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.

Optical-lattice clocks and the impending redefinition: the 1967 caesium-133 definition fixed the second to ~10⁻¹⁶ precision, which was state-of-the-art at the time. Strontium and ytterbium optical-lattice clocks built since 2014 (NIST, JILA, RIKEN, PTB) now demonstrate ~10⁻¹⁸ precision — 100× better than the existing definition can express. The General Conference on Weights and Measures has approved a roadmap toward redefining the second around an optical transition, targeting 2030 or shortly after. The redefinition will preserve the existing second to within measurement noise but will let GPS, financial trading systems, and fundamental-physics experiments anchor to a more precise reference.

The second sits at the centre of the SI graph: since 2019, all seven SI base units are defined via fundamental constants, and most of those constants implicitly depend on the second. The metre depends on the speed of light (a measurement involving time). The kilogram depends on Planck’s constant (in joule-seconds). The ampere depends on the elementary charge per second. The candela depends on watts per steradian (energy per second). Only the kelvin (Boltzmann), mole (Avogadro), and second itself are independent of seconds. If atomic-clock precision improves, every other SI unit’s precision improves with it. Reference: BIPM SI Brochure — The second.

Worked example

GPS depends on the second in a load-bearing way. Each satellite carries a caesium or rubidium clock; trilateration of position uses the travel time of radio signals from at least four satellites at the speed of light (~299,792,458 m/s). A timing error of just 1 nanosecond corresponds to a position error of ~30 cm: 299,792,458 m/s × 10⁻⁹ s ≈ 0.30 m. A 1-microsecond error becomes 300 m, which would make GPS useless for road navigation. The satellites must also correct for general relativity (clocks higher in Earth’s gravity well tick ~38 microseconds per day faster than ground clocks) and special relativity (orbital velocity makes them tick ~7 microseconds per day slower). The net correction (~38 − 7 = ~38 µs/day faster) is hard-coded into the satellite’s clock at launch; without it, GPS would drift ~11 km/day.

When and why it matters

Beyond GPS, the second underpins financial-market timestamping (MiFID II in Europe mandates microsecond-precision clock synchronisation for trading venues, traceable to UTC), power-grid phase synchronisation (PMUs sample at hundreds of times per second to detect grid instability before it cascades), telecommunications (LTE/5G base stations must be time-synced to within 1 µs of each other for handoff), and scientific measurement (LIGO’s gravitational-wave detections rely on cross-correlating signals from sites in Washington and Louisiana to within nanoseconds). The PTP protocol (IEEE 1588) and GPS-disciplined oscillators are how data centres get sub-microsecond time across thousands of servers without each needing its own atomic clock. Reference: NIST — Time and Frequency Division.