The second (symbol: s) is a unit of time derived from the division of the day first into 24 , then to 60 , and finally to 60 seconds each (24 × 60 × 60 = 86400). The current and formal definition in the International System of Units (SI) is more precise:
The second ... is defined by taking the fixed numerical value of the caesium frequency, Δ νCs, the unperturbed Ground state hyperfine transition frequency of the caesium 133 atom, to be when expressed in the unit Hertz, which is equal to s−1.
This current definition was adopted in 1967 when it became feasible to define the second based on fundamental properties of nature with . As the speed of Earth's rotation varies and is slowing ever so slightly, a leap second is added at irregular intervals to civil time to keep clocks in sync with Earth's rotation.
The definition that is based on of a rotation of the earth is still used by the Universal Time (UT1) system.
are frequently combined with the word second to denote subdivisions of the second: milliseconds (thousandths), microseconds (millionths), nanoseconds (billionths), and sometimes smaller units of a second. Multiples of seconds are usually counted in hours and minutes. Though SI prefixes may also be used to form multiples of the second such as kiloseconds (thousands of seconds), such units are rarely used in practice. An everyday experience with small fractions of a second is a 1-gigahertz microprocessor that has a cycle time of 1 nanosecond. Camera are often expressed in fractions of a second, such as second or second.
Sexagesimal divisions of the day from a calendar based on astronomical observation have existed since the third millennium BC, though they were not seconds as we know them today. Small divisions of time could not be measured back then, so such divisions were mathematically derived. The first timekeepers that could count seconds accurately were pendulum clocks invented in the 17th century. Starting in the 1950s, became better timekeepers than Earth's rotation, and they continue to set the standard today.
The difference between apparent solar time and mean time was recognized by astronomers since antiquity, but prior to the invention of accurate mechanical clocks in the mid-17th century, sundials were the only reliable timepieces, and apparent solar time was the only generally accepted standard.
Some common units of time in seconds are: a minute is 60 seconds; an hour is 3,600 seconds; a day is 86,400 seconds; a week is 604,800 seconds; a year (other than ) is 31,536,000 seconds; and a (Gregorian) century averages 3,155,695,200 seconds; with all of the above excluding any possible . In astronomy, a Julian year is precisely 31,557,600 seconds.
Some common events in seconds are: a stone falls about 4.9 meters from rest in one second; a pendulum of length about one meter has a swing of one second, so pendulum clocks have pendulums about a meter long; the fastest human sprinters run 10 meters in a second; an ocean wave in deep water travels about 23 meters in one second; sound travels about 343 meters in one second in air; light takes 1.3 seconds to reach Earth from the surface of the Moon, a distance of 384,400 kilometers.
Moreover, most other SI base units are defined by their relationship to the second: the meter is defined by setting the speed of light (in vacuum) to be 299 792 458 m/s, exactly; definitions of the SI base units kilogram, ampere, kelvin, and candela also depend on the second. The only base unit whose definition does not depend on the second is the mole, and only two of the 22 named derived units, radian and steradian, do not depend on the second either.
Civil time is defined to agree with the rotation of the Earth. The international standard for timekeeping is Coordinated Universal Time (UTC). This time scale "ticks" the same atomic seconds as TAI, but inserts or omits as necessary to correct for variations in the rate of rotation of the Earth.
A time scale in which the seconds are not exactly equal to atomic seconds is UT1, a form of universal time. UT1 is defined by the rotation of the Earth with respect to the Sun, and does not contain any leap seconds. UT1 always differs from UTC by less than a second.
There are references to "second" as part of a lunar month in the writings of natural philosophers of the Middle Ages, which were mathematical subdivisions that could not be measured mechanically.
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. The earliest spring-driven timepiece with a second hand that marked seconds is an unsigned clock depicting Orpheus in the Fremersdorf collection, dated between 1560 and During the third quarter of the 16th century, Taqi al-Din built a clock with marks every minute.
In 1579, Jost Bürgi built a clock for William of Hesse that marked seconds. In 1581, Tycho Brahe redesigned clocks that had displayed only minutes at his observatory so they also displayed seconds, even though those seconds were not accurate. In 1587, Tycho complained that his four clocks disagreed by plus or minus four seconds.In 1656, Dutch scientist Christiaan Huygens invented the first pendulum clock. It had a pendulum length of just under a meter, giving it a swing of one second, and an escapement that ticked every second. It was the first clock that could accurately keep time in seconds. By the 1730s, 80 years later, John Harrison's maritime chronometers could keep time accurate to within one second in 100 days.
In 1832, Gauss proposed using the second as the base unit of time in his millimeter–milligram–second 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 in 1874, although this system was gradually replaced over the next 70 years by MKS units. Both the CGS and MKS systems used the same second as their base unit of time. MKS was adopted internationally during the 1940s, defining the second as of a mean solar day.
The Earth's motion was described in Newcomb's Tables of the Sun (1895), which provided a formula for estimating the motion of the Sun relative to the epoch 1900 based on astronomical observations made between 1750 and 1892. This resulted in adoption of an ephemeris time scale expressed in units of the sidereal year at that epoch by the IAU in 1952.
This extrapolated timescale brings the observed positions of the celestial bodies into accord with Newtonian dynamical theories of their motion. In 1955, the tropical year, considered more fundamental than the sidereal year, was chosen by the IAU as the unit of time. The tropical year in the definition was not measured but calculated from a formula describing a mean tropical year that decreased linearly over time.
In 1956, the second was redefined in terms of a year relative to that epoch. The second was thus defined as "the fraction of the tropical year for 1900 January 0 at 12 hours ephemeris time". This definition was adopted as part of the International System of Units in 1960.
Atomic clocks now set the length of a second and the time standard for the world.
+ Evolution of the Second | ||
That according to the decisions of the 8th General Assembly of the International Astronomical Union (Rome, 1952), the second of ephemeris time (ET) is the fraction
of the tropical year for 1900 January 0 at 12 h ET. | The second is the fraction of the tropical year for 1900 January 0 at 12 hours ephemeris time. | 1956 CIPM 11th CGPM 1960 Resolution 9 |
The standard to be employed is the transition between the hyperfine levels F=4, M=0 and F=3, M=0 of the ground state of the caesium 133 atom, unperturbed by external fields, and that the frequency of this transition is assigned the value 9192631770 hertz. | 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 | 13th CGPM Resolution 1 CIPM 1967 |
This definition implies that the caesium atom is at rest and unperturbed. In consequence, in its practical realization, measurements must be corrected for velocity of the atoms with respect to the clock reference frame, for magnetic and electric fields including ambient black-body radiation, for spin-exchange effects and for other possible perturbations. | At its 1997 meeting, the CIPM affirmed that: This definition refers to a caesium atom at rest at a temperature of 0 K. This note was intended to make it clear that the definition of the SI second is based on a Cs atom unperturbed by black-body radiation, that is, in an environment whose temperature is 0 K, and that the frequencies of primary frequency standards should therefore be corrected for the shift due to ambient radiation, as stated at the meeting of the CCTF in 1999. | footnote added by the 14th meeting of the Consultative Committee for Time and Frequency in 1999 the footnote was added at the 86th (1997) meeting of the CIPM GCPM 1998 7th Edition SI Brochure |
The definition of a unit refers to an idealized situation that can be reached in the practical realization with some uncertainty only. In this spirit, the definition of the second has to be understood as referring to atoms free of any perturbation, at rest and in the absence of electric and magnetic fields.
A future re-definition of the second would be justified if these idealized conditions can be achieved much easier than with the current definition.
The definition of the second should be understood as the definition of the unit of proper time: it applies in a small spatial domain that shares the motion of the caesium atom used to realize the definition. In a laboratory sufficiently small to allow the effects of the non-uniformity of the gravitational field to be neglected when compared to the uncertainties of the realization of the second, the proper second is obtained after application of the special relativistic correction for the velocity of the atom in the laboratory. It is wrong to correct for the local gravitational field. | The second, symbol s, is the SI unit of time. It is defined by taking the fixed numerical value of the caesium frequency, Δ νCs, the unperturbed ground-state hyperfine transition frequency of the caesium 133 atom, to be 9 192 631 770 when expressed in the unit Hz, which is equal to s−1.
The reference to an unperturbed atom is intended to make it clear that the definition of the SI second is based on an isolated caesium atom that is unperturbed by any external field, such as ambient black-body radiation. The second, so defined, is the unit of proper time in the sense of the general theory of relativity. To allow the provision of a coordinated time scale, the signals of different primary clocks in different locations are combined, which have to be corrected for relativistic caesium frequency shifts (see section 2.3.6). The CIPM has adopted various secondary representations of the second, based on a selected number of spectral lines of atoms, ions or molecules. The unperturbed frequencies of these lines can be determined with a relative uncertainty not lower than that of the realization of the second based on the 133Cs hyperfine transition frequency, but some can be reproduced with superior stability. | Current Definition resolved in 2018 effective after the 26th GCPM approved the redefinition May 20, 2019. SI Brochure 9 |
are based on forbidden optical transitions in ions or atoms. They have frequencies around , with a natural linewidth of typically 1 Hz, so the Q-factor is about , or even higher. They have better stabilities than microwave clocks, which means that they can facilitate evaluation of lower uncertainties. They also have better time resolution, which means the clock "ticks" faster. Optical clocks use either a single ion, or an optical lattice with – atoms.
The only viable way to fix the Rydberg constant involves trapping and cooling hydrogen. This is difficult because it is very light and the atoms move very fast, causing Doppler shifts. The radiation needed to cool the hydrogen – – is also difficult. Another hurdle involves improving the uncertainty in QED calculations, specifically the Lamb shift in the 1s-2s transition of the hydrogen atom.
+SI multiples for second (s) ! colspan="3" | Submultiples | ! colspan="4"Multiples | ||||
10−1 s | ds | decisecond | 101 s | das | decasecond | 10 seconds |
10−2 s | cs | centisecond | 102 s | hs | hectosecond | 1 minute, 40 seconds |
10−3 s | ms | millisecond | 103 s | ks | kilosecond | 16 minutes, 40 seconds |
10−6 s | μs | microsecond | 106 s | Ms | megasecond | 1 week, 4 days, 13 hours, 46 minutes, 40 seconds |
10−9 s | ns | nanosecond | 109 s | Gs | gigasecond | 31.7 years |
10−12 s | ps | picosecond | 1012 s | Ts | terasecond | 31,700 years |
10−15 s | fs | femtosecond | 1015 s | Ps | petasecond | 31.7 million years |
10−18 s | as | attosecond | 1018 s | Es | exasecond | 31.7 billion years |
10−21 s | zs | zeptosecond | 1021 s | Zs | zettasecond | 31.7 trillion years |
10−24 s | ys | yoctosecond | 1024 s | Ys | yottasecond | 31.7 quadrillion years |
10−27 s | rs | rontosecond | 1027 s | Rs | ronnasecond | 31.7 quintillion years |
10−30 s | qs | quectosecond | 1030 s | Qs | quettasecond | 31.7 sextillion years |
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