- (second serving''): second helping
- Finnish: lisäannos
- third-person singular of second
- Plural of second
The second (SI symbol: s), sometimes abbreviated sec., is the name of a unit of time, and is the International System of Units (SI) base unit of time.
SI prefixes are frequently combined with the word second to denote subdivisions of the second, e.g., the millisecond (one thousandth of a second) and nanosecond (one billionth of a second). Though SI prefixes may also be used to form multiples of the second (such as “kilosecond,” or one thousand seconds), such units are rarely used in practice. More commonly encountered, non-SI units of time such as the minute, hour, and day increase by multiples of 60 and 24 (rather than by powers of ten as in the SI system).
International secondUnder the International System of Units, the second is currently defined as This definition refers to a caesium atom at rest at a temperature of 0 K (absolute zero). The ground state is defined at zero magnetic field. The second thus defined is equivalent to the ephemeris second, which was based on astronomical measurements. (See History below.)
The international standard symbol for a second is s (see ISO 31-1)
The realization of the standard second is described briefly in NIST Special Publication 330; Appendix 2, pp. 53 ff, and in detail by National Research Council of Canada.
Equivalence to other units of time1 international second is equal to:
HistoryThe Egyptians had subdivided daytime and nighttime into twelve hours each since at least 2000 BC, hence their hours varied seasonally. The Hellenistic astronomers Hipparchus (c. 150 BC) and Ptolemy (c. AD 150) subdivided the day sexagesimally and also used a mean hour ( day), but did not use distinctly named smaller units of time. Instead they used simple fractions of an hour.
The day was subdivided sexagesimally, that is by , by of that, by of that, etc., to at least six places after the sexagesimal point by the Babylonians after 300 BC, but they did not sexagesimally subdivide smaller units of time. For example, six fractional sexagesimal places of a day was used in their specification of the length of the year, although they were unable to measure such a small fraction of a day in real time. As another example, they specified that the mean synodic month was 29;31,50,8,20 days (four fractional sexagesimal positions), which was repeated by Hipparchus and Ptolemy sexagesimally, and is currently the mean synodic month of the Hebrew calendar, though restated as 29 days 12 hours 793 halakim (where 1 hour = 1080 halakim). They did not use the hour, but did use a double-hour, a time-degree lasting four of our minutes, and a barleycorn lasting 3⅓ of our seconds (the helek of the modern Hebrew calendar).
In 1000, the Muslim scholar al-Biruni gave the times of the new moons of specific weeks as a number of days, hours, minutes, seconds, thirds, and fourths after noon Sunday. In 1267, the medieval scientist Roger Bacon stated the times of full moons as a number of hours, minutes, seconds, thirds, and fourths (horae, minuta, secunda, tertia, and quarta) after noon on specified calendar dates. Although a third for of a second remains in some languages, for example Polish (tercja) and Arabic (ثالثة), the modern second is subdivided decimally.
The second first became measurable with the development of pendulum clocks keeping mean time (as opposed to the apparent time displayed by sundials), specifically in 1670 when William Clement added a seconds pendulum to the original pendulum clock of Christian Huygens. The seconds pendulum has a period of two seconds, one second for a swing forward and one second for a swing back, enabling the longcase clock incorporating it to tick seconds. From this time, a second hand that rotated once per minute in a small subdial began to be added to the clock faces of precision clocks.
In 1956 the second was defined in terms of the period of revolution of the Earth around the Sun for a particular epoch, because by then it had become recognized that the Earth's rotation on its own axis was not sufficiently uniform as a standard of time. The Earth's motion was described in Newcomb's Tables of the Sun, which provides a formula for the motion of the Sun at the epoch 1900 based on astronomical observations made between 1750 and 1892. The second thus defined is
- the fraction 1/31,556,925.9747 of the tropical year for 1900 January 0 at 12 hours ephemeris time. they determined the orbital motion of the Moon about the Earth, from which the apparent motion of the Sun could be inferred, in terms of time as measured by an atomic clock. As a result, in 1967 the Thirteenth General Conference on Weights and Measures defined the second of atomic time in the International System of Units as
- the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom.
The definition of the second was later refined at the 1997 meeting of the BIPM to include the statement
- This definition refers to a caesium atom at rest at a temperature of 0 K.
The revised definition would seem to imply that the ideal atomic clock would contain a single caesium atom at rest emitting a single frequency. In practice, however, the definition means that high-precision realizations of the second should compensate for the effects of the ambient temperature (black-body radiation) within which atomic clocks operate and extrapolate accordingly to the value of the second as defined above.
For approximately twenty years, it has been possible to confine an ion to a region of space smaller than one cubic micron (10-6 m)3. Such an ion is almost completely isolated from the surrounding environment and suggests a frequency or time standard with a reproducibility and stability several orders of magnitude superior to the best caesium time standards. Such standards are under development. See magneto-optical trap and
ApproximationsIt is a common belief that saying one one thousand, two one thousand, three one thousand... can be used to time events in seconds (one Mississippi, two Mississippi is another common sequence)
SI prefixes are commonly used to measure time less than a second, but rarely for multiples of a second, instead the non-SI units minutes, hours and days are used.
- NIST: Definition of the second; notice the cesium atom must be in its ground state at 0 K
- Official BIPM definition of the second
- National Physical Laboratory: Trapped ion optical frequency standards
- National Research Council of Canada: Optical frequency standard based on a single trapped ion
- What is a Cesium atom clock?
- Seconds and leap seconds by the USNO
- The leap second: its history and possible future
seconds in Tosk Albanian: Sekunde
seconds in Arabic: ثانية
seconds in Official Aramaic (700-300 BCE): ܪܦܦܐ
seconds in Asturian: Segundu
seconds in Belarusian: Секунда
seconds in Belarusian (Tarashkevitsa): Сэкунда (адзінка вымярэньня часу)
seconds in Bengali: সেকেন্ড
seconds in Bulgarian: Секунда
seconds in Catalan: Segon
seconds in Czech: Sekunda
seconds in Danish: Sekund
seconds in German: Sekunde
seconds in Estonian: Sekund
seconds in Modern Greek (1453-): Δευτερόλεπτο
seconds in Spanish: Segundo
seconds in Esperanto: Sekundo
seconds in Basque: Segundo
seconds in Persian: ثانیه
seconds in French: Seconde (temps)
seconds in Western Frisian: Sekonde
seconds in Irish: Soicind
seconds in Galician: Segundo
seconds in Korean: 초 (시간)
seconds in Hindi: सैकण्ड
seconds in Croatian: Sekunda
seconds in Indonesian: Detik
seconds in Interlingua (International Auxiliary Language Association): Secunda
seconds in Icelandic: Sekúnda
seconds in Italian: Secondo
seconds in Hebrew: שנייה
seconds in Georgian: წამი
seconds in Swahili (macrolanguage): Sekunde
seconds in Haitian: Segonn
seconds in Latin: Secundum
seconds in Latvian: Sekunde
seconds in Luxembourgish: Sekonn
seconds in Lithuanian: Sekundė
seconds in Lojban: snidu
seconds in Hungarian: Másodperc
seconds in Marathi: सेकंद
seconds in Malay (macrolanguage): Saat
seconds in Dutch: Seconde
seconds in Japanese: 秒
seconds in Norwegian: Sekund
seconds in Norwegian Nynorsk: Sekund
seconds in Occitan (post 1500): Segonda
seconds in Low German: Sekunn
seconds in Polish: Sekunda
seconds in Portuguese: Segundo
seconds in Kölsch: Sekůndt
seconds in Romanian: Secundă
seconds in Quechua: Sikundu
seconds in Russian: Секунда
seconds in Albanian: Sekonda
seconds in Sicilian: Secunnu
seconds in Simple English: Second
seconds in Slovak: Sekunda
seconds in Slovenian: Sekunda
seconds in Serbian: Секунд
seconds in Finnish: Sekunti
seconds in Swedish: Sekund
seconds in Tamil: நொடி (கால அளவு)
seconds in Thai: วินาที
seconds in Vietnamese: Giây
seconds in Turkish: Saniye
seconds in Ukrainian: Секунда
seconds in Yiddish: סעקונדע
seconds in Chinese: 秒
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