Information On Second

The second (International System of Units symbol: s , sometimes abbreviated sec. is the name of a units of measurement of time and is the International System of Units (SI) SI base unit of time. lt;/ref> It may be measured using a clock Early definitions of the second were based on the apparent motion of the sun around the earth. The solar day was divided into 24 hours, each of which contained 60 minutes of 60 seconds each, so the second was of the mean solar day. However, nineteenth- and twentieth-century astronomical observations revealed that this average time is lengthening, and thus the motion of the earth is no longer considered a suitable standard for definition. With the advent of atomic clock , it became feasible to define the second based on fundamental properties of nature. Since 1967, the second has been defined to be periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.}} SI prefix s are frequently combined with the word secondto denote subdivisions of the second, e.g. the 1 E-3 s (one thousandth of a second), the 1 E-6 s (one millionth of a second), and the 1 E-9 s (one billionth of a second). Though SI prefixes may also be used to form multiples of the second such as 1 E3 s (one thousand seconds), such units are rarely used in practice. The more common larger non-SI units of time are not formed by powers of ten; instead, the second is multiplied by 60 to form a minute which is multiplied by 60 to form an hour which is multiplied by 24 to form a day The second is also the base unit of time in the Centimetre gram second system of units Mks system of units Metre-tonne-second system of units and Imperial units systems of units.

International second

Under the International System of Units, the second is currently defined as periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.}} This definition refers to a caesium atom at rest at a temperature of 0 Kelvin (absolute zero , and with appropriate corrections for gravitational time dilation The ground state is defined at zero electric field and magnetic field . The second thus defined is consistent with the ephemeris second which was based on astronomical measurements. (See #History below.) The realization of the standard second is described briefly in a special publication from the National Institute of Science and Technology lt;/ref> and in detail by the National Research Council of Canada lt;/ref>

Equivalence to other units of time

1 international second is equal to: * 1/60 minute (but see also leap second * 1/3,600 hour * 1/86,400 day (International Astronomical Union system of units) * 1/31,557,600 Julian year (astronomy) (IAU system of units)

History

Before mechanical clocks

The Egyptians subdivided daytime and nighttime into twelve hours each since at least 2000 BC, hence the seasonal variation of their hours. The Hellenistic astronomers Hipparchus (c.150 BC) and Ptolemy (c.AD 150) subdivided the day sexagesimal y 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 (a precision of less than 2 microseconds) by the Babylonia s 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 helek (where 1 hour 1080 halakim). lt;/ref> The Babylonians did not use the hour, but did use a double-hour lasting 120 modern minutes, a time-degree lasting four modern minutes, and a barleycorn lasting 3modern seconds (the helekof the modern Hebrew calendar).See page 325 in lt;/ref> In 1000, the Persian people 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. lt;/ref> 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. lt;/ref> Although a thirdfor of a second remains in some languages, for example Polish language (tercja and Turkish language (salise, the modern second is subdivided decimally.

Seconds measured by mechanical clocks

In 1577 Taqi al-Din Muhammad ibn Ma'ruf built a mechanical clock for the Istanbul observatory of Taqi al-Din that had three dials showing hours, minutes, and seconds (marked every five seconds, not every second).Sevim Tekeli, http://books.google.com/books?idraKRY3KQspsC&pgPA934 "Taqi al-Din"], Encyclopaedia of the history of science, technology, and medicine in non-Western cultures934–935. The observatory and its instruments were destroyed in 1580. The first mechanical clock displaying seconds in Western Europe was constructed in Switzerland at the beginning of the 17th century. The second first became accurately measurable with the development of pendulum clock keeping mean time(as opposed to the apparent timedisplayed by sundials), specifically in 1670 when William Clement added a seconds pendulum to the original pendulum clock of Christian Huygens See page 2 in 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 face of precision clocks.

Modern measurements

In 1956 the second was defined in terms of the period of revolution of the Earth around the Sun for a particular epoch (astronomy) because by then it had become recognized that the Earths rotation on its own axis was not sufficiently uniform as a standard of time. The Earths motion was described in Newcomb's Tables of the Sun (1895), which provide a formula estimating the motion of the Sun relative to the epoch 1900 based on astronomical observations made between 1750 and 1892. lt;/ref> The second thus defined is This definition was ratified by the Eleventh General Conference on Weights and Measuresin 1960. The [[tropical year]]in the definition was not measured, but calculated from a formula describing a mean tropical year that decreased linearly over time, hence the curious reference to a specific instantaneoustropical year. This definition of the second was in conformity with the ephemeris time scale adopted by the International Astronomical Union in 1952,Explanatory Supplement to the Astronomical Ephemeris and the American Ephemeris and Nautical Almanac(prepared jointly by the Nautical Almanac Offices of the United Kingdom and the United States of America, HMSO, London, 1961), at Sect. 1C, p.9), stating that at a conference "in March 1950 to discuss the fundamental constants of astronomy ... the recommendations with the most far-reaching consequences were those that defined ephemeris time and brought the lunar ephemeris into accordance with the solar ephemeris in terms of ephemeris time. These recommendations were addressed to the International Astronomical Union and were formally adopted by Commission 4 and the General Assembly of the Union in Rome in September 1952." defined as the measure of time that brings the observed positions of the celestial bodies into accord with the Newtonian dynamical theories of their motion (those accepted for use during most of the twentieth century being Newcomb's Tables of the Sun used from 1900 through 1983, and Ernest William Brown#Work on the motion of the Moon used from 1923 through 1983). 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. Following several years of work, Louis Essen from the National Physical Laboratory, UK (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. lt;/ref> Using a common-view measurement method based on the received signals from radio station WWV (radio station) lt;/ref> 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. They found that the second of ephemeris time (ET) had the duration of 9,192,631,770 ± 20 cycles of the chosen caesium frequency. As a result, in 1967 the Thirteenth Conférence Générale des Poids et Mesures defined the second of International Atomic Time in the International System of Units as This SI second, referred to atomic time, was later verified to be in agreement, within 1 part in 1010, with the second of ephemeris time as determined from lunar observations. lt;/ref> (Nevertheless, this SI second was already, when adopted, a little shorter than the then-current value of the second of mean solar time. lt;/ref>In the late 1950s, the caesium standard was used to measure both the current mean length of the second of mean solar time (UT2) ( and also the second of ephemeris time (ET) (, see As noted in page 162, the figure was chosen for the SI second. L Essen in the same 1968 article stated that this value "seemed reasonable in view of the variations in UT2".) During the 1970s it was realized that gravitational time dilation caused the second produced by each atomic clock to differ depending on its altitude 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 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.See page 515 in lt;/ref> The definition of the second was later refined at the 1997 meeting of the Bureau International des Poids et Mesures to include the statement The revised definition seems to imply that the ideal atomic clock contains 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 within which atomic clocks operate, and extrapolate accordingly to the value of the second at a temperature of absolute zero Today, the atomic clock operating in the microwave region is challenged by atomic clocks operating in the optical region. To quote Ludlow et al.lt;ref nameLudlow> }} “In recent years, optical atomic clocks have become increasingly competitive in performance with their microwave counterparts. The overall accuracy of single trapped ion based optical standards closely approaches that of the state-of-the-art caesium fountain standards. Large ensembles of ultracold alkaline earth atoms have provided impressive clock stability for short averaging times, surpassing that of single-ion based systems. So far, interrogation of neutral atom based optical standards has been carried out primarily in free space, unavoidably including atomic motional effects that typically limit the overall system accuracy. An alternative approach is to explore the ultranarrow optical transitions of atoms held in an optical lattice. The atoms are tightly localized so that Doppler and photon-recoil related effects on the transition frequency are eliminated.” The http://inms-ienm.nrc-cnrc.gc.ca/research/optical_frequency_projects_e.html#optical NRC] attaches a "relative uncertainty" of 2.5 (limited by day-to-day and device-to-device reproducibility) to their atomic clock based upon the 127I2 molecule, and is advocating use of an 88Sr ion trap instead (relative uncertainty due to linewidth of 2.2. See magneto-optical trap and Such uncertainties rival that of the NIST F-1 caesium atomic clock in the microwave region, estimated as a few parts in 1016 averaged over a day. lt;/ref> lt;/ref>

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 minute , hour , day , Julian year (astronomy) , Julian centuries, and Julian millennia are used.

See also

*Atomic clock *Becquerel *Hertz *International Atomic Time *International System of Units *Leap second *Magneto-optical trap *Orders of magnitude (time) *Time standard

References

External links

* http://www.npl.co.uk/server.php?showConWebDoc.1086 National Physical Laboratory: Trapped ion optical frequency standards] * http://resource.npl.co.uk/docs/networks/time/meeting3/klein.pdf High-accuracy strontium ion optical clock National Physical Laboratory (2005)] * http://inms-ienm.nrc-cnrc.gc.ca/research/optical_frequency_projects_e.html#optical National Research Council of Canada: Optical frequency standard based on a single trapped ion * http://physics.nist.gov/cuu/Units/second.html NIST: Definition of the second notice the cesium atom must be in its ground state at 0 K] * http://www.bipm.org/en/si/si_brochure/chapter2/2-1/second.html Official BIPM definition of the second] * http://tycho.usno.navy.mil/leapsec.html Seconds and leap seconds by the USNO]* http://www.cl.cam.ac.uk/~mgk25/time/metrologia-leapsecond.pdf The leap second: its history and possible future] * http://inms-ienm.nrc-cnrc.gc.ca/faq_time_e.html#10 What is a Cesium atom clock? Category:Centimetre gram second system of units Category:Orders of magnitude (time) Category:SI base units Category:Units of time als:Sekunde ar:ثانية an:Segundo arc:ܪܦܦܐ ast:Segundu bn:সেকেন্ড be:Секунда be-x-old:Сэкунда (адзінка вымярэньня часу) bo:སྐར་ཆ། bs:Sekunda br:Eilenn (amzer) bg:Секунда ca:Segon cs:Sekunda cy:Eiliad da:Sekund de:Sekunde et:Sekund el:Δευτερόλεπτο es:Segundo eo:Sekundo eu:Segundo fa:ثانیه fr:Seconde (temps) fy:Sekonde ga:Soicind gl:Segundo gan:秒 gu:સેકન્ડ hak:Miéu ko:초 (시간) hi:सैकण्ड hr:Sekunda io:Sekundo id:Detik ia:Secunda is:Sekúnda it:Secondo he:שנייה krc:Секунд ka:წამი sw:Sekunde ht:Segonn la:Secundum lv:Sekunde lb:Sekonn lt:Sekundė li:Secónd jbo:snidu hu:Másodperc mk:Секунда mg:Segondra ml:സെക്കന്റ് mr:सेकंद arz:ثانيه ms:Saat nl:Seconde ja:秒 no:Sekund nn:Sekund oc:Segonda uz:Soniya pnb:سکنٹ nds:Sekunn pl:Sekunda pt:Segundo ksh:Sekůndt ro:Secundă qu:Sikundu ru:Секунда sah:Сөкүүндэ sco:Seicont sq:Sekonda scn:Secunnu simple:Second sk:Sekunda sl:Sekunda szl:Sekůnda sr:Секунд fi:Sekunti sv:Sekund ta:நொடி (கால அளவு) th:วินาที tr:Saniye uk:Секунда ur:ثانیہ vi:Giây war:Segundo wo:Saa yi:סעקונדע zh-yue:秒 (時間) bat-smg:Sekondė zh:秒

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