Contents 1 Clocks and solar time 2 Events and units of time in seconds 3 Other units incorporating seconds 4 Timekeeping standards 5 Optical lattice clock 6 History of definition 6.1 Sexagesimal divisions of calendar time and day 6.2 Fraction of solar day 6.3 Fraction of an ephemeris year 6.4 "Atomic" second 7 SI multiples 8 See also 9 Notes 10 References 11 External links

Clocks and solar time 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 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 3600 seconds; a day is 86,400 seconds, a week is 604,800 seconds; a year is about 31.6 million seconds; and a century is a little over 3 billion (3.16×109) 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 a fraction over one second to reach Earth from the surface of the Moon.

Other units incorporating seconds 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 centigrade, 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 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 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]

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 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

Notes ^ 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. ^ 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. ^ 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] ^ 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] ^ 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.

References ^ "Unit of time (second)". SI Brochure. BIPM. Retrieved December 22, 2013.  ^ Second. Merriam Webster Learner's Dictionary.  ^ "Base unit definitions: Second". physics.nist.gov. Retrieved September 9, 2016.  ^ McCarthy, Dennis D.; Seidelmann, P. Kenneth (2009). Time: From Earth Rotation to Atomic Physics. Weinheim: Wiley. pp. 207–218.  ^ McCarthy, Dennis D.; Seidelmann, P. Kenneth (2009). Time: From Earth Rotation to Atomic Physics. Weinheim: Wiley. pp. 16–17, 207.  ^ McCarthy, Dennis D.; Seidelmann, P. Kenneth (2009). Time: From Earth Rotation to Atomic Physics. Weinheim: Wiley. pp. 68, 232.  ^ Vincent, James. "The most accurate clock ever built only loses one second every 15 billion years". TheVerge.  ^ 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.  ^ 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.  ^ a b c Landes, David S. (1983). Revolution in Time. Cambridge, Massachusetts: Harvard University Press. ISBN 0-674-76802-7.  ^ 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). ^ 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.  ^ Jenkin, Henry Charles Fleeming, ed. (1873). Reports of the committee on electrical standards. British Association for the Advancement of Science. p. 90.  ^ a b c "Leap Seconds". Time Service Department, United States Naval Observatory. Retrieved November 22, 2015.  ^ 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." ^ McCarthy, Dennis D.; Seidelmann, P. Kenneth (2009). Time: From Earth Rotation to Atomic Physics. Weinheim: Wiley. pp. 231–232.