Second
SI unit of time, defined as 9 192 631 770 periods of the ground-state hyperfine transitions of caesium-133
The second (symbol: s) is a unit of time derived from the division of the day, first into 24 hours, then into 60 minutes, and lastly into 60 seconds, for a total of 24 × 60 × 60 = 86400 seconds per day. That definition, based on 1⁄86400 of a rotation of the Earth, is still used by the Universal Time 1 (UT1) system.
The current and formal definition in the International System of Units (SI) is more precise:The second [...] is defined by taking the fixed numerical value of the caesium (Cs) frequency, ΔνCs, the unperturbed ground-state hyperfine transition frequency of the Cs-133 atom, to be 9192631770 when expressed in the unit hertz, which is equal to s−1.
This current definition was adopted in 1967 when it became feasible to define the second based on fundamental properties of nature with caesium clocks. As the speed of Earth's rotation varies and is slowing ever so slightly, a leap second is added at irregular intervals to civil time to keep clocks in sync with Earth's rotation.
Etymology
"Minute" comes from Latin: pars minuta prima, meaning 'first small part', i.e. first division of the hour – dividing it into 60 parts, and "second" comes from pars minuta secunda, 'second small part', dividing again into 60 parts.
Uses
Analog clocks and watches often have sixty tick marks on their faces, representing seconds (and minutes), and a "second hand" to mark the passage of time in seconds. Digital clocks and watches often have a two-digit seconds counter.
SI prefixes are frequently combined with the word second to denote subdivisions of the second: milliseconds (thousandths), microseconds (millionths), nanoseconds (billionths), and sometimes smaller units of a second.
The public source identifies “Second” as sI unit of time, defined as 9 192 631 770 periods of the ground-state hyperfine transitions of caesium-133. This brief keeps that definition visible, then builds a research path around Second, unit and time.
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This entry incorporates text from “Second” on English Wikipedia. Contributors are listed in the page history. Text is available under the Creative Commons Attribution-ShareAlike 4.0 License. Selected authority identifiers and statements are retrieved from Wikidata under CC0; their references and qualifiers remain part of the verification path.