cesium clock

instrument

Also known as: cesium atomic clock

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Last updated

June 24, 2026

Quick Summary

Atomic time NIST-F1 cesium fountain atomic clock, the primary time and frequency standard for the United States, Boulder, Colorado.

A cesium clock is a highly accurate instrument that uses the natural vibration frequency of cesium atoms to keep time. These clocks are a type of atomic clock, which rely on the resonance frequencies of atoms like cesium or rubidium.12

The accuracy of a cesium clock stems from the fact that all cesium-133 atoms are identical. When they absorb or release energy, they produce radiation of a precise and consistent frequency. This frequency is used to regulate the electronic components of the clock. In 1967, the definition of the second, the SI unit of time, was established based on the cesium-133 atom's transition frequency: 9,192,631,770 periods of radiation. This makes the cesium clock the most accurate timekeeping standard developed, with the potential to be off by less than a second in over 50 million years.23

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