- The 19th letter of the English alphabet

Translated by: FARHAD PEZESHKI

- The 60th part of a minute (of time). Historically based on the period of rotation of Earth, the standard day counts 86400 seconds. However its new definition based on a fixed number of periods of a wave of light emitted during the transition between two stable states of a reference atom, requires does not fit with the variability of the terrestrial rotation. Both definitions are synchronized in the UTC clock by adding or substracting 1 or 2 seconds to the last minute of a month, every 6 months or 1 year. these adjustments are unpredictable in the long term, and are announced one month before by the BIPM normalization body in Paris, by comparing the measures performed by different worldwide observatories. These adjustements allow the terrestrial clock to never differ by more than 0,9 second from the official UTC time used for official civil calendar applications. So some days in the official calendar count between 86398 and 86402 seconds. In scientific applications, where datation requires days of fixed duration, the UTC clock is not used, but is based on the fixed definition of the day by atomic clocks. These clocks differ from UTC by only an integral number of fixed seconds, depending only on the official date in UTC, so that conversion between them is easy. For scientific applications, the abbreviation in the SI system is "s". For civil applications related to official dates and hours, the second is abbreviated by a colon separating it from the count of minutes, or by double primes or apostrophes