What *Exactly* Is a Second?

This article delves into the evolving definition of a second. Initially defined as 1/86400th of a day, the slowing rotation of the Earth rendered this definition imprecise. In 1967, the second was redefined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the cesium-133 atom. Cesium was chosen for its ease of isolation and purification, while the specific number of periods ensured backward compatibility with the length of the tropical year in 1900. Although the astronomical definition has been discarded, the cesium-based definition remains in use today.