Second

The second is the SI base unit of time. It is defined by fixing the cesium-133 hyperfine transition frequency at exactly 9,192,631,770 hertz.

Cesium standard: A frequency standard that uses the 9,192,631,770-hertz transition of cesium-133 atoms to realize the second. Its counted oscillations realize the second in atomic clocks.
International System of Units: The internationally agreed system of measurement units, symbols, and defining constants. The second is one of its seven base units.
Babylonian timekeeping: Ancient systems of time measurement that used sexagesimal arithmetic to divide hours and circles. The base-60 divisions inherited from ancient practice shaped the modern second’s subdivisions.
Coordinated Universal Time: The primary international time standard, maintained using atomic clocks and adjusted when needed to stay close to Earth rotation. Its atomic-clock timescales are measured and coordinated in seconds.
Minute: A unit of time equal to 60 seconds. It is a conventional multiple of the second, not an SI base unit.
Atomic clock: A clock that measures time by tracking the frequency of an atomic transition. Atomic clocks use the defining cesium frequency to keep time.
Time: The dimension in which events are ordered and durations between them are measured. The second is the SI base unit used to measure duration.
Mean solar time: Time measured by the average apparent motion of the Sun, smoothed to account for seasonal variation. The second was once defined as a fraction of the mean solar day.
Global Positioning System: A satellite navigation system that determines position from the timed arrival of radio signals. Position calculations depend on measuring signal travel times with extreme precision.
Day: A unit of time associated with one rotation of Earth relative to the Sun or stars. Unlike the atomic second, its duration is tied to Earth’s rotation.

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