The SECOND (symbol: S) (abbreviated S or SEC) is the base unit of
time in the
International System of Units
SI prefixes are 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 nonSI 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 other systems of measurement : the centimetre–gram–second , metre–kilogram–second , metre–tonne–second , and foot–pound–second systems of units. CONTENTS * 1 International second * 2 Equivalence to other units * 2.1
Time
* 3
History
* 3.1 Early civilizations * 3.2 Based on subdivisions of the moon cycle * 3.3 Based on mechanical clocks * 3.4 Based on a fraction of a year * 3.5 Based on caesium microwave atomic clock * 3.6 Proposed: based on optical atomic clock * 3.7 Modern folklore * 4 SI multiples * 5 Other current definitions * 6 See also * 7 Notes and references * 8 External links INTERNATIONAL SECOND Under the
International System of Units
The second thus defined is consistent with the ephemeris second ,
which was based on astronomical measurements. (See
History
EQUIVALENCE TO OTHER UNITS TIME UNITS Main article:
Time
1 international second is equal to: * 1⁄60 minute (but see also leap second ) * 1⁄3,600 hour * 1⁄86,400 day (IAU system of units) * 1⁄31,557,600 Julian year (IAU system of units) FREQUENCY UNITS Main article: Reciprocal second * 1⁄(1 hertz ); more generally, (period of wave in seconds) =
1⁄(frequency of wave in hertz), where (period of
wave)×(wavenumber ) = 1⁄(velocity of wave) in seconds per metre
(SI ) or in kayser seconds (
CGS
HISTORY OF DEFINITION EARLY CIVILIZATIONS Early civilizations constructed divisions in the day, but none used the term second, and none was a precursor to the modern second: * The Egyptians since 2000 BC subdivided daytime and nighttime into
twelve hours each, hence the seasonal variation of the length of their
hours, and the differences in length between daytime and nighttime
hours in any given day.
* The
Hellenistic astronomers
Hipparchus
BASED ON SUBDIVISIONS OF THE MOON CYCLE * Circa 1000, the Persian scholar alBiruni , writing in Arabic, used the term second, and defined the division of time between new moons of certain specific weeks as a number of days, hours, minutes, seconds, thirds, and fourths after noon Sunday. * In 1267, the medieval scientist Roger Bacon , writing in Latin, defined the division of time between full moons as a number of hours, minutes, seconds, thirds, and fourths (horae, minuta, secunda, tertia, and quarta) after noon on specified calendar dates. * The modern second is subdivided using decimals  although the term third ( 1⁄60 of a second) remains in some languages, for example Polish (tercja) and Turkish (salise). BASED ON MECHANICAL CLOCKS The earliest clocks to display seconds appeared during the last half
of the 16th century. The second became accurately measurable with the
development of mechanical clocks keeping mean time, as opposed to the
apparent time displayed by sundials . The earliest springdriven
timepiece with a second hand which marked seconds is an unsigned clock
depicting
Orpheus
In 1644, Marin Mersenne calculated that a pendulum with a length of 39.1 inches (0.994 m) would have a period at one standard gravity of precisely two seconds, one second for a swing forward and one second for the return swing, enabling such a pendulum to tick in precise seconds. In 1670,
London
In 1832, Gauss proposed using the second as the base unit of time in
his millimetermilligramsecond system of units . The British
Association for the Advancement of Science (BAAS) in 1862 stated that
"All men of science are agreed to use the second of mean solar time as
the unit of time." BAAS formally proposed the
CGS
BASED ON A FRACTION OF A YEAR In 1956, the second was redefined in terms of a year (the period of
the
Earth
The second was thus defined as: the fraction 1⁄31,556,925.9747 of the tropical year for 1900 January 0 at 12 hours ephemeris time. This definition was ratified by the Eleventh General Conference on Weights and Measures in 1960, which also established the International System of Units . The tropical year in the 1960 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 instantaneous tropical year. This was in conformity with the ephemeris time scale adopted by the IAU in 1952. This definition brings the observed positions of the celestial bodies into accord with Newtonian dynamical theories of their motion. Specifically, those tables used for most of the 20th century were Newcomb\'s Tables of the Sun (used from 1900 through 1983) and Brown\'s Tables of the Moon (used from 1923 through 1983). Thus, the 1960 SI definition abandoned any explicit relationship between the scientific second and the length of a day , as most people understand the term. BASED ON CAESIUM MICROWAVE ATOMIC CLOCK With the development of the atomic clock in the early 1960s, it was
decided to use atomic time as the basis of the definition of the
second, rather than the revolution of the
Earth
Following several years of work,
Louis Essen from the National
Physical Laboratory (Teddington, England) and
William Markowitz
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 caesium133 atom. 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. (Nevertheless, this SI second was already, when adopted, a little shorter than the thencurrent value of the second of mean solar time. ) 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. The definition of the second was later refined at the 1997 meeting of
the
BIPM
This definition refers to a caesium atom at rest at a temperature of 0 K. 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 highprecision realizations of the second should compensate for the effects of the ambient temperature (blackbody radiation ) within which atomic clocks operate, and extrapolate accordingly to the value of the second at a temperature of absolute zero . PROPOSED: BASED ON OPTICAL ATOMIC CLOCK Today, the atomic clock operating in the microwave region is challenged by atomic clocks operating in the optical region. To quote Ludlow et al., “In recent years, optical atomic clocks have become increasingly competitive in performance with their microwave counterparts. The overall accuracy of singletrappedionbased optical standards closely approaches that of the stateoftheart caesium fountain standards. Large ensembles of ultracold alkaline earth atoms have provided impressive clock stability for short averaging times, surpassing that of singleionbased systems. So far, interrogation of neutralatombased 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 photonrecoil related effects on the transition frequency are eliminated.” The Canadian National Research Council attaches a "relative uncertainty" of 2.5×10−11 (limited by daytoday and devicetodevice 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 magnetooptical trap and "Trapped ion optical frequency standards". National Physical Laboratory . Such uncertainties rival that of the NISTF1 caesium atomic clock in the microwave region, estimated as a few parts in 1016 averaged over a day. MODERN FOLKLORE * The phrase "One Mississippi, Two Mississippi" is one of several similar phrases used to measure time verbally. SI MULTIPLES SI prefixes are commonly used to measure time less than a second, but rarely for multiples of a second (which is known as metric time ). Instead, the nonSI units minutes , hours , days , Julian years , Julian centuries, and Julian millennia are used. SI multiples for second (s) SUBMULTIPLES MULTIPLES VALUE SI SYMBOL NAME VALUE SI 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 Thus a megasecond is 11 days, 13 hours, 46 minutes and 40 seconds, which is roughly of the order of a week. A kilosecond is 16 minutes, 40 seconds, or the length of a short break. A gigasecond is 31.7 years, so typical human lifespans are 2 to 3 gigaseconds. OTHER CURRENT DEFINITIONS For specialized purposes, a second may be used as a unit of time in time scales where the precise length differs slightly from the SI definition. One such time scale is UT1, a form of universal time . McCarthy and Seidelmann refrain from stating that the SI second is the legal standard for timekeeping throughout the world, saying only that "over the years UTC has become either the basis for legal time of many countries, or accepted as the de facto basis for standard civil time". SEE ALSO *
Time
*
SI unit
NOTES AND REFERENCES * ^ A B C D "
Unit of time (second)". SI Brochure.
BIPM
