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 ...

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, 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 a ...

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 objects. * Sun-based: the

year A year or annus is the orbital period of a planetary body, for example, the Earth, moving in 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 h ...

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 A millennium (plural millennia or millenniums) is a period of one thousand years, sometimes called a kiloannus, kiloannum (ka), or kiloyear (ky). Normally, the word is used specifically for periods of a thousand years that begin at the starting ...

. *

Moon The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...

-based: the month was based on the Moon's

orbital period The orbital period (also revolution period) is the amount of time a given astronomical object takes to complete one orbit around another object. In astronomy, it usually applies to planets or asteroids orbiting the Sun, moons orbiting pla ...

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 sur ...

-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 f ...

. Units originally derived from this base include the

week A week is a unit of time equal to seven days. It is the standard time period used for short cycles of days in most parts of the world. The days are often used to indicate common work days and rest days, as well as days of worship. Weeks are of ...

(seven

days 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 ...

), and the fortnight (14 days). Subdivisions of the day include the hour (1/24 of a day), which was further subdivided into minutes and finally seconds. The second became the international standard unit (SI units) for science. * Celestial sphere-based: as in sidereal time, 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. 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 calendars, a lunar month is the time between two successive syzygies of the same type: new moons or full moons. The precise definition varies, especially for the beginning of the month. Variations In Shona, Middle Eastern, and Eur ...

(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 d ...

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.

Historical

The natural units for timekeeping used by most historical societies are the day, the solar year and the

lunation In lunar calendars, a lunar month is the time between two successive syzygies of the same type: new moons or full moons. The precise definition varies, especially for the beginning of the month. Variations In Shona, Middle Eastern, and ...

. Such calendars include the Sumerian, Egyptian, Chinese, Babylonian, 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 ma ...

, Icelandic,

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 ...

, 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 dictator Julius Caesar and emperor Augustus in the late 1stcenturyBC and some ...

, 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 mathematicians and astronomers such as Sosigenes of Alexandri ...

, 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 (later naturalized American) physicist and the creator of the world's first nuclear reactor, the Chicago Pile-1. He has been called the "architect of the nuclear age" and t ...

(about the size of a nucleon) 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 The galactic year, also known as a cosmic year, is the duration of time required for the Sun to orbit once around the center of the Milky Way Galaxy.

, 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 layers ( strata) and layering (stratification). It is primarily used in the study of sedimentary and layered volcanic rocks. Stratigraphy has three related subfields: lithost ...

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 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 mass extinction of three-quarters of the plant and animal species on Earth, approximately 66 million years ago. With the ...

. 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 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

, 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