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This article is about the unit of time. For other uses, see Second (disambiguation).

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. For other uses see Time (disambiguation Time is a component of a measuring system used to sequence events to compare the durations of The International System of Units (SI defines seven dimensionally independent SI base units.

SI prefixes are frequently combined with the word second to denote subdivisions of the second, e. An SI prefix (also known as a metric prefix) is a name or associated symbol that precedes a unit of measure (or its symbol to form a Decimal multiple or g. , the millisecond (one thousandth of a second) and nanosecond (one billionth of a second). A millisecond (from Milli- and Second; abbreviation ms is one thousandth of a Second. A nanosecond ( ns) is one billionth of a second See also times of other orders of magnitude. 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. A kilosecond is 1000 seconds (16 minutes 40 seconds so there are 86 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). A minute is a Unit of measurement of Time or of Angle. The minute is a unit of Time equal to 1/60th of an Hour or 60 The hour (symbol h) is a unit of Time. It is not an SI unit but is accepted for use with the SI A day (symbol d is a unit of Time equivalent to 24 Hours and the duration of a single Rotation of planet Earth with respect to the

Contents

International second

Under the International System of Units, the second is currently defined 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. Frequency is a measure of the number of occurrences of a repeating event per unit Time. In Atomic physics, hyperfine coupling is the weak magnetic interaction between Electrons and nuclei. In Quantum mechanics, a stationary state is an Eigenstate of a Hamiltonian, or in other words a state of definite energy Caesium or cesium (ˈsiːziəm is the Chemical element with the symbol Cs and Atomic number 55 [1] ”

This definition refers to a caesium atom at rest at a temperature of 0 K (absolute zero). The kelvin (symbol K) is a unit increment of Temperature and is one of the seven SI base units The Kelvin scale is a thermodynamic Absolute zero is the point at which molecules do not move (relative to the rest of the body more than they are required to by a quantum mechanical effect called Zero-point The ground state is defined at zero magnetic field. In Physics, a magnetic field is a Vector field that permeates space and which can exert a magnetic force on moving Electric charges 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[2] (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. ISO 31-1 is the part of International standard ISO 31 that defines names and symbols for quantities and units related to Space

Equivalence to other units of time

1 international second is equal to:

History

The Egyptians had subdivided daytime and nighttime into twelve hours each since at least 2000 BC, hence their hours varied seasonally. A minute is a Unit of measurement of Time or of Angle. The minute is a unit of Time equal to 1/60th of an Hour or 60 The hour (symbol h) is a unit of Time. It is not an SI unit but is accepted for use with the SI A day (symbol d is a unit of Time equivalent to 24 Hours and the duration of a single Rotation of planet Earth with respect to the In Astronomy, a Julian year (symbol a) is a unit of measurement of Time defined The Hellenistic astronomers Hipparchus (c. This article focuses on the cultural aspects of the Hellenistic age for the historical aspects see Hellenistic period. Hipparchus ( Greek; ca 190 BC &ndash ca 120 BC was a Greek Astronomer, Geographer, and Mathematician of the Hellenistic 150 BC) and Ptolemy (c. Claudius Ptolemaeus ( Greek: Klaúdios Ptolemaîos; after 83 &ndash ca AD 150) subdivided the day sexagesimally and also used a mean hour (1⁄24 day), but did not use distinctly named smaller units of time. Sexagesimal ( base-sixty) is a Numeral system with sixty as the base. Instead they used simple fractions of an hour.

The day was subdivided sexagesimally, that is by 1⁄60, by 1⁄60 of that, by 1⁄60 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. Babylonia was an Amorite state in lower Mesopotamia (modern southern Iraq) with Babylon as its capital 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). The Hebrew calendar (הלוח העברי ha'luach ha'ivri) or Jewish calendar is a Lunisolar calendar used by Jews for predominantly religious The helek ( Hebrew, meaning "portion" plural halakim) is a unit of Time used in the calculation of the Hebrew calendar. [3] 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). [4]

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. A Muslim (مسلم pronounced Muslim, not Muzlim) is an adherent of the Religion [5] 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. For the Nova Scotia premier see Roger Bacon (politician. Roger Bacon, O [6] Although a third for 1⁄60 of a second remains in some languages, for example Polish (tercja) and Arabic (ثالثة), the modern second is subdivided decimally. Polish ( język polski, polszczyzna) is the Official language of Poland. Arabic (ar الْعَرَبيّة (informally ar عَرَبيْ) in terms of the number of speakers is the largest living member of the Semitic language

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. A pendulum clock is a Clock that uses a Pendulum, a swinging weight as its Timekeeping element A seconds pendulum is a Pendulum whose period is precisely two Seconds one second for a swing forward and one second for a swing back Christiaan Huygens (ˈhaɪgənz in English ˈhœyɣəns in Dutch) ( April 14, 1629 &ndash July 8, 1695) was a Dutch [7] 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. A longcase clock, also tall-case clock, grandfather clock or floor clock, is a freestanding weight-driven Pendulum clock with the 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. A clock face is the part of an Analog clock that displays the time through the use of a fixed numbered dial or dials and moving hands

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. EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001 In Astronomy, an epoch is a moment in time used as a reference for the Orbital elements of a Celestial body. 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. Newcomb's Tables of the Sun is the short title for a work by the American astronomer and mathematician Simon Newcomb entitled "Tables of the Motion [8] 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. A tropical year (also known as a solar year) is the length of time that the Sun takes to return to the same position in the cycle of seasons as seen from Earth Ephemeris Time ( ET) is a time scale used in Ephemerides of celestial bodies in particular the Sun (as observed from the Earth Moon planets and other members of [8]

This definition was ratified by the Eleventh General Conference on Weights and Measures in 1960. The tropical year in the definition was not measured, but calculated from a formula describing a mean tropical year which decreased linearly over time, hence the curious reference to a specific instantaneous tropical year. Because this second was the independent variable of time used in ephemerides of the Sun and Moon during most of the twentieth century (Newcomb's Tables of the Sun were used from 1900 through 1983, and Brown's Tables of the Moon were used from 1920 through 1983), it was called the ephemeris second. An ephemeris (plural ephemerides; from the Greek word ἐφήμερος ephemeros "daily" is a table of values that gives the positions of Ernest William Brown ( November 29 1866 &ndash July 22 1938) was an English Mathematician and Astronomer, who [8]

With the development of the atomic clock, it was decided to use atomic clocks as the basis of the definition of the second, rather than the revolution of the Earth around the Sun. An atomic clock is a type of Clock that uses an Atomic resonance Frequency standard as its timekeeping element

Following several years of work, Louis Essen from the National Physical Laboratory (Teddington, England) and William Markowitz from the United States Naval Observatory (USNO) determined the relationship between the hyperfine transition frequency of the caesium atom and the ephemeris second. Louis Essen ( September 6, 1908 &ndash August 24, 1997) was an English Physicist whose most notable achievements were in The National Physical Laboratory (NPL is the national Measurement standards laboratory for the United Kingdom, based at Bushy Park in Teddington William Markowitz ( February 8, 1907 in Austrian Empire - October 10, 1998 in Pompano Beach Florida) was an American The United States Naval Observatory ( USNO) is one of the oldest Scientific agencies in the United States. Caesium or cesium (ˈsiːziəm is the Chemical element with the symbol Cs and Atomic number 55 [8] Using a common-view measurement method based on the received signals from radio station WWV,[9] 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. This article is about radio broadcasting for other uses see Radio (disambiguation. WWV is the Callsign of NIST 's Shortwave Radio station located in Fort Collins Colorado. 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 General Conference on Weights and Measures is the English name of the Conférence générale des poids et mesures ( CGPM, never GCWM International Atomic Time ( TAI, from the French name Temps Atomique International) is a high-precision atomic Time standard that tracks [8]

During the 1970s it was realized that gravitational time dilation caused the second produced by each atomic clock to differ depending on its altitude. Gravitational time dilation is the effect of time passing at different rates in regions of different Gravitational potential; the higher the local distortion of Spacetime Altitude is the Elevation of a point or object from a known level or datum (plural data A uniform second was produced by correcting the output of each atomic clock to mean sea level (the rotating geoid), lengthening the second by about 1×10−10. Mean sea level (MSL is the average (mean height of the Sea, with reference to a suitable reference surface The geoid is that Equipotential surface which would coincide exactly with the mean ocean surface of the Earth if the oceans were in equilibrium at rest and extended through This correction was applied at the beginning of 1977 and formalized in 1980. In relativistic terms, the SI second is defined as the proper time on the rotating geoid. In relativity, proper time is Time measured by a single Clock between events that occur at the same place as the clock [10]

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 International Bureau of Weights and Measures ( Bureau international des poids et mesures, in French) is an international Standards organization, one

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. In Physics, a black body is an object that absorbs all light that falls on it

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 Trapped ion optical frequency standards. A magneto-optical trap (abbreviated MOT) is a device that cools down non-charged atoms to temperatures near Absolute zero and traps them at a certain place using magnetic National Physical Laboratory.

Approximations

It 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 multiples

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. A minute is a Unit of measurement of Time or of Angle. The minute is a unit of Time equal to 1/60th of an Hour or 60 The hour (symbol h) is a unit of Time. It is not an SI unit but is accepted for use with the SI A day (symbol d is a unit of Time equivalent to 24 Hours and the duration of a single Rotation of planet Earth with respect to the

SI multiples for second (s)
Submultiples Multiples
Value Symbol Name Value Symbol Name
10–1 s ds decisecond 101 s das decasecond
10–2 s cs centisecond 102 s hs hectosecond
10–3 s ms millisecond 103 s ks kilosecond
10–6 s µs microsecond 106 s Ms megasecond
10–9 s ns nanosecond 109 s Gs gigasecond
10–12 s ps picosecond 1012 s Ts terasecond
10–15 s fs femtosecond 1015 s Ps petasecond
10–18 s as attosecond 1018 s Es exasecond
10–21 s zs zeptosecond 1021 s Zs zettasecond
10–24 s ys yoctosecond 1024 s Ys yottasecond
Common prefixes are in bold

See also

References

  1. ^ Official BIPM definition
  2. ^ ISO 31-1
  3. ^ Neugebauer Otto (1975). The hertz (symbol Hz) is a measure of Frequency, informally defined as the number of events occurring per Second. The becquerel (symbol Bq) is the SI derived unit of radioactivity. A leap second is a one- Second adjustment that keeps broadcast standards for time of day close to mean solar time. A magneto-optical trap (abbreviated MOT) is a device that cools down non-charged atoms to temperatures near Absolute zero and traps them at a certain place using magnetic A time standard An atomic clock is a type of Clock that uses an Atomic resonance Frequency standard as its timekeeping element International Atomic Time ( TAI, from the French name Temps Atomique International) is a high-precision atomic Time standard that tracks A history of ancient mathematical astronomy. Berlin: Springer.  
  4. ^ Neugebauer Otto (1949). "The astronomy of Maimonides and its sources". Hebrew Union College Annual 22: 321–360.  
  5. ^ al-Biruni (1879). The chronology of ancient nations: an English version of the Arabic text of the Athâr-ul-Bâkiya of Albîrûnî, or "Vestiges of the Past". London: W. H. Allen, 147-149. OCLC 9986841. The OCLC Online Computer Library Center is according to its website a "nonprofit membership computer library service and research organization dedicated to the public purpose  
  6. ^ Bacon Roger (2000). The Opus Majus of Roger Bacon. Philadelphia: University of Pennsylvania Press, table facing page 231. ISBN 9781855068568.  
  7. ^ Long Case Clock: Pendulum
  8. ^ a b c d e Leap Seconds. Time Service Department, United States Naval Observatory. The United States Naval Observatory ( USNO) is one of the oldest Scientific agencies in the United States. Retrieved on 2006-12-31. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 406 – Vandals, Alans and Suebians cross the Rhine, beginning an invasion of Gallia.
  9. ^ Leschiutta Sigfrido (2005-06-07). "The definition of the 'atomic' second". Metrologia 42 (3): S10–S19. doi:10.1088/0026-1394/42/3/S03. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document.  
  10. ^ Nelson RA et al. , (2000). "The leap second: its history and possible future" (PDF 381KB). Metrologia 38: 509–529.  

External links

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