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A pendulum-governed escapement of a clock, ticking every second
Unit information
Unit system SI base unit
Unit of Time
Symbol s 

The second is the SI base unit of time, commonly understood and historically defined as 1/86,400 of a day – this factor derived from the division of the day first into 24 hours, then to 60 minutes and finally to 60 seconds each. Another intuitive understanding is that it is about the time between beats of a human heart.[nb 1] Mechanical and electric clocks and watches usually have a face with 60 tickmarks representing seconds and minutes, traversed by a second hand and minute hand. Digital clocks and watches often have a two-digit counter that cycles through seconds; in common parlance, a "clock tick" is a second, though most modern clocks are digital electronic, and do not actually tick.[citation needed] The second is also part of several other units of measurement like velocity, acceleration, and frequency.

Though the historical definition of the unit was based upon this division of the Earth's rotation cycle, the formal definition in the International System of Units SI is a much steadier timekeeper: 1 second is defined to be exactly 9 192 631 770 cycles of a Caesium atomic clock.[1][2][3] Because the Earth's rotation varies and is also slowing ever so slightly, a leap second is added to clock time[nb 2] to keep clocks in sync with Earth's rotation.

Multiples of seconds are usually counted in hours and minutes. Fractions of a second are usually counted in tenths or hundredths; in scientific work, small fractions of a second are counted in milliseconds (thousandths), microseconds (millionths), nanoseconds (billionths), and sometimes smaller units of a second. An everyday experience with small fractions of a second is a 1-gigahertz microprocessor which has a cycle time of 1 nanosecond. Camera shutter speeds usually range from 1/60 second to 1/250 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 counted back then, so such divisions were figurative, the first timekeepers that could count seconds accurately were pendulum clocks invented in the 17th century. Starting in the 1950s, atomic clocks became better timekeepers than earth's rotation, and they continue to set the standard today.

Clocks and solar time[edit]

A mechanical clock, one which does not depend on measuring the relative rotational position of the earth, keeps uniform time called mean time, within whatever accuracy is intrinsic to it, that means that every second, minute and every other division of time counted by the clock will be the same duration as any other identical division of time. But a sundial which measures the relative position of the sun in the sky called apparent time, does not keep uniform time, the time kept by a sundial varies by time of year, meaning that seconds, minutes and every other division of time is a different duration at different times of the year. The time of day measured with mean time versus apparent time may differ by as much as 15 minutes, but a single day will differ from the next by only a small amount; 15 minutes is a cumulative difference over a part of the year. The effect is due chiefly to the obliqueness of earth's axis with respect to its orbit around the sun.

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 generally accepted standard.

Events and units of time in seconds[edit]

Fractions of a second are usually denoted in decimal notation, i.e. 2.01 seconds, or two and one hundredth seconds. Multiples of seconds are usually expressed as minutes and seconds, or hours, minutes and seconds of clock time, separated by colons, such as 11:23:24, or 45:23 (the latter notation can give rise to ambiguity, because the same notation is used to denote hours and minutes), it rarely makes sense to express longer periods of time like hours or days in seconds, because they are awkwardly large numbers. For the metric unit of second, there are decimal prefixes representing 10−24 to 1024 seconds.

Some common units of time in seconds are: an hour is 3,600 seconds; a day is 86,400 seconds, a week is 604,800 seconds; a year is 31,536,000 seconds; and a century is 3,153,600,000 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.

Other units incorporating seconds[edit]

A second is part of other units, such as frequency measured in hertz (inverse seconds or second−1), speed (meters/second) and acceleration (meters/second2), the metric system unit becquerel, a measure of radioactive decay, is measured in inverse seconds. The meter is defined in terms of the speed of light and the second; definitions of the metric base units ampere and candela also depend on the second. Of the 22 named derived units of the SI, only three: degree Celsius, radian, and steradian, do not depend on the second. Many derivative units for everyday things are reported in terms of larger units of time, not seconds, such as clock time in hours and minutes, velocity of a car in miles/hour or kilometers/hour, kilowatt hours of electricity usage, and speed of a turntable in rotations per minute.

Timekeeping standards[edit]

A set of atomic clocks throughout the world keeps time by consensus: the clocks "vote" on the correct time, and all voting clocks are steered to agree with the consensus, which is called International Atomic Time (TAI). TAI "ticks" atomic seconds.[4]

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 leap seconds as necessary to correct for variations in the rate of rotation of the earth.[5]

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.[6] UT1 always differs from UTC by less than a second.

Optical lattice clock[edit]

While they are not yet part of any timekeeping standard, optical lattice clocks with frequencies in the visible light spectrum now exist and are the most accurate timekeepers of all. A strontium clock with frequency 430 THz, in the red range of visible light, now holds the accuracy record: it will gain or lose less than a second in 15 billion years, which is longer than the estimated age of the universe. Such a clock can measure a change in its height of as little as 2 cm by the change in its rate due to gravitational time dilation.[7]

History of definition[edit]

There have only ever been three definitions of the second: as a fraction of the day, as a fraction of an extrapolated year, and as the microwave frequency of a caesium atomic clock, and they have realized a sexagesimal division of the day from ancient astronomical calendars.

Sexagesimal divisions of calendar time and day[edit]

Civilizations in the classic period and before constructed divisions of the calendar as well as arcs according to a sexagesimal system of counting, but none used the term second, and none was a precursor to the modern second. Sundials and water clocks were among the earliest timekeeping devices, and units of time were measured in degrees of arc. Conceptual units of time smaller than realizable on sundials were also used.

There are references to 'second' as part of a lunar month in the writings of natural philosophers of the Middle Ages, but no evidence that 'seconds' were ever realizable or adopted as part of timekeeping based on the lunar calendar.[nb 3][nb 4]

Fraction of solar day[edit]

The earliest mechanical clocks which appeared starting in the 14th century had displays that divided the hour into halves, thirds, quarters and sometimes even 12 parts, but never by 60; in fact, the hour was not commonly understood to be the duration of 60 minutes. It was not practical for timekeepers to consider minutes until the first mechanical clocks that displayed minutes appeared near the end of the 16th century. By that time, sexagesimal divisions of time were well established in Europe.[nb 5]

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 spring-driven timepiece with a second hand which marked seconds is an unsigned clock depicting Orpheus in the Fremersdorf collection, dated between 1560 and 1570.[10]:417–418[11] During the 3rd quarter of the 16th century, Taqi al-Din built a clock with marks every 1/5 minute.[12] In 1579, Jost Bürgi built a clock for William of Hesse that marked seconds.[10]:105 In 1581, Tycho Brahe redesigned clocks that displayed minutes at his observatory so they also displayed seconds. However, they were not yet accurate enough for seconds; in 1587, Tycho complained that his four clocks disagreed by plus or minus four seconds.[10]:104

In 1656, Dutch scientist Christiaan Huygens invented the first pendulum clock, it had a pendulum length of just under a meter which gave 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."[13] BAAS formally proposed the CGS system 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 ​186,400 of a mean solar day.

Fraction of an ephemeris year[edit]

Some time in the late 1940s, quartz crystal oscillator clocks with an operating frequency of ~100 kHz advanced to keep time with accuracy better than 1 part in 108 over an operating period of a day. It became apparent that a consensus of such clocks kept better time than the rotation of the Earth. Metrologists also knew that Earth's orbit around the Sun (a year) was much more stable than earth's rotation, this led to proposals as early as 1950 to define the second as a fraction of a year.

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,[14] this resulted in adoption of an ephemeris time scale expressed in units of the sidereal year at that epoch by the IAU in 1952.[15] This extrapolated timescale brings the observed positions of the celestial bodies into accord with Newtonian dynamical theories of their motion;[14] 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 ​131,556,925.9747 of the tropical year for 1900 January 0 at 12 hours ephemeris time".[14] This definition was adopted as part of the International System of Units in 1960.

"Atomic" second[edit]

But even the best mechanical, electric motorized and quartz crystal-based clocks develop discrepancies, and virtually none are good enough to realize an ephemeris second. Far better for timekeeping is the natural and exact "vibration" in an energized atom, the frequency of vibration (i.e., radiation) is very specific depending on the type of atom and how it is excited. Since 1967, the second has been defined as exactly 9,192,631,770 times the period of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom. This length of a second was selected to correspond exactly to the length of the ephemeris second previously defined. Atomic clocks use such a frequency to measure seconds by counting cycles per second at that frequency. Radiation of this kind is one of the most stable and reproducible phenomena of nature, the current generation of atomic clocks is accurate to within one second in a few hundred million years.

Atomic clocks now set the length of a second and the time standard for the world.[16]

SI multiples[edit]

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 non-SI 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

See also[edit]


  1. ^ The typical adult heart rate is usually between 72 and 80 beats per minute, so a second is just a little longer than a heartbeat for most people.
  2. ^ Clock time (that is, civil time) is set, directly or indirectly, to Coordinated Universal Time, which includes leap seconds. Other time scales are used in scientific and technical fields which do not contain leap seconds.
  3. ^ Circa 1000, the Persian scholar al-Biruni, 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.[8]
  4. ^ In 1267, the medieval English 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.[9]
  5. ^ It may be noted that 60 is the smallest multiple of the first 6 counting numbers. So a clock with 60 divisions would have a mark for thirds, fourths, fifths, sixths and twelfths (the hours); whatever units the clock would likely keep time in, would have marks.


  1. ^ "Unit of time (second)". SI Brochure. BIPM. Retrieved December 22, 2013. 
  2. ^ Second. Merriam Webster Learner's Dictionary. 
  3. ^ "Base unit definitions: Second". Retrieved September 9, 2016. 
  4. ^ McCarthy, Dennis D.; Seidelmann, P. Kenneth (2009). Time: From Earth Rotation to Atomic Physics. Weinheim: Wiley. pp. 207–218. 
  5. ^ McCarthy, Dennis D.; Seidelmann, P. Kenneth (2009). Time: From Earth Rotation to Atomic Physics. Weinheim: Wiley. pp. 16–17, 207. 
  6. ^ McCarthy, Dennis D.; Seidelmann, P. Kenneth (2009). Time: From Earth Rotation to Atomic Physics. Weinheim: Wiley. pp. 68, 232. 
  7. ^ Vincent, James. "The most accurate clock ever built only loses one second every 15 billion years". TheVerge. 
  8. ^ 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". translated by Sachau C. Edward. pp. 147–149. 
  9. ^ Bacon, Roger (2000) [1928]. The Opus Majus of Roger Bacon. translated by Robert Belle Burke. University of Pennsylvania Press. table facing page 231. ISBN 978-1-85506-856-8. 
  10. ^ a b c Landes, David S. (1983). Revolution in Time. Cambridge, Massachusetts: Harvard University Press. ISBN 0-674-76802-7. 
  11. ^ Willsberger, Johann (1975). Clocks & watches. New York: Dial Press. ISBN 0-8037-4475-7.  full page color photo: 4th caption page, 3rd photo thereafter (neither pages nor photos are numbered).
  12. ^ Selin, Helaine (July 31, 1997). Encyclopaedia of the History of Science, Technology, and Medicine in Non-Western Cultures. Springer Science & Business Media. p. 934. ISBN 978-0-7923-4066-9. 
  13. ^ Jenkin, Henry Charles Fleeming, ed. (1873). Reports of the committee on electrical standards. British Association for the Advancement of Science. p. 90. 
  14. ^ a b c "Leap Seconds". Time Service Department, United States Naval Observatory. Retrieved November 22, 2015. 
  15. ^ Explanatory Supplement to the Astronomical Ephemeris and the American Ephemeris and Nautical Almanac (1961), Sec. 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 that defined ephemeris time and brought the lunar ephemeris into accordance with the solar ephemeris in terms of ephemeris time. These recommendations were adopted by the International Astronomical Union in Sept. 1952."
  16. ^ McCarthy, Dennis D.; Seidelmann, P. Kenneth (2009). Time: From Earth Rotation to Atomic Physics. Weinheim: Wiley. pp. 231–232. 

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