A unit of time is any particular

time Time is the continued sequence of existence and events that occurs in an apparently irreversible succession from the past, through the present, into the future. It is a component quantity of various measurements used to sequence events, t ...

interval, used as a standard way of measuring or expressing duration. The 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 pleonastically as the SI system, is the modern form of the metric system and the world's most widely used system of measurement. ...

(SI) and by extension most of the

Western world The Western world, also known as the West, primarily refers to the various nations and states in the regions of Europe, North America, and Oceania.

, is the

second The second (symbol: s) is the unit of time in the International System of Units (SI), historically defined as 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 ...

, defined as about 9 billion oscillations of the

caesium Caesium ( IUPAC spelling) (or cesium in American English) is a chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-golden alkali metal with a melting point of , which makes it one of only five elemental metals that ...

atom. The exact modern SI definition is "

he second He or HE may refer to: Language * He (pronoun), an English pronoun * He (kana), the romanization of the Japanese kana へ * He (letter), the fifth letter of many Semitic alphabets * He (Cyrillic), a letter of the Cyrillic script called ''He'' in ...

is defined by taking the fixed numerical value of the cesium frequency, , 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 of

astronomical object An astronomical object, celestial object, stellar object or heavenly body is a naturally occurring physical entity, association, or structure that exists in the observable universe. In astronomy, the terms ''object'' and ''body'' are often us ...

s. * Sun-based: the year was the time for the Earth to revolve around the Sun. Historical year-based units include the Olympiad (four years), the lustrum (five years), the indiction (15 years), the decade, the century, and the millennium. * Moon-based: the month was based on the Moon's orbital period around the Earth. *

Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surf ...

-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 (referred to as civil time in modern usage) when direct sunlight shines by the apparent position of the Sun in the sky. In the narrowest sense of the word, it consists of a flat ...

. Units originally derived from this base include the week (seven days), and the fortnight (14 days). Subdivisions of the day include the

hour An hour (symbol: h; also abbreviated hr) is a unit of time conventionally reckoned as of a day and scientifically reckoned between 3,599 and 3,601 seconds, depending on the speed of Earth's rotation. There are 60 minutes in an hour, and 24 ho ...

(1/24 of a day), which was further subdivided into

minute The minute is a unit of time usually equal to (the first sexagesimal fraction) of an hour, or 60 seconds. In the UTC time standard, a minute on rare occasions has 61 seconds, a consequence of leap seconds (there is a provision to insert a nega ...

s and finally seconds. The second became the international standard unit (SI units) for science. * Celestial sphere-based: as in

sidereal time Sidereal time (as a unit also sidereal day or sidereal rotation period) (sidereal ) is a timekeeping system that astronomers use to locate celestial objects. Using sidereal time, it is possible to easily point a telescope to the proper coord ...

, where the apparent movement of the stars and 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 Intercalation may refer to: * Intercalation (chemistry), insertion of a molecule (or ion) into layered solids such as graphite *Intercalation (timekeeping), insertion of a leap day, week or month into some calendar years to make the calendar foll ...

. 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 (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 used in most parts of the world. It was introduced in October 1582 by Pope Gregory XIII as a modification of, and replacement for, the Julian calendar. The principal change was to space leap years di ...

as days has to be adjusted with leap days and leap seconds. Consequently, these units are now all defined for scientific purposes as multiples of seconds. Units of time based on orders of magnitude of the second include the nanosecond and the

millisecond A millisecond (from '' milli-'' and second; symbol: ms) is a unit of time in the International System of Units (SI) equal to one thousandth (0.001 or 10−3 or 1/1000) of a second and to 1000 microseconds. A unit of 10 milliseconds may be called ...

Historical

The natural units for timekeeping used by most historical societies are the

day A day is the time period of a full rotation of the Earth with respect to the Sun. On average, this is 24 hours, 1440 minutes, or 86,400 seconds. In everyday life, the word "day" often refers to a solar day, which is the length between two so ...

, the solar year and the lunation. Such calendars include the

Sumerian Sumerian or Sumerians may refer to: *Sumer, an ancient civilization **Sumerian language **Sumerian art **Sumerian architecture **Sumerian literature **Cuneiform script, used in Sumerian writing *Sumerian Records, an American record label based in ...

, Egyptian, Chinese,

Babylonian Babylonian may refer to: * Babylon, a Semitic Akkadian city/state of ancient Mesopotamia founded in 1894 BC * Babylonia, an ancient Akkadian-speaking Semitic nation-state and cultural region based in central-southern Mesopotamia (present-day Iraq) ...

, ancient Athenian,

Buddhist Buddhism ( , ), also known as Buddha Dharma and Dharmavinaya (), is an Indian religion or philosophical tradition based on teachings attributed to the Buddha. It originated in northern India as a -movement in the 5th century BCE, and ...

Hindu Hindus (; ) are people who religiously adhere to Hinduism.Jeffery D. Long (2007), A Vision for Hinduism, IB Tauris, , pages 35–37 Historically, the term has also been used as a geographical, cultural, and later religious identifier for ...

Islamic Islam (; ar, ۘالِإسلَام, , ) is an Abrahamic monotheistic religion centred primarily around the Quran, a religious text considered by Muslims to be the direct word of God (or '' Allah'') as it was revealed to Muhammad, the mai ...

, Icelandic, Mayan, and French Republican calendars. The modern calendar has its origins in the

Roman calendar The Roman calendar was the calendar used by the Roman Kingdom and Roman Republic. The term often includes the Julian calendar established by the reforms of the Roman dictator, dictator Julius Caesar and Roman emperor, emperor Augustus in the ...

, which evolved into the

Julian calendar The Julian calendar, proposed by Roman consul Julius Caesar in 46 BC, was a reform of the Roman calendar. It took effect on , by edict. It was designed with the aid of Greek mathematics, Greek mathematicians and Ancient Greek astronomy, as ...

, and then the Gregorian.

Scientific

* The Jiffy is the amount of time light takes to travel one fermi (about the size of a

nucleon In physics and chemistry, a nucleon is either a proton or a neutron, considered in its role as a component of an atomic nucleus. The number of nucleons in a nucleus defines the atom's mass number (nucleon number). Until the 1960s, nucleons w ...

) in a vacuum. * The Planck time is the time light takes to travel one Planck length. * The TU (for Time Unit) is a unit of time defined as 1024 µs for use in engineering. * The Svedberg is a time unit used for sedimentation rates (usually of proteins). It is defined as 10⁻¹³ seconds (100 fs). * The galactic year, 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 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 ...

to time. The deep time 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 that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...

period and the

Paleogene The Paleogene ( ; British English, also spelled Palaeogene or Palæogene; informally Lower Tertiary or Early Tertiary) is a geologic period, geologic period and system that spans 43 million years from the end of the Cretaceous Period million yea ...

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 million y ...

. The largest unit is the supereon, composed of eons. Eons are divided into eras, which are in turn divided into periods, epochs and

ages Ages may refer to: *Advanced glycation end-products, known as AGEs *Ages, Kentucky, census-designated place, United States * ''Ages'' (album) by German electronic musician Edgar Froese *The geologic time scale, a system of chronological measuremen ...

. 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 spelled light year, is a large unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (), or 5.88 trillion miles ().One trillion here is taken to be 1012 ...

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, usually 365 days. The other units used are multiples or divisions of these three.

References

{{Time measurement and standards

Historical

Scientific

List

Interrelation

See also

References