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Encyclopedia > Second

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. Second has a number of different meanings: Angle: Unit of Angle: The arcsecond or second of arc (symbol: â€³) Unit of Angle: The second of right ascension (symbol: s) Democracy: In relation to democratic procedure: To second is to support a proposal. ... â€œSIâ€ redirects here. ... The former Weights and Measures office in Middlesex, England. ... Look up time in Wiktionary, the free dictionary. ... â€œSIâ€ redirects here. ... The SI system of units defines seven SI base units: physical units defined by an operational definition. ...

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). 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 submultiple. ... To help compare orders of magnitude of different times this page lists times between 10&#8722;3 seconds and 10&#8722;2 seconds (1. ... To help compare orders of magnitude of different times this page lists times between 10-9 seconds and 10-8 seconds (1 nanosecond and 10 nanoseconds) See also times of other orders of magnitude. ... To help compare orders of magnitude of different times this page lists times between 103 seconds (a kilosecond) and 104 seconds (16. ... A minute is a unit of time equal to 1/60th of an hour and to 60 seconds. ... The hour (symbol: h) is a unit of time. ... Look up day in Wiktionary, the free dictionary. ...

## Contents

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, which was based on astronomical measurements. (See Historical origin below.) Periodicity is the quality of occurring at regular intervals (e. ... In atomic physics, hyperfine structure is a small perturbation in the energy levels (or spectra) of atoms or molecules due to the magnetic dipole-dipole interaction, arising from the interaction of the nuclear magnetic dipole with the magnetic field of the electron. ... In physics, the ground state of a quantum mechanical system is its lowest-energy state. ... General Name, Symbol, Number caesium, Cs, 55 Chemical series alkali metals Group, Period, Block 1, 6, s Appearance silvery gold Standard atomic weight 132. ... For other uses, see Atom (disambiguation). ... For other uses, see Kelvin (disambiguation). ... Absolute zero is the lowest possible temperature where nothing could be colder, and no heat energy remains in a substance. ... Magnetic field lines shown by iron filings In physics, the space surrounding moving electric charges, changing electric fields and magnetic dipoles contains a magnetic field. ...

The international standard symbol for a second is s (see ISO 31-1) ISO 31-1 is the part of international standard ISO 31 that defines names and symbols for quantities and units related to space and time. ...

## Equivalence to other units of time

1 international second is equal to:

A minute is a unit of time equal to 1/60th of an hour and to 60 seconds. ... The hour (symbol: h) is a unit of time. ... Look up day in Wiktionary, the free dictionary. ... Logo of the IAU The International Astronomical Union (French: Union astronomique internationale) unites national astronomical societies from around the world. ... In astronomy, a Julian year is a unit of time defined as exactly 365. ...

## Historical origin

Originally,[clarify] the hour was the subdivided sexagesimally (by 60) into pars minuta prima (first small part, our modern minute), pars minuta secunda (second small part, our modern second), pars minuta tertia (third small part) and so on. Although a third for 1/60 of a second remains in some languages, for example Polish (tercja) and Arabic (ثالثة), the modern second is now subdivided decimally. The sexagesimal (base-sixty) is a numeral system with sixty as the base. ... A minute is a unit of time equal to 1/60th of an hour and to 60 seconds. ...

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 This article is about Earth as a planet. ... In astronomy, an epoch is a moment in time for which celestial coordinates or orbital elements are specified. ... Newcombs Tables of the Sun is the short title for a work by the American astronomer and mathematician Simon Newcomb entitled Tables of the Motion of the Earth on its Axis and Around the Sun on pages 1-169 of volume VI of the serial publication Astronomical Papers prepared...

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 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.[1] A tropical year is the length of time that the Sun, as viewed from the Earth, takes to return to the same position along the ecliptic (its path among the stars on the celestial sphere). ... Ephemeris Time (ET) is a now obsolete time scale used in ephemerides of celestial bodies, in particular the Sun (as observed from the Earth), Moon, planets, and other members of the solar system. ... An ephemeris (plural: ephemerides) (from the Greek word ephemeros = daily) is a device giving the positions of astronomical objects in the sky. ... Ernest William Brown (November 29, 1866 &#8211; July 22, 1938) was a British astronomer. ...

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. Chip-scale atomic clock unveiled by NIST An Atomic Clock is a type of clock that uses an atomic resonance frequency standard to feed its counter. ...

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]

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.[4] Gravitational time dilation is a consequence of Albert Einsteins theories of relativity and related theories which causes time to pass at different rates in regions of a different gravitational potential; the higher the local distortion of spacetime due to gravity, the slower time passes. ... Altitude is the elevation of an object from a known level or datum. ... For considerations of sea level change, in particular rise associated with possible global warming, see sea level rise. ... The GOCE project will measure high-accuracy gravity gradients and provide an accurate geoid model based on the Earths gravity field. ... In relativity, proper time is time measured by a single clock between events that occur at the same place as the clock. ...

The definition of the second was later refined at the 1997 meeting of the BIPM to include the statement The Bureau International des Poids et Mesures (International Bureau of Weights and Measures, or BIPM) is a standards organization, one of the three organizations established to maintain the SI system under the terms of the Metre Convention. ...

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. As the temperature decreases, the peak of the black body radiation curve moves to lower intensities and longer wavelengths. ...

Look up second in Wiktionary, the free dictionary.

Wikipedia does not have an article with this exact name. ... Wiktionary (a portmanteau of wiki and dictionary) is a multilingual, Web-based project to create a free content dictionary, available in over 150 languages. ... A leap second is a one-second adjustment to civil time in order to keep it close to the mean solar time. ... The pages linked in the right-hand column contain lists of times that are of the same order of magnitude (power of ten). ... ... This article is about the SI unit of frequency. ... The becquerel (symbol Bq) is the SI derived unit of radioactivity, defined as the activity of a quantity of radioactive material in which one nucleus decays per second. ...

## References

1. ^ a b c d e f g Leap Seconds. Time Service Department, United States Naval Observatory. Retrieved on 2006-12-31.
2. ^ The seconds pendulum
3. ^ Leschiutta, Sigfrido. "The definition of the 'atomic' second". Metrologia 42 (3): S10–S19.
4. ^ R. A. Nelson et al., "The leap second: its history and possible futurePDF (381 KiB)", Metrologia 38 (2000) 509-529, p. 515.

Aerial view of USNO. The United States Naval Observatory (USNO) is one of the oldest scientific agencies in the United States. ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... is the 365th day of the year (366th in leap years) in the Gregorian calendar. ... â€œPDFâ€ redirects here. ... A kibibyte (a contraction of kilo binary byte) is a unit of information or computer storage, commonly abbreviated KiB (never kiB). 1 kibibyte = 210 bytes = 1,024 bytes The kibibyte is closely related to the kilobyte, which can be used either as a synonym for kibibyte or to refer to...

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 Second Site (638 words) Second Site is a genealogy web site builder for use with TMG. See Second Site News #031 for a description of recent changes, see Second Site News #029 for a description of the major new features in SS2, and see the Change Log for details. Second Site does not publish the data for you.
 Second - Wikipedia, the free encyclopedia (897 words) 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. 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. 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.
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