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Unit of time

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Measurement unit for time

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Table showing quantitative relationships between common units of time

Aunit of time is any particulartime interval, used as a standard way of measuring or expressing duration. Thebase unit of time in theInternational System of Units (SI), and by extension most of theWestern world, is thesecond, defined as about 9 billion oscillations of thecaesium atom. The exact modern SI definition is "[The second] is defined by taking the fixed numerical value of the cesium frequency,Δν_Cs, the unperturbed ground-state hyperfine transition frequency of the cesium 133 atom, to be9192631770 when expressed in the unit Hz, which is equal to s⁻¹."^[1]

Historically, many units of time were defined by the movements ofastronomical objects.

Sun-based: theyear is based on theEarth's orbital period around the sun. Historical year-based units include theOlympiad (four years), thelustrum (five years), theindiction (15 years), thedecade, thecentury, and themillennium.
Moon-based: themonth is based on theMoon's orbital period around the Earth.
Earth-based: theday is based on the time it takes for the Earth to rotate on its own axis, relative to the Sun. Units originally derived from this base include theweek (seven days), and thefortnight (14 days). Subdivisions of the day include thehour (1/24 of a day), which is further subdivided intominutes and seconds. The second is the international standard unit (SI unit) for science.
Celestial sphere-based: as insidereal time, where the apparent movement of the stars andconstellations across the sky is used to calculate the length of a year.

These units do not have a consistent relationship with each other and requireintercalation. For example, the year cannot be divided into twelve 28-day months since 12 times 28 is 336, well short of 365. Thelunar month (as defined by the moon's rotation) is not 28 days but 28.3 days. The year, defined in theGregorian calendar as365.2425 days has to be adjusted withleap days andleap seconds. Consequently, these units are now all defined for scientific purposes as multiples of seconds.

Units of time based onorders of magnitude of the second follow the system ofmetric prefixes.

Historical

[edit]

Main article:History of calendars

The natural units for timekeeping used by most historical societies are theday, thesolar year and thelunation. Such calendars include theSumerian,Egyptian,Chinese,Babylonian,ancient Athenian,Buddhist,Hindu,Islamic,Icelandic,Mayan, andFrench Republican calendars.

The modern calendar has its origins in theRoman calendar, which evolved into theJulian calendar, and then theGregorian calendar.

Horizontallogarithmic scale marked with units of time in theGregorian calendar

Scientific

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ThePlanck time is the time that light takes to travel onePlanck length.
TheJiffy is the amount of time light takes to travel onefemtometre (about the diameter of anucleon).
Theatomic time relates to the orbital period of a ground stateelectron around ahydrogen atom and is about 24.2 attoseconds.
Thesvedberg is a time unit used forsedimentation rates (usually of proteins). It is defined as 10⁻¹³ seconds (100 fs).
TheTU (fortime unit) is a unit of time defined as 1024 μs for use in engineering.
Thegalactic year, based on the rotation of the galaxy and usually measured in million years.^[2]
Thegeological time scale relatesstratigraphy to time. Thedeep time of Earth's past is divided into units according to events that took place in each period. For example, the boundary between theCretaceous period and thePaleogene period is defined by theCretaceous–Paleogene extinction event. The largest unit is the supereon, composed of eons. Eons are divided intoeras, which are in turn divided intoperiods,epochs andages. It is not a true mathematical unit, as all ages, epochs, periods, eras, or eons don't have the same length; instead, their length is determined by the geological and historical events that define them individually.

Note: Thelight-year is not a unit of time, but a unit of length of about 9.5 petametres (9454254955488 km).

Note: Theparsec is not a unit of time, but a unit of length of about 30.9 trillion kilometres, despitemovie references otherwise.

List

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Units of time
Name	Length	Notes
Planck time	~5.39×10⁻⁴⁴ s	The amount of time light takes to travel onePlanck length.
quectosecond	10⁻³⁰ s	One nonillionth of a second.
rontosecond	10⁻²⁷ s	One octillionth of a second.
yoctosecond	10⁻²⁴ s	One septillionth of a second.
jiffy (physics)	3×10⁻²⁴ s	The amount of time light takes to travel onefermi (about the size of a nucleon) in a vacuum.
zeptosecond	10⁻²¹ s	One sextillionth of a second. Time measurement scale of the NIST and JILA strontium atomic clock. Smallest fragment of time currently measurable is 247 zeptoseconds.^[3]
attosecond	10⁻¹⁸ s	One quintillionth of a second.
atomic time	~24.2×10⁻¹⁸ s	Derived from atomic theory of hydrogen.
femtosecond	10⁻¹⁵ s	One quadrillionth of a second.
svedberg	10⁻¹³ s	Time unit used for sedimentation rates (usually of proteins).
picosecond	10⁻¹² s	One trillionth of a second.
nanosecond	10⁻⁹ s	One billionth of a second. Time for molecules to fluoresce.
shake	10⁻⁸ s	10 nanoseconds, also a casual term for a short period of time.
microsecond	10⁻⁶ s	One millionth of a second. Symbol is μs
millisecond	10⁻³ s	One thousandth of a second. Shortest time unit used on stopwatches.
centisecond	10⁻² s	One hundredth of a second.
jiffy (electronics)	~2×10⁻² s	Used to measure the time between alternating power cycles.
decisecond	10⁻¹ s	One tenth of a second.
second	1 s	SI base unit for time.
decasecond	10 s	Ten seconds (one sixth of a minute)
minute	60 s
hectosecond	100 s
milliday	1/1000 d (0.001 d)	1.44 minutes, or 86.4 seconds. Also marketed as a ".beat" by the Swatch corporation.
moment	1/40 solar hour (90 s on average)	Medieval unit of time used by astronomers to compute astronomical movements, length varies with the season.^[4] Also colloquially refers to a brief period of time.
centiday	0.01 d (1 % of a day)	14.4 minutes, or 864 seconds. One-hundredth of a day is 1 cd (centiday), also called "kè" in traditionalChinese timekeeping. The unit was also proposed by Lagrange and endorsed by Rey-Pailhade^[5] in the 19th century, named "centijours" (from Frenchcenti- 'hundred' andjour 'day').
kilosecond	10³ s	About 17 minutes.
hour	60 min
deciday	0.1 d (10 % of a day)	2.4 hours, or 144 minutes. One-tenth of a day is 1 dd (deciday), also called "gēng" in traditionalChinese timekeeping.
day	24 h	Longest unit used on stopwatches and countdowns. The SI day is exactly 86 400 seconds.
week	7 d	Historically sometimes also called "sennight".
decaday	10 d (1 Dd)	10 days. A period of time analogous to the concept of "week", used bydifferent societies around the world: the ancientEgyptian calendar, the ancientChinese calendar, and also theFrench Republican calendar (in which it was called adécade).
megasecond	10⁶ s	About 11.6 days.
fortnight	2 weeks	14 days
lunar month	27 d 4 h 48 min – 29 d 12 h	Various definitions oflunar month exist; sometimes also called a "lunation".
month	28–31 d	Occasionally calculated as 30 days.
quarantine	40d (approximately 5.71weeks)	To retain in obligatory isolation or separation, as a sanitary measure to prevent the spread of contagious disease. Historically it meant to be isolated for40 days. From Middle English quarentine, fromItalianquarantina ("forty days"), the period Venetians customarily kept ships from plague-ridden countries waiting off port, fromquaranta ("forty"), fromLatinquadrāgintā.
hectoday	100 d (1 hd)	100 days, roughly equivalent to 1/4 of a year (91.25 days). In Chinese tradition "bǎi rì" (百日) is the hundredth day after one's birth, also called Baby's 100 Days Celebration.
semester	18 weeks	A division of the academic year.^[6] Literally "six months", also used in this sense.
lunar year	354.37 d
year	12 mo	365 or 366 d
common year	365 d	52 weeks and 1 day.
tropical year	365 d 5 h 48 min 45.216 s^[7]	Average.
Gregorian year	365 d 5 h 49 min 12 s	Average.
sidereal year	365 d 6 h 9 min 9.7635456 s
leap year	366 d	52 weeks and2 d
olympiad	4 yr	A quadrennium (plural: quadrennia or quadrenniums) is also a period of four years, most commonly used in reference to the four-year period between eachOlympic Games.^[8] It is also used in reference to the four-year interval betweenleap years, for example when wishing friends and family a "happy quadrennium" onFebruary 29.^{[citation needed]}
lustrum	5 yr	In early Roman times, the interval between censuses.
decade	10 yr
indiction	15 yr	Interval for taxation assessments (Roman Empire).
gigasecond	10⁹ s	About 31.7 years.
jubilee	50 yr
century	100 yr
millennium	1000 yr	Also called "kiloannum".
Age	2 148 and 2/3 of a year	Asuperstitious unit of time used inastrology, each of them representing a star sign.
terasecond	10¹² s	About 31,709 years.
megaannum	10⁶ yr	Also called "megayear". 1000 millennia (plural of millennium), or 1 million years (ingeology, abbreviated asMa).
petasecond	10¹⁵ s	About31709791 years.
galactic year	2.3×10⁸ yr	The amount of time it takes the Solar System to orbit the center of the Milky Way Galaxy (approx230000000 years^[2]).
cosmological decade	logarithmic (varies)	10 times the length of the previous cosmological decade, with CD 1 beginning either 10 seconds or 10 years after the Big Bang, depending on the definition.
eon	10⁹ yr	Also refers to an indefinite period of time, otherwise is1000000000 years.
kalpa	4.32×10⁹ yr	Used inHindu mythology. About4320000000 years.
exasecond	10¹⁸ s	About31709791983 years. Approximately 2.3 times the currentage of the universe.
zettasecond	10²¹ s	about31709791983764 years.
yottasecond	10²⁴ s	About31709791983764586 years.
ronnasecond	10²⁷ s	About31709791983764586504 years.
quettasecond	10³⁰ s	About31709791983764586504312 years.

Interrelation

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Flowchart illustrating selected units of time. The graphic also shows the three celestial objects that are related to the units of time.

All of the formal units of time are scaled multiples of each other. The most common units are the second, defined in terms of an atomic process; the day, an integral multiple of seconds; and theyear, usually 365 days. The other units used are multiples or divisions of these 3.

References

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^"The International System of Units – 9th edition – Complete text in English and French (2019)".BIPM. Retrieved22 July 2022.
^^a ^bhttp://starchild.gsfc.nasa.gov/docs/StarChild/questions/question18.html NASA – StarChild Question of the Month for February 2000
^"Meet the zeptosecond, the shortest unit of time ever measured". Retrieved2020-10-17.
^Milham, Willis I. (1945).Time and Timekeepers. New York: MacMillan. p. 190.ISBN 0-7808-0008-7.
^Gamez, Christophe (2020-06-17).La décimalisation du temps au prisme du Bureau des longitudes (1875-1901). Entre patriotisme, rationalité et politique (other thesis) (in French). Université de Lorraine.
^"Semester". Webster's Dictionary. Retrieved3 December 2014.
^McCarthy, Dennis D.; Seidelmann, P. Kenneth (2009).Time: from Earth rotation to atomic physics. Wiley-VCH. p. 18.ISBN 978-3-527-40780-4.,Extract of page 18
^"Merriam-Webster Dictionary". Merriam-Webster Incorporated. Retrieved29 November 2016.

Time

Key concepts

Measurement
andstandards

Chronometry	UTC UT TAI Unit of time Orders of magnitude (time)
Measurement systems	Italian six-hour clock Thai six-hour clock 12-hour clock 24-hour clock Relative hour Daylight saving time Chinese Decimal Hexadecimal Hindu Metric Roman Sidereal Solar Time zone
Calendars	Main types Solar Lunar Lunisolar Gregorian Julian Hebrew Islamic Solar Hijri Chinese Hindu Panchang Maya List
Clocks	Main types astronomical astrarium atomic quantum hourglass marine sundial watch mechanical stopwatch water-based Cuckoo clock Digital clock Grandfather clock History Timeline

Philosophy of time

Human experience
anduse of time

Time inscience

Geology	Geological time age chron eon epoch era period Geochronology Geological history of Earth
Physics	Absolute space and time Arrow of time Chronon Coordinate time Instant Proper time Spacetime Theory of relativity Time domain Time translation symmetry Time reversal symmetry
Other fields	Chronological dating Chronobiology Circadian rhythms Clock reaction Glottochronology Time geography

v t e Time measurement andstandards
Chronometry Orders of magnitude Metrology
International standards	Coordinated Universal Time offset UT ΔT DUT1 International Earth Rotation and Reference Systems Service ISO 31-1 ISO 8601 International Atomic Time 12-hour clock 24-hour clock Barycentric Coordinate Time Barycentric Dynamical Time Civil time Daylight saving time Geocentric Coordinate Time International Date Line IERS Reference Meridian Leap second Solar time Terrestrial Time Time zone 180th meridian
Obsolete standards	Ephemeris time Greenwich Mean Time Prime meridian
Time in physics	Absolute space and time Spacetime Chronon Continuous signal Coordinate time Cosmological decade Discrete time and continuous time Proper time Theory of relativity Time dilation Gravitational time dilation Time domain Time-translation symmetry T-symmetry
Horology	Clock Astrarium Atomic clock Complication History of timekeeping devices Hourglass Marine chronometer Marine sandglass Radio clock Watch stopwatch Water clock Sundial Dialing scales Equation of time History of sundials Sundial markup schema
Calendar	Gregorian Hebrew Hindu Holocene Islamic (lunar Hijri) Julian Solar Hijri Astronomical Dominical letter Epact Equinox Intercalation Julian day Leap year Lunar Lunisolar Solar Solstice Tropical year Weekday determination Weekday names
Archaeology and geology	Chronological dating Geologic time scale International Commission on Stratigraphy
Astronomical chronology	Galactic year Nuclear timescale Precession Sidereal time
Otherunits of time	Instant Flick Shake Jiffy Second Minute Moment Hour Day Week Fortnight Month Year Olympiad Lustrum Decade Century Saeculum Millennium
Related topics	Chronology Duration music Mental chronometry Decimal time Metric time System time Time metrology Time value of money Timekeeper