But starting in 1972, radio signals began broadcasting atomic seconds. By definition, a SI second is the duration of 9,192,631,770 periods of radiation from the transition between two hyperfine levels of the ground state of the cesium-133 atom, a physical phenomenon distinct from the rotation of Earth.īefore atomic timekeeping, clocks were set to the skies. It is maintained by a continuous count of SI seconds, the metric system’s fundamental unit of duration. It is the basis for legal timekeeping in most of the world, including the United States.
In January 2012, a United Nations–affiliated organization known as the Radiocommunication Sector of the International Telecommunications Union (ITU-R) could permanently break this link by redefining Coordinated Universal Time.Ĭoordinated Universal Time is better known by its international acronym UTC, the modern successor to traditional Greenwich Mean Time. But what was certain then-the necessity of linking civil time to the motion of celestial bodies-may soon be abandoned.
American astronomers Edgar Woolard and Gerald Clemence published those words in 1966, not that long ago in the history of human timekeeping.
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