Unit of time

Table showing quantitative relationships between common units of time

Aunit of timeis any particulartimeinterval, used as a standard way of measuring or expressing duration. Thebase unitof time in theInternational System of Units(SI), and by extension most of theWestern world,is thesecond,defined as about 9 billion oscillations of thecaesiumatom. 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 be 9 192 631 770 when expressed in the unit Hz, which is equal to s⁻¹."

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

• Sun-based: theyearwas the time for the Earth to revolve around the Sun. Historical year-based units include theOlympiad(four years), thelustrum(five years), theindiction(15 years), thedecade,thecentury,and themillennium.
• Moon-based: the month was based on the Moon'sorbital periodaround the Earth.
• Earth-based: the time it took for the Earth to rotate on its own axis, as observed on asundial.Units originally derived from this base include theweek(sevendays), and thefortnight(14 days). Subdivisions of the day include thehour(1/24 of a day), which was further subdivided intominutesand finally seconds. The second became the international standard unit (SI units) for science.
• Celestial sphere-based: as insidereal time,where the apparent movement of the stars andconstellationsacross 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 calendaras365.2425days has to be adjusted withleap daysandleap seconds.Consequently, these units are now all defined for scientific purposes as multiples of seconds.

Units of time based onorders of magnitudeof the second include thenanosecondand themillisecond.

Historical

Main article:History of calendars

The natural units for timekeeping used by most historical societies are theday,thesolar yearand 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.

Horizontallogarithmic scalemarked with units of time in theGregorian calendar

Scientific

• TheJiffyis the amount of time light takes to travel onefermi(about the size of anucleon) in a vacuum.
• ThePlanck timeis the time light takes to travel onePlanck length.
• TheTU(fortime unit) is a unit of time defined as 1024 μs for use in engineering.
• TheSvedbergis a time unit used forsedimentationrates (usually of proteins). It is defined as 10⁻¹³seconds (100 fs).
• Thegalactic year,based on the rotation of the galaxy and usually measured in million years.
• Thegeological time scalerelatesstratigraphyto time. Thedeep timeof Earth's past is divided into units according to events that took place in each period. For example, the boundary between theCretaceousperiod and thePaleogeneperiod 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,epochsandages.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-yearis not a unit of time, but a unit of length of about 9.5 petametres (9 454 254 955 488 km).

Note: Theparsecis not a unit of time, but a unit of length of about 30.9 trillion kilometres, despitemovie referencesotherwise.

List

Units of time

Name	Length	Notes
Planck time	5.39×10−44s	The amount of time light takes to travel onePlanck length.
quectosecond	10−30s	One nonillionth of a second.
rontosecond	10−27s	One octillionth of a second.
yoctosecond	10−24s	One septillionth of a second.
jiffy(physics)	3×10−24s	The amount of time light takes to travel onefermi(about the size of a nucleon) in a vacuum.
zeptosecond	10−21s	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.
attosecond	10−18s	One quintillionth of a second.
femtosecond	10−15s	One quadrillionth of a second. Pulse time on fastest lasers.
Svedberg	10−13s	Time unit used for sedimentation rates (usually of proteins).
picosecond	10−12s	One trillionth of a second.
nanosecond	10−9s	One billionth of a second. Time for molecules to fluoresce.
shake	10−8s	10 nanoseconds, also a casual term for a short period of time.
microsecond	10−6s	One millionth of a second. Symbol is μs
millisecond	10−3s	One thousandth of a second. Shortest time unit used on stopwatches.
jiffy(electronics)	~10−3s	Used to measure the time between alternating power cycles. Also a casual term for a short period of time.
centisecond	10−2s	One hundredth of a second.
decisecond	10−1s	One tenth of a second.
second	1 s	SI base unitfor time.
decasecond	10s	Ten seconds (one sixth of a minute)
minute	60s
hectosecond	100s
milliday	1/1000d	Also marketed as a ".beat" by the Swatch corporation.
moment	1/40solar hour(90son average)	Medieval unit of time used by astronomers to compute astronomical movements, length varies with the season.Also colloquially refers to a brief period of time.
kilosecond	103s	About 17 minutes.
hour	60min
day	24h	Longest unit used on stopwatches and countdowns.
week	7d	Historically sometimes also called "sennight".
megasecond	106s	About 11.6 days.
fortnight	2weeks	14 days
lunar month	27d4h48min–29d12h	Various definitions oflunar monthexist; sometimes also called a "lunation."
month	28–31d	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 for40days. From Middle English quarentine, fromItalianquarantina( “forty days” ), the period Venetians customarily kept ships from plague-ridden countries waiting off port, fromquaranta( “forty” ), fromLatinquadrāgintā.
semester	18weeks	A division of the academic year.Literally "six months", also used in this sense.
lunar year	354.37d
year	12mo	365 or 366d
common year	365d	52 weeks and 1 day.
tropical year	365d5h48min45.216s	Average.
Gregorian year	365d5h49min12s	Average.
sidereal year	365d6h9min9.7635456s
leap year	366d	52weeksand2d
olympiad	4yr	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.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.
lustrum	5yr	In early Roman times, the interval between censuses.
decade	10yr
indiction	15yr	Interval for taxation assessments (Roman Empire).
gigasecond	109s	About 31.7 years.
jubilee	50yr
century	100yr
millennium	1000yr	Also called "kiloannum".
Age	2,148 and two thirds of a year	a unit used in astrology, each of them represent a star sign
terasecond	1012s	About 31,709 years.
megaannum	106yr	Also called "Megayear."1,000 millennia (plural of millennium), or 1 million years (ingeology,abbreviated asMa).
petasecond	1015s	About 31,709,791 years.
galactic year	2.3×108yr	The amount of time it takes the Solar System to orbit the center of the Milky Way Galaxy (approx 230,000,000 years).
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	109yr	Also refers to an indefinite period of time, otherwise is 1,000,000,000 years.
kalpa	4.32×109yr	Used inHindu mythology.About 4,320,000,000 years.
exasecond	1018s	About 31,709,791,983 years. Approximately 2.3 times the currentage of the universe.
zettasecond	1021s	About 31,709,791,983,764 years.
yottasecond	1024s	About 31,709,791,983,764,586 years.
ronnasecond	1027s	About 31,709,791,983,764,586,504 years.
quettasecond	1030s	About 31,709,791,983,764,586,504,312 years.

Interrelation

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.

See also

• Unit of frequency
• Orders of magnitude (time)