Midnight to 1 a.m. on a 24-hour clock with a digital face
Anhour (symbol:h;[1] also abbreviatedhr) is aunit oftime historically reckoned as1⁄24 of aday and defined contemporarily as exactly 3,600seconds (SI). There are 60 minutes in an hour, and 24 hours in a day.
Equal hours orequinoctial hours were taken as1⁄24 of the day as measured from noon to noon; the minor seasonal variations of this unit were eventually smoothed by making it1⁄24 of themean solar day. Since this unit was not constant due tolong term variations in the Earth's rotation, the hour was finally separated from the Earth's rotation and defined in terms of theatomic or physical second.
It is anon-SI unit that is accepted for use with SI.[2] In the modernmetric system, one hour is defined as 3,600 atomic seconds. However, on rare occasions an hour may incorporate a positive or negativeleap second,[a] effectively making it appear to last 3,599 or 3,601 seconds, in order to keepUTC within 0.9 seconds ofUT1, the latter of which is based on measurements of themean solar day.
Thetime of day is typically expressed in English in terms of hours. Whole hours on a12-hour clock are expressed using the contracted phraseo'clock, from the olderof the clock.[7] (10 am and 10 pm are both read as "ten o'clock".)
Hours on a24-hour clock ("military time") are expressed as "hundred" or "hundred hours".[8] (1000 is read "ten hundred" or "ten hundred hours"; 10 pm would be "twenty-two hundred".)
Fifteen and thirty minutes past the hour is expressed as "a quarter past" or "after"[9] and "half past", respectively, from their fraction of the hour. Fifteen minutes before the hour may be expressed as "a quarter to", "of", "till", or "before" the hour.[9] (9:45 may be read "nine forty-five" or "a quarter till ten".)
48 hour day scaled to the 24 hour day
12 hour time (48 hour day)
48 hour time (48 hour day)
12 hour time (24 hour day)
24 hour time (24 hour day)
12:00 night – 11:59 night
00:00 – 11:59
12:00am – 5:59am
00:00 – 05:59
12:00 morning – 11:59 morning
12:00 – 23:59
6:00am – 11:59am
06:00 – 11:59
12:00 afternoon – 11:59 afternoon
24:00 – 35:59
12:00pm – 5:59pm
12:00 – 17:59
12:00 evening – 11:59 evening
36:00 – 47:59
6:00pm – 11:59pm
18:00 – 23:59
Time based on the 48 hour day moves twice as fast to synchronise with the time of the 24 hour day.
History
Antiquity
Ancient Egypt
In ancient Egypt theflooding of the Nile was, and still is, an important annual event, crucial for agriculture. It was accompanied by the rise ofSirius before the sunrise, and the appearance of 12 constellations across the night sky, to which the Egyptians assigned some significance. Influenced by this, the Egyptians divided the night into 12 equal intervals.[10] These wereseasonal hours, shorter in the summer than in the winter. Subsequently, the day was divided into intervals as well, which eventually became more important than the nightly intervals. These subdivisions of a day spread to Greece, and later to Rome.
The ancient Greeks kept time differently than is done today. Instead of dividing the time between one midnight and the next into 24 equal hours, they divided the time from sunrise to sunset into 12 "seasonal hours" (their actual duration depending on season), and the time from sunset to the next sunrise again in 12 "seasonal hours".[11] Initially, only the day was divided into 12 seasonal hours and the night into three or four night watches.[12]
By theHellenistic period the night was also divided into 12 hours.[13] The day-and-night (νυχθήμερον) was probably first divided into 24 hours byHipparchus of Nicaea.[14] The Greek astronomerAndronicus of Cyrrhus oversaw the construction of a horologion called theTower of the Winds in Athens during the first century BCE. This structure tracked a 24-hour day using both sundials and mechanical hour indicators.[15]
Thecanonical hours were inherited intoearly Christianity fromSecond Temple Judaism.By AD 60, theDidache recommends disciples to pray theLord's Prayer three times a day; this practice found its way into the canonical hours as well. By the second and third centuries, suchChurch Fathers asClement of Alexandria,Origen, andTertullian wrote of the practice of Morning and Evening Prayer, and of the prayers at the third, sixth and ninth hours.In the early church, during the night before every feast, avigil was kept. The word "Vigils", at first applied to the Night Office, comes from a Latin source, namely theVigiliae or nocturnal watches or guards of the soldiers. The night from six o'clock in the evening to six o'clock in the morning was divided into four watches or vigils of three hours each, the first, the second, the third, and the fourth vigil.[16]
TheHorae were originally personifications of seasonal aspects of nature, not of the time of day.The list of 12Horae representing the 12 hours of the day is recorded only inLate Antiquity, byNonnus.[17] The first and twelfth of theHorae were added to the original set of ten:
Medieval astronomers such asal-Biruni[19] andSacrobosco,[20] divided the hour into 60minutes, each of 60seconds; this derives fromBabylonian astronomy, where the corresponding terms[clarification needed] denoted the time required for the Sun's apparent motion through theecliptic to describe one minute or second of arc, respectively. In present terms, the Babylonian degree of time was thus four minutes long, the "minute" of time was thus four seconds long and the "second" 1/15 of a second.[21][22]
In medieval Europe, the Roman hours continued to be marked onsundials but the more important units of time were thecanonical hours of theOrthodox andCatholic Church. During daylight, these followed the pattern set by the three-hour bells of theRoman markets, which were succeeded by thebells of local churches. They rangprime at about 6am,terce at about 9am,sext at noon,nones at about 3pm, andvespers at either 6pm orsunset.Matins andlauds precede these irregularly in the morning hours;compline follows them irregularly before sleep; and themidnight office follows that.Vatican II ordered their reformation for the Catholic Church in 1963,[23] though they continue to be observed in the Orthodox churches.
When mechanicalclocks began to be used to show hours of daylight or nighttime, their period needed to be changed every morning and evening (for example, by changing the length of theirpendula). The use of 24 hours for the entire day meant hours varied much less and the clocks needed to be adjusted only a few times a month.
The minor irregularities of the apparent solar day were smoothed by measuring time using themean solar day, using the Sun's movement along thecelestial equator rather than along theecliptic. The irregularities of this time system were so minor that most clocks reckoning such hours did not need adjustment. However, scientific measurements eventually became precise enough to note the effect oftidal deceleration of theEarth by theMoon, which gradually lengthens the Earth's days.
During theFrench Revolution, ageneral decimalisation of measures was enacted, includingdecimal time between 1794 and 1800. Under its provisions, the French hour (French:heure) was1⁄10 of the day and divided formally into 100 decimal minutes (minute décimale) and informally into 10 tenths (décime). Mandatory use for all public records began in 1794, but was suspended six months later by the same 1795 legislation that first established the metric system. In spite of this, a few localities continued to use decimal time for six years for civil status records, until 1800, after Napoleon's Coup of 18 Brumaire.
Themetric system bases its measurements of time upon thesecond, defined since 1952 in terms of the Earth's rotation in AD1900. Its hours are a secondary unit computed as precisely 3,600 seconds.[24] However, an hour ofCoordinated Universal Time (UTC), used as the basis of most civil time, has lasted 3,601 seconds 27 times since 1972 in order to keep it within 0.9 seconds ofuniversal time, which is based on measurements of themean solar day at0° longitude. The addition of these seconds accommodates the very gradual slowing of therotation of theEarth.
In modern life, the ubiquity of clocks and other timekeeping devices means that segmentation of days according to their hours is commonplace. Most forms ofemployment, whetherwage orsalaried labour, involve compensation based upon measured or expected hours worked. The fight for aneight-hour day was a part oflabour movements around the world. Informalrush hours andhappy hours cover the times of day when commuting slows down due to congestion or alcoholic drinks being available at discounted prices. Thehour record for the greatest distance travelled by a cyclist within the span of an hour is one ofcycling's greatest honours.
Counting hours
Top view of an equatorial sundial. The hour lines are spaced equally about the circle, and the shadow of the gnomon (a thin cylindrical rod) rotates uniformly. The height of the gnomon is5⁄12 the outer radius of the dial. This animation depicts the motion of the shadow from 3 a.m. to 9 p.m. on mid-summer's day, when the Sun is at its highest declination (roughly 23.5°). Sunrise and sunset occur at 3 a.m. and 9 p.m. respectively on that day at geographical latitudes near 57.5°, roughly the latitude of Aberdeen or Sitka, Alaska.Planispheric astrolabe designed for the latitude of Varese (Italy)
Many different ways of counting the hours have been used. Because sunrise, sunset, and, to a lesser extent, noon, are the conspicuous points in the day, starting to count at these times was, for most people in most early societies, much easier than starting at midnight. However, with accurate clocks and modern astronomical equipment (and the telegraph or similar means to transfer a time signal in a split-second), this issue is much less relevant.
In ancient and medieval cultures, the counting of hours generally started with sunrise. Before the widespread use of artificial light, societies were more concerned with the division between night and day, and daily routines often began when light was sufficient.[25]
"Babylonian hours" divide the day and night into 24 equal hours, reckoned from the time of sunrise.[26] They are so named from the false belief of ancient authors that the Babylonians divided the day into 24 parts, beginning at sunrise. In fact, they divided the day into 12 parts (calledkaspu or "double hours") or into 60 equal parts.[27]
Sunrise marked the beginning of the first hour, the middle of the day was at the end of the sixth hour and sunset at the end of the twelfth hour. This meant that the duration of hours varied with the season. In the Northern hemisphere, particularly in the more northerly latitudes, summer daytime hours were longer than winter daytime hours, each being one twelfth of the time between sunrise and sunset. These variable-length hours were variously known as temporal, unequal, or seasonal hours and were in use until the appearance of the mechanical clock, which furthered the adoption of equal length hours.[25]
This is also the system used inJewish law and frequently called "Talmudic hour" (Sha'a Zemanit) in a variety of texts. The Talmudic hour is one twelfth of time elapsed from sunrise to sunset, day hours therefore being longer than night hours in the summer; in winter they reverse.
The Indic day began at sunrise. The termhora was used to indicate an hour. The time was measured based on the length of the shadow at day time. Ahora translated to 2.5pe. There are 60pe per day, 60 minutes perpe and 60kshana (snap of a finger or instant) per minute.Pe was measured with a bowl with a hole placed in still water. Time taken for this graduated bowl was onepe. Kings usually had an officer in charge of this clock.
In so-called "Italian time", "Italian hours", or "old Czech time", the first hour started with the sunsetAngelus bell (or at the end of dusk, i.e., half an hour after sunset, depending on local custom and geographical latitude). The hours were numbered from 1 to 24. For example, in Lugano, the sun rose in December during the 14th hour and noon was during the 19th hour; in June the sun rose during the 7th hour and noon was in the 15th hour. Sunset was always at the end of the 24th hour. The clocks in church towers struck only from 1 to 12, thus only during night or early morning hours.
This manner of counting hours had the advantage that everyone could easily know how much time they had to finish their day's workwithout artificial light. It was already widely used inItaly by the 14th century and lasted until the mid-18th century; it was officially abolished in 1755, or in some regions customary until the mid-19th century.[c][28]
The system of Italian hours can be seen on a number of clocks in Europe, where the dial is numbered from 1 to 24 in either Roman or Arabic numerals. TheSt Mark's Clock in Venice, and theOrloj in Prague are famous examples. It was also used inPoland,Silesia, andBohemia until the 17th century.
Its replacement by the more practical division intotwice twelve (equinoctial) hours (also called small clock or civic hours) began as early as the 16th century.
TheIslamic day begins at sunset. The first prayer of the day (maghrib) is to be performed between just after sunset and the end of twilight. Until 1968 Saudi Arabia used the system of counting 24 equal hours with the first hour starting at sunset.[29]
Counting from noon
For many centuries, up to 1925, astronomers counted the hours and days from noon, because it was the easiest solar event to measure accurately. An advantage of this method (used in theJulian Date system, in which a new Julian Day begins at noon) is that the date doesn't change during a single night's observing.
Counting from midnight
In the modern12-hour clock, counting the hours starts at midnight and restarts at noon. Hours are numbered 12, 1, 2, ..., 11.Solar noon is always close to 12 noon (ignoring artificial adjustments due totime zones anddaylight saving time), differing according to theequation of time by as much as fifteen minutes either way. At theequinoxes sunrise is around 6 a.m. (Latin:ante meridiem, before noon), and sunset around 6 p.m. (Latin:post meridiem, after noon).
In the modern24-hour clock, counting the hours starts at midnight, and hours are numbered from 0 to 23. Solar noon is always close to 12:00, again differing according to the equation of time. At the equinoxes sunrise is around 06:00, and sunset around 18:00.
Theancient Egyptians began dividing the night intownwt at some time before the compilation of theDynasty VPyramid Texts[30] in the 24thcenturyBC.[31] By 2150BC (Dynasty IX), diagrams of stars inside Egyptiancoffin lids—variously known as "diagonal calendars" or "star clocks"—attest that there were exactly 12 of these.[31]Clagett writes that it is "certain" this duodecimal division of the night followed the adoption of theEgyptian civil calendar,[30] usually placedc. 2800BC on the basis of analyses of theSothic cycle, but alunar calendar presumably long predated this[32] and also would have had 12months in each of its years. The coffin diagrams show that the Egyptians took note of theheliacal risings of 36 stars orconstellations (now known as "decans"), one for each of the ten-day "weeks" of their civil calendar.[33] (12 sets of alternate "triangle decans" were used for the 5epagomenal days between years.)[34] Each night, the rising of eleven of these decans were noted, separating the night into 12 divisions whose middle terms would have lasted about 40minutes each. (Another seven stars were noted by the Egyptians during the twilight and predawn periods,[citation needed] although they were not important for the hour divisions.) The original decans used by the Egyptians would have fallen noticeably out of their proper places over a span of several centuries. By the time ofAmenhotep III (c. 1350BC), the priests atKarnak were usingwater clocks to determine the hours. These were filled to the brim at sunset and the hour determined by comparing the water level against one of its 12 gauges, one for each month of the year.[35] During theNew Kingdom, another system of decans was used, made up of 24 stars over the course of the year and 12 within any one night.
The later division of the day into 12 hours was accomplished bysundials marked with ten equal divisions. The morning and evening periods when the sundials failed to note time were observed as the first and last hours.[36]
The Egyptian hours were closely connected both with the priesthood of the gods and with their divine services. By theNew Kingdom, each hour was conceived as a specific region of the sky orunderworld through whichRa'ssolar barge travelled.[37] Protective deities were assigned to each and were used as the names of the hours.[37] As the protectors and resurrectors of the sun, the goddesses of the night hours were considered to hold power over all lifespans[37] and thus became part of Egyptian funerary rituals. Two fire-spitting cobras were said to guard the gates of each hour of the underworld, andWadjet and the rearingcobra (uraeus) were also sometimes referenced aswnwt from their role protecting the dead through these gates. The Egyptian word forastronomer, used as a synonym for priest, waswnwty, "one of thewnwt", as it were "one of the hours".[d] The earliest forms ofwnwt include one or three stars, with the later solar hours including thedeterminativehieroglyph for "sun".[30]
A Chinese diagram fromSu Song's AD1092Xinyi Xiangfa Yao illustrating his clocktower atKaifeng.A reconstruction of another kind of Chineseclepsydra inBeijing'sDrum Tower
Imperial China continued to useke andgeng but also began to divide the day into 12 "double hours"(t時,s时,oc*də,[48]pshí,lit. "time[s]") named after theearthly branches and sometimes also known by the name of the corresponding animal of theChinese zodiac.[55] The firstshi originally ran from 11pm to 1am but was reckoned as starting at midnight by the time of theHistory of Song, compiled during the earlyYuan.[56] These apparently began to be used during theEastern Han that preceded theThree Kingdoms era, but the sections that would have covered them are missing from their official histories; they first appear in official use in theTang-eraBook of Sui.[52] Variations of all these units were subsequently adopted byJapan[54] and the other countries of theSinosphere.
The 12shi supposedly began to be divided into 24 hours under theTang,[54] although they are first attested in theMing-eraBook of Yuan.[49] In that work, the hours were known by the sameearthly branches as theshi, with the first half noted as its "starting" and the second as "completed" or "proper"shi.[49] In modern China, these are instead simply numbered and described as "littleshi". The modernke is now used to count quarter-hours, rather than a separate unit.
As with the Egyptian night and daytime hours, the division of the day into 12shi has been credited to the example set by the rough number of lunar cycles in a solar year,[57] although the 12-yearJovian orbital cycle was more important totraditional Chinese[58] and Babylonian reckoning of the zodiac.[59][f]
InThailand,Laos, andCambodia, the traditional system of noting hours is thesix-hour clock. This reckons each of a day's 24 hours apart fromnoon as part of a fourth of the day. The first hour of the first half of daytime was 7 am; 1 pm the first hour of the latter half of daytime; 7 pm the first hour of the first half of nighttime; and 1 am the first hour of the latter half of nighttime. This system existed in theAyutthaya Kingdom, deriving its current phrasing from the practice of publicly announcing the daytime hours with agong and the nighttime hours with adrum.[61] It was abolished in Laos and Cambodia during theirFrench occupation and is uncommon there now. The Thai system remains in informal use in the form codified in 1901 byKing Chulalongkorn.[62]
TheVedas andPuranas employedunits of time based on thesidereal day (nakṣatra ahorātra). This was variously divided into 30muhūrta-s of 48 minutes each[63] or 60dandas[citation needed] ornadī-s of 24 minutes each.[64] Thesolar day was later similarly divided into 60ghaṭikás of about the same duration, each divided in turn into 60vinadis.[64] TheSinhalese followed a similar system but called their sixtieth of a day apeya.
^There is a trace of that system, for instance, inVerdi'soperas where inRigoletto or inUn ballo in maschera midnight is announced by the bell striking six times, not 12. But in his last opera,Falstaff, strangely, he abandoned that style, perhaps under influence of contemporary trends at end of 19thcentury when he composed it, and the midnight bell strikes 12 times.
^According to the 2nd-centuryShuowen Jiezi, "A water clock holds the water in a copper pot and notes the marks [kè] by a rule. There are 100 marks which represent the day".
^The late classical Indians also began to reckonyears based on the Jovian cycle, but this was much later than their lunar calendar and initially named after it.[60]
Citations
^"Resolution 7",Resolutions of the CGPM: 9th Meeting, Paris: International Bureau of Weights and Measures, October 1948
^ This article incorporates text from a publication now in thepublic domain: Cabrol, Fernand (1911). "Matins". In Herbermann, Charles (ed.).Catholic Encyclopedia. Vol. 10. New York: Robert Appleton Company.
^Nothaft, C. Philipp E. (2018),Scandalous Error: Calendar Reform and Calendrical Astronomy in Medieval Europe, Oxford: Oxford University Press, p. 126,ISBN9780198799559
^"Nach langem stillen Stauen trennten wir uns, da es fernher 7 schlug, nach unserer Uhr 12 Uhr Mitternacht." (Carl Oesterley, am 10. Dezember 1826 aus Rom nach einem nächtlichen Besuch des Kolosseums vier Tage zuvor). In:Herrmann Zschoche (Hrsg.):Carl Oesterley – Briefe aus Italien 1826-1828. Frankfurt am Main 2013, S. 33.
Baxter, William H.; Laurent Sagart (2014).Old Chinese: A New Reconstruction. Oxford, UK: Oxford University Press."parts available here".The Baxter-Sagart reconstruction of Old Chinese. University of Michigan. 20 September 2014. version 1.1.
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