Caesium was discovered in 1860 by Robert Bunsen (1811-1899) and Gustav Kirchhoff (1824-1887) with the aid of a spectroscope they had invented the year previously. It was the first element to be identified through spectrum analysis.
Caesium-133 is a stable isotope of Caesium widely used in atomic clocks. Time is measured as the transition between two electron spin energy levels. The SI unit, of a second is 9,192,631,770 cycles between transition states of Caesium-133.