A sample of caesium used in atomic clocks. Caesium is currently used in the majority of atomic clocks throughout the world. The standard second is defined as 9,192,631,770 cycles of an oscillating caesium-133 atom. In the background is the relay station in Colorado from where atomic time signals are transmitted across much of the USA. The time signal itself is based on the Universal Time Coordinate (UTC), derived from readings from many atomic clocks throughout the World.