How is a second defined?
One second is the duration of 9,192,631,770 periods of the radiation from a transition between two energy levels of the cesium-133 atom. This atomic definition was adopted in 1967. Before that, the second was a fraction of the day, and then of the year.
Key facts
| Definition | 9,192,631,770 periods of the cesium-133 hyperfine transition |
|---|---|
| Adopted | 1967, 13th General Conference on Weights and Measures (CGPM) |
| Wording updated | 2019, as part of the SI redefinition (fixed value of ΔνCs) |
| Before 1960 | 1/86,400 of a mean solar day |
| 1960–1967 | Ephemeris second: 1/31,556,925.9747 of the year 1900 |
| Maintained by | BIPM, Sèvres, France |
| Possible next definition | An optical transition, around 2030 |
Three definitions in one century
The solar second. For most of history the second was simply 1/86,400 of a day (24 × 60 × 60). But by the 1930s, quartz clocks showed that Earth's rotation isn't uniform. Tidal friction is slowly lengthening the day, by about 1.8 milliseconds per century on average, and it also wobbles by milliseconds from year to year.
The ephemeris second (1960). Astronomers switched to Earth's orbit around the sun, defining the second as 1/31,556,925.9747 of the tropical year 1900. It was more stable, but almost impossible to measure in a lab.
The atomic second (1967). After Louis Essen and Jack Parry's 1955 cesium clock at the UK's National Physical Laboratory, the second was tied to the cesium atom, with the number of cycles chosen to match the ephemeris second as closely as possible. Since the 2019 revision of the SI, the definition is written as fixing the cesium frequency at exactly 9,192,631,770 hertz.
Why cesium?
Every cesium-133 atom is identical, the transition is easy to reach with microwaves, and cesium is stable and not radioactive. Any lab in the world can build a clock that realizes the same second. See how atomic clocks work.
What's next
Optical clocks are now about 100 times more precise than the best cesium clocks, so the second will probably be redefined using an optical transition, such as one in strontium or ytterbium, with a decision expected around 2030. The new definition will be chosen so that a second stays the same length; only the precision improves.
Questions people ask
Why exactly 9,192,631,770?
That number of cesium cycles matched the length of the existing ephemeris second as measured in the late 1950s, so the switch didn't change the length of a second.
Is a second the same everywhere?
Locally, yes. But because of relativity, a second on a mountaintop and a second at sea level pass at very slightly different rates when compared with each other.
Who decides the definition of a second?
The General Conference on Weights and Measures (CGPM), advised by the International Bureau of Weights and Measures (BIPM).
Sources
Last updated October 18, 2026. Found an error? Tell us.