The flow of sand in an hourglass can be used to measure the passage of time. It also concretely represents the present as being between the past and the future.

Time is the indefinite continued progress of existence and events that occur in an apparently irreversible succession from the past, through the present, into the future.[1][2][3] Time is a component quantity of various measurements used to sequence events, to compare the duration of events or the intervals between them, and to quantify rates of change of quantities in material reality or in the conscious experience.[4][5][6][7] Time is often referred to as a fourth dimension, along with three spatial dimensions.[8]

Time has long been an important subject of study in religion, philosophy, and science, but defining it in a manner applicable to all fields without circularity has consistently eluded scholars.[2][6][7][9][10][11]Nevertheless, diverse fields such as business, industry, sports, the sciences, and the performing arts all incorporate some notion of time into their respective measuring systems.[12][13][14]

Time in physics is operationally defined as "what a clock reads".[6][15][16] See Units of Time. Time is one of the seven fundamental physical quantities in both the International System of Units and International System of Quantities. Time is used to define other quantities – such as velocity – so defining time in terms of such quantities would result in circularity of definition.[17] An operational definition of time, wherein one says that observing a certain number of repetitions of one or another standard cyclical event (such as the passage of a free-swinging pendulum) constitutes one standard unit such as the second, is highly useful in the conduct of both advanced experiments and everyday affairs of life. To describe observations of an event, a location (position in space) and time are typically noted.

The operational definition of time does not address what the fundamental nature of it is. It does not address why events can happen forward and backwards in space, whereas events only happen in the forward progress of time. Investigations into the relationship between space and time led physicists to define the spacetime continuum. General Relativity is the primary framework for understanding how spacetime works. Through advances in both theoretical and experimental investigations of space-time, it has been shown that time can be distorted, particularly at the edges of blackholes.

Temporal measurement has occupied scientists and technologists, and was a prime motivation in navigation and astronomy. Periodic events and periodic motion have long served as standards for units of time. Examples include the apparent motion of the sun across the sky, the phases of the moon, the swing of a pendulum, and the beat of a heart. Currently, the international unit of time, the second, is defined by measuring the electronic transition frequency of caesium atoms (see below). Time is also of significant social importance, having economic value ("time is money") as well as personal value, due to an awareness of the limited time in each day and in human life spans.

Temporal measurement

Generally speaking, methods of temporal measurement, or chronometry, take two distinct forms: the calendar, a mathematical tool for organising intervals of time,[18] and the clock, a physical mechanism that counts the passage of time. In day-to-day life, the clock is consulted for periods less than a day whereas the calendar is consulted for periods longer than a day. Increasingly, personal electronic devices display both calendars and clocks simultaneously. The number (as on a clock dial or calendar) that marks the occurrence of a specified event as to hour or date is obtained by counting from a fiducial epoch – a central reference point.

History of the calendar

Artifacts from the Paleolithic suggest that the moon was used to reckon time as early as 6,000 years ago.[19] Lunar calendars were among the first to appear, with years of either 12 or 13 lunar months (either 354 or 384 days). Without intercalation to add days or months to some years, seasons quickly drift in a calendar based solely on twelve lunar months. Lunisolar calendars have a thirteenth month added to some years to make up for the difference between a full year (now known to be about 365.24 days) and a year of just twelve lunar months. The numbers twelve and thirteen came to feature prominently in many cultures, at least partly due to this relationship of months to years. Other early forms of calendars originated in Mesoamerica, particularly in ancient Mayan civilization. These calendars were religiously and astronomically based, with 18 months in a year and 20 days in a month, plus five epagomenal days at the end of the year.[20]

The reforms of Julius Caesar in 45 BC put the Roman world on a solar calendar. This Julian calendar was faulty in that its intercalation still allowed the astronomical solstices and equinoxes to advance against it by about 11 minutes per year. Pope Gregory XIII introduced a correction in 1582; the Gregorian calendar was only slowly adopted by different nations over a period of centuries, but it is now by far the most commonly used calendar around the world.

During the French Revolution, a new clock and calendar were invented in an attempt to de-Christianize time and create a more rational system in order to replace the Gregorian calendar. The French Republican Calendar's days consisted of ten hours of a hundred minutes of a hundred seconds, which marked a deviation from the base 12 (duodecimal) system used in many other devices by many cultures. The system was abolished in 1806.[21]

History of time measurement devices

Horizontal sundial in Taganrog
An old kitchen clock

A large variety of devices have been invented to measure time. The study of these devices is called horology.[22]

An Egyptian device that dates to c. 1500 BC, similar in shape to a bent T-square, measured the passage of time from the shadow cast by its crossbar on a nonlinear rule. The T was oriented eastward in the mornings. At noon, the device was turned around so that it could cast its shadow in the evening direction.[23]

A sundial uses a gnomon to cast a shadow on a set of markings calibrated to the hour. The position of the shadow marks the hour in local time. The idea to separate the day into smaller parts is credited to Egyptians because of their sundials, which operated on a duodecimal system. The importance of the number 12 is due to the number of lunar cycles in a year and the number of stars used to count the passage of night.[24]

The most precise timekeeping device of the ancient world was the water clock, or clepsydra, one of which was found in the tomb of Egyptian pharaoh Amenhotep I. They could be used to measure the hours even at night, but required manual upkeep to replenish the flow of water. The Ancient Greeks and the people from Chaldea (southeastern Mesopotamia) regularly maintained timekeeping records as an essential part of their astronomical observations. Arab inventors and engineers in particular made improvements on the use of water clocks up to the Middle Ages.[25] In the 11th century, Chinese inventors and engineers invented the first mechanical clocks driven by an escapement mechanism.

A contemporary quartz watch, 2007

The hourglass uses the flow of sand to measure the flow of time. They were used in navigation. Ferdinand Magellan used 18 glasses on each ship for his circumnavigation of the globe (1522).[26]

Incense sticks and candles were, and are, commonly used to measure time in temples and churches across the globe. Waterclocks, and later, mechanical clocks, were used to mark the events of the abbeys and monasteries of the Middle Ages. Richard of Wallingford (1292–1336), abbot of St. Alban's abbey, famously built a mechanical clock as an astronomical orrery about 1330.[27][28]

Great advances in accurate time-keeping were made by Galileo Galilei and especially Christiaan Huygens with the invention of pendulum driven clocks along with the invention of the minute hand by Jost Burgi.[29]

The English word clock probably comes from the Middle Dutch word klocke which, in turn, derives from the medieval Latin word clocca, which ultimately derives from Celtic and is cognate with French, Latin, and German words that mean bell. The passage of the hours at sea were marked by bells, and denoted the time (see ship's bell). The hours were marked by bells in abbeys as well as at sea.

Chip-scale atomic clocks, such as this one unveiled in 2004, are expected to greatly improve GPS location.[30]

Clocks can range from watches, to more exotic varieties such as the Clock of the Long Now. They can be driven by a variety of means, including gravity, springs, and various forms of electrical power, and regulated by a variety of means such as a pendulum.

Alarm clocks first appeared in ancient Greece around 250 BC with a water clock that would set off a whistle. This idea was later mechanized by Levi Hutchins and Seth E. Thomas.[29]

A chronometer is a portable timekeeper that meets certain precision standards. Initially, the term was used to refer to the marine chronometer, a timepiece used to determine longitude by means of celestial navigation, a precision firstly achieved by John Harrison. More recently, the term has also been applied to the chronometer watch, a watch that meets precision standards set by the Swiss agency COSC.

The most accurate timekeeping devices are atomic clocks, which are accurate to seconds in many millions of years,[31] and are used to calibrate other clocks and timekeeping instruments.

Atomic clocks use the frequency of electronic transitions in certain atoms to measure the second. One of the atoms used is caesium, most modern atomic clocks probe caesium with microwaves to determine the frequency of these electron vibrations.[32] Since 1967, the International System of Measurements bases its unit of time, the second, on the properties of caesium atoms. SI defines the second as 9,192,631,770 cycles of the radiation that corresponds to the transition between two electron spin energy levels of the ground state of the 133Cs atom.

Today, the Global Positioning System in coordination with the Network Time Protocol can be used to synchronize timekeeping systems across the globe.

In medieval philosophical writings, the atom was a unit of time referred to as the smallest possible division of time. The earliest known occurrence in English is in Byrhtferth's Enchiridion (a science text) of 1010–1012,[33] where it was defined as 1/564 of a momentum (1½ minutes),[34] and thus equal to 15/94 of a second. It was used in the computus, the process of calculating the date of Easter.

As of May 2010, the smallest time interval uncertainty in direct measurements is on the order of 12 attoseconds (1.2 × 10−17 seconds), about 3.7 × 1026 Planck times.[35]

Units of time

The second (s) is the SI base unit. A minute (min) is 60 seconds in length, and an hour is 60 minutes in length. A day is 24 hours or 86,400 seconds in length.

Other Languages
Afrikaans: Tyd
Alemannisch: Zeit
አማርኛ: ጊዜ
Аҧсшәа: Аамҭа
العربية: زمن
aragonés: Tiempo
অসমীয়া: সময়
asturianu: Tiempu
Atikamekw: Tipahikanicic
Avañe'ẽ: Ára
azərbaycanca: Zaman (fizika)
تۆرکجه: زامان
বাংলা: সময়
Bân-lâm-gú: Sî-kan
Basa Banyumasan: Wektu
башҡортса: Ваҡыт
беларуская: Час
беларуская (тарашкевіца)‎: Час
भोजपुरी: समय
български: Време
Boarisch: Zeid
བོད་ཡིག: དུས་ཚོད།
brezhoneg: Amzer
буряад: Саг хугасаа
català: Temps
Чӑвашла: Вăхăт
Cebuano: Panahon
čeština: Čas
Chi-Chewa: Nthawi
chiShona: Nguva
Cymraeg: Amser
dansk: Tid
Deutsch: Zeit
eesti: Aeg
Ελληνικά: Χρόνος
emiliàn e rumagnòl: Tèimp
эрзянь: Шка
español: Tiempo
Esperanto: Tempo
estremeñu: Tiempu
euskara: Denbora
فارسی: زمان
français: Temps
Frysk: Tiid
furlan: Timp
Gaeilge: Am
galego: Tempo
ГӀалгӀай: Ха
客家語/Hak-kâ-ngî: Sṳ̀-kiên
хальмг: Цаг
한국어: 시간
Hausa: Lokaci
հայերեն: Ժամանակ
हिन्दी: समय
Ido: Tempo
Bahasa Indonesia: Waktu
interlingua: Tempore
íslenska: Tími
italiano: Tempo
עברית: זמן
Jawa: Wektu
Kabɩyɛ: Alɩwaatʋ
ಕನ್ನಡ: ಕಾಲ
ქართული: დრო
қазақша: Уақыт
Kiswahili: Wakati
Kreyòl ayisyen: Tan
Кыргызча: Убакыт
Latina: Tempus
latviešu: Laiks
Lëtzebuergesch: Zäit
lietuvių: Laikas
Ligure: Tempo
Limburgs: Tied
Livvinkarjala: Aigu
lumbaart: Temp
magyar: Idő
मैथिली: समय
македонски: Време
മലയാളം: കാലം
मराठी: काळ
Bahasa Melayu: Masa
Mìng-dĕ̤ng-ngṳ̄: Sì-găng
монгол: Хугацаа
မြန်မာဘာသာ: အချိန်
Nāhuatl: Cahuitl
Na Vosa Vakaviti: Gauna
Nederlands: Tijd
Nedersaksies: Tied
नेपाली: समय
नेपाल भाषा: ई (काल)
日本語: 時間
Nordfriisk: Tidj
Norfuk / Pitkern: Tiem
norsk: Tid
norsk nynorsk: Tid
Novial: Tempe
occitan: Temps
ଓଡ଼ିଆ: ସମୟ (କାଳ)
oʻzbekcha/ўзбекча: Vaqt
ਪੰਜਾਬੀ: ਸਮਾਂ
پنجابی: ویلا
پښتو: وخت
Patois: Taim
Picard: Tans
Piemontèis: Temp
Plattdüütsch: Tiet
polski: Czas
português: Tempo
Qaraqalpaqsha: Waqıt
română: Timp
Runa Simi: Mit'awi
русиньскый: Час
русский: Время
саха тыла: Кэм
ᱥᱟᱱᱛᱟᱲᱤ: ᱚᱠᱛᱚ
Scots: Time
Seeltersk: Tied
Sesotho sa Leboa: Nako
shqip: Koha
sicilianu: Tempu
සිංහල: කාලය
Simple English: Time
سنڌي: وقت
slovenčina: Čas (fyzika)
slovenščina: Čas
словѣньскъ / ⰔⰎⰑⰂⰡⰐⰠⰔⰍⰟ: Врѣмѧ
ślůnski: Czas
Soomaaliga: Wakhti
کوردی: کات
српски / srpski: Време
srpskohrvatski / српскохрватски: Vrijeme
Sunda: Waktu
suomi: Aika
svenska: Tid
Tagalog: Panahon
தமிழ்: நேரம்
Taqbaylit: Akud
татарча/tatarça: Вакыт
తెలుగు: సమయం
ไทย: เวลา
тоҷикӣ: Замон
Türkçe: Zaman
Türkmençe: Wagt
українська: Час
اردو: وقت
Vahcuengh: Seizgan
vèneto: Tenpo
vepsän kel’: Aig
Tiếng Việt: Thời gian
Volapük: Tim
Võro: Aig
Winaray: Oras (pisika)
吴语: 辰光
ייִדיש: צייט
Yorùbá: Àsìkò
粵語: 時間
Zazaki: Zeman
žemaitėška: Čiesos
中文: 时间
kriyòl gwiyannen: Tan