Second

The second is the SI base unit of time. Currently defined (since 1967) as 9,192,631,770 oscillations of the radiation emitted by a specific transition in the caesium-133 atom at absolute zero, ground state.

Historical definitions:

• Pre-1956: 1/86,400 of a mean solar day. Tied to Earth’s rotation, which is irregular.
• 1956-1967: 1/31,556,925.9747 of the tropical year 1900. Solved the rotation-irregularity problem but still arbitrary.
• 1967-present: caesium-133 atomic clock definition above.

Modern atomic clocks (the BIPM’s ensemble, NIST’s NIST-F2) are accurate to about one part in 10¹⁶ — enough that they would gain or lose less than a second over the age of the universe. Cs-133 clocks are now being supplemented by even more accurate optical-lattice clocks (one part in 10¹⁸ or better), which may form the basis of a future redefinition.

The second is the most fundamental SI unit in practical terms: meter is defined in terms of light travel time, kilogram in terms of Planck’s constant which involves time, and most modern measurements depend ultimately on accurate time-keeping.

For datetime conversions and the related concept of leap seconds, see our datetime tools.