SISekunde

SISekunde is a term used to refer to the SI unit of time, commonly known in English as the second. The standard symbol for the unit is s. In some German-language and multilingual contexts, SISekunde is used as a direct rendering of “SI-second” to denote this base unit of time within the International System of Units (SI). The SISekunde is defined by a precise physical process rather than a celestial cycle, ensuring stability and universality across cultures and technologies.

Historically, the second was once defined as 1/86,400 of mean solar time. Since 1967, it has been defined by the cesium-133 atom: one SISekunde is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the atom’s ground state. This atomic definition underpins modern timekeeping and is realized in practice by atomic clocks around the world. The SI time scale is maintained by the International Bureau of Weights and Measures (BIPM), and practical timescales such as International Atomic Time (TAI) and Coordinated Universal Time (UTC) are offset to remain in step with Earth's rotation, occasionally using leap seconds.

Applications of the SISekunde are widespread and essential. It serves as the fundamental unit for all time measurements, enabling precise navigation (for example, satellite systems), telecommunications, scientific experiments, and synchronization of computer networks. As optical and microwave clocks continue to advance, they improve the realization of the SI second and contribute to more accurate timekeeping globally.

See also: International System of Units, cesium-133, atomic clock, UTC, TAI.