Atomic definition of the second
Definición atómica del segundo
- Date
- 1967 · confidence high
- Place
- Paris, Île-de-France, France
- Credited to
- 13th General Conference on Weights and Measures
- Field
- Knowledge & symbols
What it is
In 1967 the CGPM defined the second as 9,192,631,770 periods of the radiation of a caesium-133 transition, replacing the fraction of the mean solar day. It is the first SI base unit defined by an atomic property.
The ephemeris second (set in 1956 and ratified in 1960), based on the tropical year, had replaced the fraction of the solar day; in 1958 Markowitz and Hall measured the caesium frequency in ephemeris seconds, and that value was used in the definition.
What it led to
- International Atomic Time (TAI) and UTC with leap seconds (1972)
- synchronisation of networks, satellite navigation and finance
- definition of other units from the second, such as the metre (1983)