Second (s)
Definition
The second (symbol s) is the base unit of time in the International System of Units (SI). It is defined by fixing the numerical value of the unperturbed ground-state hyperfine transition frequency of the caesium-133 atom at exactly 9,192,631,770 hertz.
History
Subdividing time into 24 hours, 60 minutes, and 60 seconds dates to Babylonian astronomy and was popularized in Europe by Persian astronomers in the 11th century. For most of history the second was defined as 1/86,400 of a mean solar day.
Mid-twentieth-century precision astronomy revealed that the Earth's rotation is unsteady, so in 1956 the second was redefined against the orbital year and then, in 1967, against the caesium hyperfine transition. The 2019 SI revision preserved this definition while restating it as a fixed numerical value of that transition frequency.
Standard reference
Defined by the BIPM SI Brochure (9th edition, 2019) by fixing the caesium-133 hyperfine transition frequency at 9,192,631,770 Hz exactly.
Common conversions
| 1 s | = 1,000 ms |
|---|---|
| = 1,000,000 μs | |
| = 0.0166667 min | |
| = 0.000277778 h |