Second

 This article is about the unit of time. See second (disambiguation) for other uses
The second (symbol s) is a unit for time, and one of seven SI base units. It is 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 caesium133 atom at zero kelvins.
In common reckoning of time, a second is 1/60 of a minute, and 1/3600 of an hour.
Contents 
History
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 magnitude of this division was set at 1/60, that is, there are 60 minutes in one hour and 60 seconds in one minute. This may have been influenced by the Babylonians, who used a sexagesimal (base60) counting system. The hour had previously been defined by the Egyptians in terms of the rotation of the Earth as 1/24 of a mean solar day. This made the second 1/86,400 of a mean solar day.
In 1956, the International Committee for Weights and Measures, under the authority given it by the Tenth General Conference on Weights and Measures in 1954, defined the second 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 during the eighteenth and nineteenth centuries. 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.
This definition was ratified by the Eleventh General Conference on Weights and Measures in 1960. Reference to the year 1900 does not mean that this is the epoch of a mean solar day of 86,400 seconds. Rather, it is the epoch of the tropical year of 31,556,925.9747 seconds of ephemeris time. Ephemeris Time (ET) was defined as the measure of time that brings the observed positions of the celestial bodies into accord with the Newtonian dynamical theory of motion.
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 rotation of the earth.
Following several years of work, two astronomers at the United States Naval Observatory (USNO) and two astronomers at the National Physical Laboratory (Teddington, England) determined the relationship between the hyperfine transition frequency of the caesium atom and the ephemeris second. Using a commonview measurement method based on the received signals from radio station WWV, 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 caesium133 atom.
The ground state is defined at zero magnetic field. The second thus defined is equivalent to the ephemeris second.
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.
In practice, this means that highprecision realizations of the second should compensate for the effects of ambient radiation to try to extrapolate to the value of the second as defined above.
Multiples and submultiples
 A millisecond is an SIderived unit of time, equal to one thousandth of a second.
 A microsecond is an SI unit of time equal to one millionth (10^{6}) of a second. It is often used for measuring things like atomic and chemical reactions, which occur in normally imperceptible lengths of time. Its symbol is µs.
 A nanosecond is an SI derived unit of time equal to 10^{9} of a second.
 It is only infrequently put into everyday use. In technical situations it is however a very common unit, especially in the line of telecommunications, pulsed lasers and some areas of electronics.
 In 1 ns, light travels exactly 29.9792458 cm in a vacuum (via the definition of the metre). But the speed of light is slower in materials, indicated by an index of refraction greater than 1. Thus in air (1.003), light travels about 29.89 cm in 1 ns, but it travels only about 22.54 cm in water (1.33) each nanosecond.
 A picosecond is an SI unit of time equal to 10^{12} of a second, or one trillionth (ie, one million millionth) of a second.
 A femtosecond is the SI unit of time equal to 10^{15} of a second.
 The waves of visible light oscillate with a period of about 1 femtosecond.
 An attosecond is an SI unit of time equal to 10^{18} of a second.
 The current shortest measurable period of time (as of February 2004) is 100 attoseconds.
(BBC News) (http://news.bbc.co.uk/2/hi/science/nature/3486160.stm)
 A zeptosecond is an SI unit of time equal to 10^{21} of a second.
 A yoctosecond is an SI unit of time equal to one septillionth (10^{24}) of a second.
See also
External links
 Official BIPM definition of the second (http://www.bipm.fr/en/si/si_brochure/chapter2/21/second.html)
 Public domain article about definition of seconds and leap seconds (http://tycho.usno.navy.mil/leapsec.html)
 wiktionary:secondca:segon
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