To fix this, time is now defined using a caesium atomic clock, based on the extremely regular periodic vibrations of radiation absorbed and emitted by caesium atoms during a specific atomic transition. Precisely: one second is defined as the time taken for exactly $9{,}192{,}631{,}770$ vibrations of the radiation (electromagnetic wave) emitted during the transition between two particular hyperfine energy states of the $\text{Cs}^{133}$ atom. Because this atomic transition frequency is a fixed property of the caesium atom (unaffec …