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From Wikipedia, the free encyclopedia

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. It may be measured using a clock.

Early definitions of the second were based on the motion of the earth: 24 hours in a day meant that the second could be defined as 1⁄86 400 of the average time required for the earth to complete one rotation about its axis. 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 clocks, it became feasible to define the second based on fundamental properties of nature. Since 1967, the second has been defined to be

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.[1]

SI prefixes are frequently combined with the word second to denote subdivisions of the second, e.g., the millisecond (one thousandth of a second), the microsecond (one millionth of a second), and the nanosecond (one billionth of a second). Though SI prefixes may also be used to form multiples of the second such as kilosecond (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 was also the base unit of time in the centimetre-gram-second, metre-kilogram-second, metre-tonne-second, and foot-pound-second systems of units.

Contents

International second

Under the International System of Units, the second is currently defined as

The second is 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.[1]

This definition refers to a caesium atom at rest at a temperature of 0 K (absolute zero), and with appropriate corrections for gravitational time dilation. The ground state is defined at zero electric and magnetic fields. The second thus defined is consistent with 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 a special publication from the National Institute of Science and Technology,[2] and in detail by the National Research Council of Canada.[3]

Equivalence to other units of time

1 international second is equal to:

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 sexagesimally and also used a mean hour (1⁄24 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 1⁄60, by 1⁄60 of that, by 1⁄60 of that, etc, to at least six places after the sexagesimal point (a precision of less than 2 microseconds) 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).[4] 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 31⁄3 modern seconds (the helek of the modern Hebrew calendar).[5]

In 1000, the Persian 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.[6] 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.[7] Although a third for 1⁄60 of a second remains in some languages, for example Polish (tercja) and Turkish (salise), the modern second is subdivided decimally.

Seconds measured by mechanical clocks

The first clock that could show time in seconds was created by Taqi al-Din at the Istanbul observatory of Taqi al-Din between 1577-1580. He called it the "observational clock" in his In the Nabik Tree of the Extremity of Thoughts, where he described it as "a mechanical clock with three dials which show the hours, the minutes, and the seconds." He used it as an astronomical clock, particularly for measuring the right ascension of the stars.[8] The first mechanical clock displaying seconds in Western Europe was constructed in Switzerland at the beginning of the 17th century.[9]

The second first became accurately 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.[10] 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.

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, 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 (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.[11] 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.[11]

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. This definition of the second was in conformity with the ephemeris time scale adopted by the IAU in 1952,[12] 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 Brown's Tables of the Moon, used from 1923 through 1983).[11]

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 (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.[13][11] Using a common-view measurement method based on the received signals from radio station WWV,[14] 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.[13] 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.[11]

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.[15] (Nevertheless, this SI second was already, when adopted, a little shorter than the then-current value of the second of mean solar time.[16][17])

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 × 10−10. 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.[18]

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 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.[19] “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 NRC attaches a "relative uncertainty" of 2.5 × 10−11 (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 × 10−15). See magneto-optical trap and "Trapped ion optical frequency standards". National Physical Laboratory. http://www.npl.co.uk/server.php?show=ConWebDoc.1086.  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.[20][21]

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, days, Julian years, Julian centuries, and Julian millennia are used.

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. ^ a b "Official BIPM definition". BIPM. http://www.bipm.org/en/si/si_brochure/chapter2/2-1/second.html. Retrieved 2008-09-19. 
  2. ^ BN Taylor, A Thompson (Eds.), ed (2008). "Appendix 2". The International System of Units (SI). NIST Special Publication. 330. pp. 53 ff. http://physics.nist.gov/Pubs/SP330/sp330.pdf. Retrieved 2009-08-19. 
  3. ^ "NRC's Cesium Fountain Clock - FCs1". National Research Council of Canada. http://www.nrc-cnrc.gc.ca/eng/projects/inms/fountain-clock.html. Retrieved 2009-08-19. 
  4. ^ O Neugebauer (1975). A history of ancient mathematical astronomy. Springer-Verlag. ISBN 038706995X. 
  5. ^ See page 325 in O Neugebauer (1949). "The astronomy of Maimonides and its sources". Hebrew Union College Annual 22: 321–360. 
  6. ^ 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". pp. 147–149. http://books.google.com/books?id=pFIEAAAAIAAJ&pg=PA148#v=onepage&q=&f=false. 
  7. ^ R Bacon (2000) [1928]. The Opus Majus of Roger Bacon. University of Pennsylvania Press. table facing page 231. ISBN 9781855068568. 
  8. ^ S Tekeli (1997). "Taqi al-Din". Encyclopaedia of the History of Science, Technology, and Medicine in Non-Western Cultures. Kluwer Academic Publishers. ISBN 0792340663. 
  9. ^ C Amalfi (18 December 2008). "Keeping Time". ABC Science. ABC News. http://www.abc.net.au/science/articles/2008/12/18/2450349.htm. Retrieved 2009-08-19. 
  10. ^ See page 2 in J Chappell (2002). "The Long Case Clock: The Science and Engineering that Goes Into a Grandfather Clock". Illumin 1 (0): 1. http://illumin.usc.edu/article.php?articleID=64&page=1. 
  11. ^ a b c d e "Leap Seconds". Time Service Department, United States Naval Observatory. http://tycho.usno.navy.mil/leapsec.html. Retrieved 2006-12-31. 
  12. ^ 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 which 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."
  13. ^ a b W Markowitz, RG Hall, L Essen, JVL Parry (1958). "Frequency of cesium in terms of ephemeris time". Physical Review Letters 1: 105–107. doi:10.1103/PhysRevLett.1.105. http://www.leapsecond.com/history/1958-PhysRev-v1-n3-Markowitz-Hall-Essen-Parry.pdf. 
  14. ^ S Leschiutta (2005). "The definition of the 'atomic' second". Metrologia 42 (3): S10–S19. doi:10.1088/0026-1394/42/3/S03. 
  15. ^ W Markowitz (1988). AK Babcock, GA Wilkins. ed. The Earth's Rotation and Reference Frames for Geodesy and Geophysics. IAU Sumposia #128. pp. 413–418. Bibcode: 1988IAUS..128..413M. 
  16. ^ DD McCarthy, C Hackman, R Nelson (2008). "The Physical Basis of the Leap Second". Astronomical Journal 136: 1906–1908. doi:10.1088/0004-6256/136/5/1906. "... the SI second is equivalent to an older measure of the second of UT1, which was too small to start with and further, as the duration of the UT1 second increases, the discrepancy widens.". 
  17. ^ In the late 1950s, the caesium standard was used to measure both the current mean length of the second of mean solar time (UT2) (9,192,631,830 cycles) and also the second of ephemeris time (ET) (9,192,631,770±20 cycles), see L Essen (1968). "Time Scales". Metrologia 4: 161–165. doi:10.1088/0026-1394/4/4/003. http://www.leapsecond.com/history/1968-Metrologia-v4-n4-Essen.pdf. . As noted in page 162, the 9,192,631,770 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".
  18. ^ See page 515 in RA Nelsonet al. (2000). "The leap second: its history and possible future". Metrologia 38: 509–529. doi:10.1088/0026-1394/38/6/6. http://www.cl.cam.ac.uk/~mgk25/time/metrologia-leapsecond.pdf. 
  19. ^ AD Ludlow et al. (2006). "Systematic study of the 87Sr clock transition in an optical lattice". Physical Review Letters 96: 033003. doi:10.1103/PhysRevLett.96.033003. arΧiv:physics/0508041. 
  20. ^ R Wynands, S Weyers (2005). "Atomic fountain clocks". Metrologia 42: S64–S79. doi:10.1088/0026-1394/42/3/S08. 
  21. ^ "NIST-F1 Cesium Fountain Atomic Clock". NIST. http://tf.nist.gov/cesium/fountain.htm. Retrieved 2009-08-19. 

External links

SI base units
Metre · Kilogram · Second · Ampere · Kelvin · Mole · Candela
SI derived units

1911 encyclopedia

Up to date as of January 14, 2010

From LoveToKnow 1911

SECOND (through Fr. from Lat. secundus, following, sequi, to follow), next after the first in order, time, rank, &c., more particularly the ordinal number corresponding to two. It is the only French ordinal in English; the older word was "other," Ger. ander, Goth. anthar, Skt. antara. The use of the word for the sixtieth part of a minute of time and of degree is from Med. Lat. secunda, abbreviation of minuta secunda, the second small division of the hour, minuta prima or minuta being the first division. Another particular meaning is for one who supports or assists another, especially the friend at a duel, who arranges for his principal the terms of the encounter and sees that all rules of the duel are carried out. In the British army an officer is said to be "seconded" (with the accent on the second syllable) when he is employed on special service outside his regiment, his name being retained on the regimental list, but his place being filled by promotion of other officers. He may rejoin his regiment when his special employment is at an end.


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Wiktionary

Up to date as of January 14, 2010
(Redirected to second article)

Definition from Wiktionary, a free dictionary

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English

Etymology 1

From Old French seconde, from Latin secundus (“‘following, next in order’”), from root of sequi (“‘follow’”), from Proto-Indo-European *sekʷ- (“‘to follow’”).

English ordinal numbers
 <  1st 2nd 3rd   > 
    Ordinal : second
    Cardinal : two

Pronunciation

  • enPR: sĕʹkənd, IPA: /ˈsɛk.ənd/, SAMPA: /"sEk.@nd/
     Audio (US)help, file
  • Hyphenation: sec‧ond

Adjective

second (not comparable)

Positive
second

Comparative
not comparable

Superlative
none (absolute)
  1. The ordinal number corresponding to the cardinal number two.
  2. Number-two; following immediately after the first one.
    He lives on Second Street.
    The second book in "The Lord of the Rings" series is called "The Two Towers".
  3. That which comes after the first.
    You take the first one, and I'll have the second.
Alternative forms
  • (number-two): 2nd, 2nd, 2d, II, IInd, IInd
Derived terms
terms derived from second (adjective)
Translations
second (numeral)
that which comes after the first
The translations below need to be checked and inserted above into the appropriate translation tables, removing any numbers. Numbers do not necessarily match those in definitions. See instructions at Help:How to check translations.
Translations to be checked

Noun

Singular
second

Plural
seconds

second (plural seconds)

  1. (usually in the plural) A manufactured item that, though still usable, fails to meet quality control standards.
    They were discounted because they contained blemishes, nicks or were otherwise factory seconds.
  2. (usually in the plural) An additional helping of food.
    That was good barbecue. I hope I can get seconds.
  3. Another chance to achieve what should have been done the first time, usually indicating success this time around. (See second-guess.)
  4. (music) The interval between two adjacent notes in a diatonic scale (either or both of them may be raised or lowered from the basic scale via any type of accidental).
  5. The second gear of an engine.
  6. (baseball) Second base.
Translations
manufactured item that fails to meet quality control standards
another chance to achieve what should have been done the first time
interval between two adjacent notes in a diatonic scale (with or without extra accidentals)
second gear
second base

Etymology 2

From Old French seconde, from Mediaeval Latin secunda, short for secunda pars minuta (“‘second diminished part (of the hour)’”)

Alternative forms

  • (SI unit of time): (abbreviations) s, sec; (symbols) s (SI and non-scientific usage), sec (in non-scientific usage only)
  • (unit of angle): (abbreviations) arcsec, "

Pronunciation

  • enPR: sĕ'kənd, IPA: /ˈsɛk.ənd/, SAMPA: /"sEk.@nd/
     Audio (US)help, file
  • Hyphenation: sec‧ond

Noun

Singular
second

Plural
seconds

second (plural seconds)

  1. The SI unit of time, defined as the duration of 9,192,631,770 periods of radiation corresponding to the transition between two hyperfine levels of caesium-133 in a ground state at a temperature of absolute zero and at rest; one-sixtieth of a minute.
  2. A unit of angle equal to one-sixtieth of a minute of arc or one part in 3600 of a degree.
  3. A short, indeterminate amount of time.
    I'll be there in a second.
Synonyms
Derived terms
Translations
SI unit of time
unit of angular measure
short, indeterminate amount of time
The translations below need to be checked and inserted above into the appropriate translation tables, removing any numbers. Numbers do not necessarily match those in definitions. See instructions at Help:How to check translations.
Translations to be checked

Etymology 3

From Middle French seconder, from Latin secundo (“‘assist, make favorable’”)

Pronunciation

Transfer temporarily
  • (UK) enPR: səkŏnd', IPA: /səˈkɒnd/, SAMPA: /"s@kQnd/
    Rhymes: -ɒnd
  • Hyphenation: sec‧ond
Assist, Agree
  • (British, US) enPR: sĕ'kənd, IPA: /ˈsɛkənd/, SAMPA: /"sEk@nd/
     Audio (US)help, file
  • Hyphenation: sec‧ond

Verb

Infinitive
to second

Third person singular
seconds

Simple past
seconded

Past participle
seconded

Present participle
seconding

to second (third-person singular simple present seconds, present participle seconding, simple past and past participle seconded)

  1. (transitive, British) Transfer temporarily to alternative employment.
    • 1998 — Paul Leonard, Dreamstone Moon, ch 9
      Daniel had still been surprised, however, to find the lab area deserted, all the scientists apparently seconded by Cleomides's military friends.
  2. (transitive) To assist.
  3. (transitive) To agree as a second person to (a proposal), usually to reach a necessary quorum of two.
    I second the motion.
Derived terms
Translations
temporary transfer of employment
to agree as a second person

Noun

Singular
second

Plural
seconds

second (plural seconds)

  1. The attendant of a contestant in a duel or boxing match, who must be ready to take over if the contestant drops out. In the case of a duel, the seconds may also fight each other at 90° to the other contestants.
  2. One who agrees in addition, or such a motion, as required in certain meetings to pass judgement etc.
    If we want the motion to pass, we will need a second.
Translations
attendant of a duel or boxing match standing in for a contestant
one who agrees in addition

See also

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Anagrams

French

rank noun
1 premier
2 deuxième (or second)
3 troisième
4 quatrième
5 cinquième
6 sixième
7 septième
8 huitième
9 neuvième
10 dixième
rank noun
11 onzième
12 douzième
13 treizième
14 quatorzième
15 quinzième
16 seizième
17 dix-septième
18 dix-huitième
19 dix-neuvième
20 vingtième

Alternative forms

  • (abbreviation) 2e f.

Etymology

From Latin secundus (“‘second’”); related to sequi (“‘follow’”).

Pronunciation

  • SAMPA: /s(@)go~/, /s(@)go~t/ (with liaison)
  •  audiohelp, file

Adjective

second m. (prepositive, feminine seconde)

  1. Second (ordinal numeral)
  2. alternate

Synonyms

Noun

second m. (plural seconds)

  1. assistant

See also

Anagrams


Simple English

The English used in this article or section may not be easy for everybody to understand.
 You can help Wikipedia by making this page or section simpler.
For other uses of "second", see second (disambiguation).

The second (symbol: s) is a unit of time, and one of the seven SI base units. It is the time taken by 9,192,631,770 cycles of radiation that comes from electrons moving between two energy levels of the caesium-133 atom.

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.

Other (non-SI) units of time are defined in terms of the second. There are 86,400 seconds in one day, 3,600 seconds in one hour, and 60 seconds in one minute.

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).

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.[1] 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.[1] The second thus defined is equivalent to the ephemeris second.

The international standard symbol for a second is s (see ISO 31-1)

Equivalence to other units of time

1 international second is equal to:

  • 1/60 minute (1 minute is equal to 60 seconds)
  • 1/3,600 hour (1 hour is equal to 3,600 seconds)
  • 1/86,400 day (1 day, in the sense of non-SI units accepted for use with the International System of Units, is equal to 86,400 seconds)

There are 31,536,000 seconds in a common year, 31,622,400 seconds in a leap year, and 31,557,600 seconds in a Julian year

Historical origin

Originally, the second was known as a "second minute", meaning the second minute (i.e. small) division of an hour. The first division was known as a "prime minute" and is equivalent to the minute we know today.

The factor of 60 comes from the Babylonians who used factors of 60 in their counting system. However, the Babylonians did not subdivide their time units sexagesimally (except for the day). The hour had been defined by the ancient Egyptians as either 1/12 of daytime or 1/12 of nighttime, hence both varied with the seasons. Hellenistic astronomers, including Hipparchus and Ptolemy, defined the hour as 1/24 of a mean solar day. Sexagesimally subdividing this mean solar hour made the second 1/86 400 of a mean solar day.[needs proof] Hellenistic time periods like the mean synodic month were usually specified quite precisely because they were calculated from carefully selected eclipses separated by hundreds of years—individual mean synodic months and similar time periods cannot be measured. Nevertheless, with the development of pendulum clocks keeping mean time (as opposed to the apparent time displayed by sundials), the second became measurable. The seconds pendulum was proposed as a unit of length as early as 1660 by the Royal Society of London. The duration of a beat or half period (one swing, not back and forth) of a pendulum one metre in length on the Earth's surface is approximately one second.[2]

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.[1] 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.[1]

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 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.[1]

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 (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.[1] Using a common-view measurement method based on the received signals from radio station WWV,[3] 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 (SI) 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.[1]

The ground state is defined at zero magnetic field. The second thus defined is equivalent to the ephemeris second.[1]

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.

The second in Role Playing Games

Sometimes in RPGs a second is used to refer to a small period of time or a single turn of combat. It is used as a standard moment of time, and does not necessarily refer to a real second, and could be shorter or longer depending on the scenario.

Trivia

  • Until modern times, degrees and hours were divided successively by 60 in pars minuta prima, pars minuta secunda, pars minuta tertia and so on. This evolved to the modern minute and second, but for smaller divisions we follow now the decimal division. In some languages, dictionaries still keep the word for third for 1/60 of a second, for example Polish (tercja) and Arabic (ثالثة).

Other pages

Simple English Wiktionary has the word meaning for:

References

  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 "Leap Seconds". Time Service Department, United States Naval Observatory. http://tycho.usno.navy.mil/leapsec.html. Retrieved 2006-12-31. 
  2. ↑ The seconds pendulum
  3. ↑ Leschiutta, Sigfrido. "The definition of the 'atomic' second". Metrologia 42 (3): S10–S19. http://stacks.iop.org/0026-1394/42/S10. 

Other websites

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