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A unit of time is any particular
time Time is the continued sequence of existence and event (philosophy), events that occurs in an apparently irreversible process, irreversible succession from the past, through the present, into the future. It is a component quantity of various me ...

time
interval, used as a standard way of measuring or expressing duration. The
base Base or BASE may refer to: Brands and enterprises * Base (mobile telephony provider), a Belgian mobile telecommunications operator *Base CRM Base CRM (originally Future Simple or PipeJump) is an enterprise software company based in Mountain Vie ...

base
unit of time in the
International System of Units The International System of Units, known by the international abbreviation SI in all languages and sometimes Pleonasm#Acronyms_and_initialisms, pleonastically as the SI system, is the modern form of the metric system and the world's most wi ...
(SI) and by extension most of the
Western world The Western world, also known as the West, refers to various regions, nations and state (polity), states, depending on the context, most often consisting of the majority of Europe, Northern America, and Australasia.
, is the
second The second (symbol: s, also abbreviated: sec) is the base unit of time Time is the continued sequence of existence and event (philosophy), events that occurs in an apparently irreversible process, irreversible succession from the past, th ...
, defined as about 9 billion oscillations of the
caesium Caesium (IUPAC spelling) (American and British English spelling differences, also spelled cesium in American English) is a chemical element with the Symbol (chemistry), symbol Cs and atomic number 55. It is a soft, silvery-golden alkal ...

caesium
atom. The exact modern definition, from the
National Institute of Standards and Technology The National Institute of Standards and Technology (NIST) is a physical sciences Physical science is a branch of natural science that studies abiotic component, non-living systems, in contrast to life science. It in turn has many branches, e ...
is: "The second, symbol s, is the SI unit of time. It is defined by taking the fixed numerical value of the cesium frequency Δ''ν''Cs, the unperturbed ground-state hyperfine transition frequency of the cesium 133 atom, to be when expressed in the unit Hz, which is equal to s−1." Historically, many units of time were defined by the movements of
astronomical object In astronomy, an astronomical object or celestial object is a naturally occurring physical object, physical entity, association, or structure that exists in the observable universe. In astronomy, the terms ''object'' and ''body'' are often us ...
s. *
Sun The Sun is the star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many othe ...

Sun
-based: the
year A year is the orbital period of a planetary body, for example, the Earth, moving in Earth's orbit, its orbit around the Sun. Due to the Earth's axial tilt, the course of a year sees the passing of the seasons, marked by change in weather, the ...
was the time for the Earth to revolve around the Sun. Historical year-based units include the
Olympiad An Olympiad ( el, Ὀλυμπιάς, ''Olympiás'') is a period of four years associated with the Olympic Games The modern Olympic Games or Olympics (french: Jeux olympiques) are leading international sporting events featuring summer and ...

Olympiad
(four years), the
lustrum A lūstrum (, plural lūstra) was a term for a five-year period in Ancient Rome In historiography Historiography is the study of the methods of historian ( 484– 425 BC) was a Greek historian who lived in the 5th century BC and o ...
(five years), the
indiction An indiction ( la, indictio, impost) was a periodic reassessment of taxation in the Roman Empire which took place every fifteen years. In Late Antiquity, this 15-year cycle began to be used to date documents and it continued to be used for this pu ...
(15 years), the
decade A decade is a period of 10 years. The word is derived (via French and Latin) from the grc, δεκάς, dekas, which means ''a group of ten''. Decades may describe any ten-year period, such as those of a person's life, or refer to specific groupin ...
, the
century A century is a period of 100 years. Centuries are numbered names of numbers in English#Ordinal numbers, ordinally in English and many other languages. The word ''century'' comes from the Latin ''centum'', meaning ''one hundred''. ''Century'' is s ...

century
, and the
millennium A millennium (plural millennia or millenniums) is a period of one thousand year A year is the orbital period of a planetary body, for example, the Earth, moving in Earth's orbit, its orbit around the Sun. Due to the Earth's axial tilt, th ...

millennium
. *
Moon The Moon is Earth's only natural satellite. At about one-quarter the diameter of Earth (comparable to the width of Australia (continent), Australia), it is the largest natural satellite in the Solar System relative to the size of its plane ...

Moon
-based: the month was based on the Moon's
orbital period The orbital period is the time a given astronomical object takes to complete one orbit around another object, and applies in astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of th ...
around the Earth. *
Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% is Water distribution on Earth, covered wi ...

Earth
-based: the time it took for the Earth to rotate on its own axis, as observed on a
sundial A sundial is a horological device that tells the time of day (in modern usage referred to as civil time In modern usage, civil time refers to statutory time scales designated by civilian authorities, or to local time indicated by clocks. M ...

sundial
. Units originally derived from this base include the
week A week is a time unit equal to seven days. It is the standard time period used for cycles of rest days in most parts of the world, mostly alongside—although not strictly part of—the Gregorian calendar. In many languages, the days of the we ...

week
(seven
days The word day has a number of meanings, depending on the context it is used such as of astronomy, physics, and various calendar systems. As a term in physics and astronomy it is approximately the period during which the Earth completes one ro ...
), and the
fortnight A fortnight is a unit of time equal to 14 day A day is approximately the period during which the Earth completes one rotation around its axis, which takes around 24 hours. A solar day is the length of time which elapses between the Sun reach ...
(14 days). Subdivisions of the day include the
hour An hour (symbol A symbol is a mark, sign, or that indicates, signifies, or is understood as representing an , , or . Symbols allow people to go beyond what is n or seen by creating linkages between otherwise very different s and s. All ...
(1/24 of a day), which was further subdivided into
minute The minute is a unit Unit may refer to: Arts and entertainment * UNIT Unit may refer to: Arts and entertainment * UNIT, a fictional military organization in the science fiction television series ''Doctor Who'' * Unit of action, a discrete piec ...
s and finally seconds. The second became the international standard unit (SI units) for science. *
Celestial sphere In astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies astronomical object, celestial objects and celestial event, phenomena. It uses ...

Celestial sphere
-based: as in
sidereal time Sidereal time ( ) is a timekeeping Time is the indefinite continued progress of existence Existence is the ability of an entity to interact with physical or mental reality Reality is the sum or aggregate of all that is real or existen ...

sidereal time
, where the apparent movement of the stars and
constellations A constellation is an area on the celestial sphere In astronomy and navigation, the celestial sphere is an abstraction, abstract sphere that has an arbitrarily large radius and is concentric objects, concentric to Earth. All objects in the ...

constellations
across the sky is used to calculate the length of a year. These units do not have a consistent relationship with each other and require intercalation. For example, the year cannot be divided into twelve 28-day months since 12 times 28 is 336, well short of 365. The
lunar month In lunar calendar A lunar calendar is a calendar based on the monthly cycles of the Moon's lunar phase, phases (Lunar month#Synodic month, synodic months, lunations), in contrast to solar calendars, whose annual cycles are based only directly o ...
(as defined by the moon's rotation) is not 28 days but 28.3 days. The year, defined in the
Gregorian calendar The Gregorian calendar is the calendar A calendar is a system of organizing days. This is done by giving names to periods of time, typically days, weeks, months and years. A calendar date, date is the designation of a single, speci ...
as days has to be adjusted with leap days and
leap seconds A leap second is a one-second adjustment that is occasionally applied to Coordinated Universal Time (UTC), to accommodate the difference between precise time (as measured by atomic clocks) and imprecise solar time#Mean solar time, observed solar ...
. Consequently, these units are now all defined for scientific purposes as multiples of seconds. Units of time based on
orders of magnitude An order of magnitude is an approximation of the logarithm In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geome ...
of the second include the
nanosecond A nanosecond (ns) is an SI unit of time equal to one billionth of a second The second (symbol: s, abbreviation: sec) is the SI base unit, base unit of time in the International System of Units (SI) (French: Système International d’unités) ...
and the
millisecond A millisecond (from ''milli- ''Milli-'' (symbol m) is a unit prefixA unit prefix is a specifier or mnemonic that is prepended to units of measurement to indicate multiples or fractions of the units. Units of various order of magnitude, sizes are ...
.


Historical

The natural units for timekeeping used by most historical societies are the
day The word day has a number of meanings, depending on the context it is used such as of astronomy, physics, and various calendar systems. As a term in physics and astronomy it is approximately the period during which the Earth completes one ro ...

day
, the
solar year A tropical year (also known as a solar year or tropical period) is the time Time is the continued sequence of existence and event (philosophy), events that occurs in an apparently irreversible process, irreversible succession from the past ...
and the lunation. Such calendars include the Sumerian,
Egyptian Egyptian describes something of, from, or related to Egypt. Egyptian or Egyptians may refer to: Nations and ethnic groups * Egyptians, a national group in North Africa ** Egyptian culture, a complex and stable culture with thousands of years of r ...
,
Chinese Chinese can refer to: * Something related to China China, officially the People's Republic of China (PRC), is a country in East Asia. It is the List of countries and dependencies by population, world's most populous country, with a populat ...
, Babylonian, ancient Athenian,
Buddhist Buddhism (, ) is the world's fourth-largest religion Religion is a social Social organisms, including humans, live collectively in interacting populations. This interaction is considered social whether they are aware of it or not, an ...
,
Hindu Hindus (; ) are persons who regard themselves as culturally, ethnically, or religiously adhering to aspects of Hinduism Hinduism () is an Indian religion Indian religions, sometimes also termed Dharmic religions or Indic re ...

Hindu
,
Islamic Islam (; ar, اَلْإِسْلَامُ, al-’Islām, "submission
o God Oh God may refer to: * An exclamation; similar to "oh no", "oh yes", "oh my", "aw goodness", "ah gosh", "ah gawd"; see interjection An interjection is a word or expression that occurs as an utterance on its own and expresses a spontaneous feeling ...
) is an Abrahamic religions, Abrahamic monotheistic religion teaching that Muhammad is a Muhammad in Islam, messenger of God.Peters, F. E. 2009. "Allāh." In , ed ...
,
Icelandic Icelandic refers to anything of, from, or related to Iceland and may refer to: *Icelandic people *Icelandic language *Icelandic alphabet *Icelandic cuisine See also

* Icelander (disambiguation) * Icelandic Airlines, a predecessor of Icelandai ...
,
Mayan Mayan most commonly refers to: * Maya peoples, various indigenous peoples of Mesoamerica and northern Central America * Maya civilization, pre-Columbian culture of Mesoamerica and northern Central America * Mayan languages, language family spoken i ...
, and
French Republican calendar The French Republican calendar (french: calendrier républicain français), also commonly called the French Revolutionary calendar (), was a calendar A calendar is a system of organizing days. This is done by giving names to periods of ...
s. The modern calendar has its origins in the
Roman calendar The Roman calendar was the calendar A calendar is a system of organizing days. This is done by giving names to periods of time, typically days, weeks, months and years. A calendar date, date is the designation of a single, specific ...
, which evolved into the
Julian calendar The Julian calendar, proposed by Julius Caesar Gaius Julius Caesar (; 12 July 100 BC – 15 March 44 BC) was a Roman Roman or Romans usually refers to: *Rome, the capital city of Italy *Ancient Rome, Roman civilization from 8th century B ...
, and then the Gregorian.


Scientific

* The jiffy is the amount of time light takes to travel one
fermi Enrico Fermi (; 29 September 1901 – 28 November 1954) was an Italian Italian may refer to: * Anything of, from, or related to the country and nation of Italy ** Italians, an ethnic group or simply a citizen of the Italian Republic ** Italia ...
(about the size of a
nucleon In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the changes they undergo during ...
) in a vacuum. *
Planck time In particle physics Particle physics (also known as high energy physics) is a branch of that studies the nature of the particles that constitute and . Although the word ' can refer to various types of very small objects (e.g. , gas particl ...
is the time light takes to travel one
Planck length In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and force. ...
. Theoretically, this is the smallest time measurement that will ever be possible. Smaller time units have no use in physics as we understand it today. * The TU (for Time Unit) is a unit of time defined as 1024 µs for use in engineering. * The
Svedberg A Svedberg unit (symbol S, sometimes Sv) is a non- SI metric unit for sedimentation coefficientThe sedimentation coefficient ''(s)'' of a particle characterizes its sedimentation during centrifugation. It is defined as the ratio of a particle's s ...
is a time unit used for
sedimentation Sedimentation is the deposition of sediments Sediment is a naturally occurring material that is broken down by processes of weathering Weathering is the deterioration of rocks In geology Geology (from the Ancient Greek γ ...
rates (usually of proteins). It is defined as 10−13 seconds (100 fs). * The
galactic year The galactic year, also known as a cosmic year, is the duration of time required for the Sun The Sun is the star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own ...
, based on the rotation of the galaxy and usually measured in million years.http://starchild.gsfc.nasa.gov/docs/StarChild/questions/question18.html NASA - StarChild Question of the Month for February 2000 * The
geological time scale The geologic time scale (GTS) is a system of chronological dating that classifies geological strata (stratigraphy through Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that ...
relates
stratigraphy Stratigraphy is a branch of geology concerned with the study of rock (geology), rock layers (Stratum, strata) and layering (stratification). It is primarily used in the study of sedimentary rock, sedimentary and layered volcanic rocks. Stratigrap ...

stratigraphy
to time. The
deep time Deep time is a term introduced and applied by John McPhee to the concept of geologic time in his ''Basin and Range'' (1981), parts of which originally appeared in the '' New Yorker'' magazine. The philosophical concept of geological time was de ...
of Earth's past is divided into units according to events that took place in each period. For example, the boundary between the
Cretaceous The Cretaceous ( ) is a geological period A geological period is one of the several subdivisions of geologic time enabling cross-referencing of rocks and geologic events from place to place. These periods form elements of a hierarchy of division ...

Cretaceous
period and the
Paleogene The Paleogene ( ; also spelled Palaeogene or Palæogene; informally Lower Tertiary or Early Tertiary) is a geologic period and system that spans 43 million years from the end of the Cretaceous The Cretaceous ( ) is a geological period A geologi ...
period is defined by the
Cretaceous–Paleogene extinction event The Cretaceous–Paleogene (K–Pg) extinction event (also known as the Cretaceous–Tertiary extinction) was a sudden extinction event, mass extinction of three-quarters of the plant and animal species on Earth, approximately 66 millio ...
. The largest unit is the supereon, composed of eons. Eons are divided into eras, which are in turn divided into periods, epochs and ages. 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: The
light-year A light-year, alternatively spelt lightyear, is a unit of length A unit of length refers to any arbitrarily chosen and accepted reference standard for measurement of length. The most common units in modern use are the metric system, metric un ...
is not a unit of time, but a unit of length of about 9.5 petametres (9 454 254 955 488 kilometers).


List


Interrelation

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 the
year A year is the orbital period of a planetary body, for example, the Earth, moving in Earth's orbit, its orbit around the Sun. Due to the Earth's axial tilt, the course of a year sees the passing of the seasons, marked by change in weather, the ...
, usually 365 days. The other units used are multiples or divisions of these three.


References

{{Time measurement and standards Units of time