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hour
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{{Other uses of}}{{redirect|Hours}}File:DigitalClock 1hour.gif|thumb|MidnightMidnightFile:AnalogClockAnimation1 2hands 1h in 6sec.gif|thumb|Midnight (or noonnoonAn hour (symbol: h;{{citation |contribution-url=http://www.bipm.org/en/CGPM/db/9/7/ |contribution=Resolution 7 |date=October 1948 |url=http://www.bipm.org/en/CGPM/db/9/ |title=Resolutions of the CGPM: 9th Meeting |publisher=International Bureau of Weights and Measures |location=Paris }} also abbreviated hr.) is a unit of time conventionally reckoned as {{frac|24}} of a day and scientifically reckoned as 3,599–3,601 seconds, depending on conditions.The hour was initially established in the ancient Near East as a variable measure of {{frac|12}} of the night or daytime. Such seasonal, temporal, or unequal hours varied by season and latitude. The hour was subsequently divided into 60 minutes, each of 60 seconds. Equal or equinoctial hours were taken as {{frac|24}} of the day as measured from noon to noon; the minor seasonal variations of this unit were eventually smoothed by making it {{frac|24}} of the mean solar day. Since this unit was not constant due to long term variations in the Earth's rotation, the hour was finally separated from the Earth's rotation and defined in terms of the atomic or physical second.In the modern metric system, hours are an accepted unit of time defined as 3,600 atomic seconds. However, on rare occasions an hour may incorporate a positive or negative leap second,{{efn|Since 1972, the 27 leap seconds added to UTC have all been additions.}} making it last 3,599 or 3,601 seconds, in order to keep it within 0.9 seconds of UT1, which is based on measurements of the mean solar day.{{anchor|Etymology|Names|Time of Day|Time of day}}

Name

(File:Equation of time.svg|thumb|200px|The equation of time—above the axis the sundial will appear fast, compared with a clock showing local mean time, and below the sundial will appear slow.)The modern English word hour is a development of the Anglo-Norman ' and Middle English ', first attested in the 13th century.{{harvnb|OED|loc=hour, n.}}{{efn|From the {{c.|1250}} sermon for Sexagesima Sunday: {{citation |title=An Old English Miscellany |url=https://archive.org/stream/anoldenglishmis00sullgoog#page/n5/mode/2up |editor-last=Morris |editor-first=Richard |location=London |publisher=N. Trübner & Co. for the Early English Text Society |date=1872 |contribution-url=https://archive.org/stream/anoldenglishmis00sullgoog#page/n47/mode/2up |contribution=Old Kentish Sermons (Laud MS 471) |p=34 }}}} It displaced the Old English "tide" (, "time"){{sfn|OED|loc=tide, n}} and "stound" (', "span of time").{{sfn|OED|loc=stound, n.¹}} The Anglo-Norman term was a borrowing of Old French ', a variant of ', which derived from Latin ' and Greek hṓrā (}}). Like Old English ' and ', hṓrā was originally a vaguer word for any span of time, including seasons and years. Its Proto-Indo-European root has been reconstructed as {{PIE|(wikt:Reconstruction:Proto-Indo-European/yeh₁-|*yeh₁-)}} ("year, summer"), making hour distantly cognate with year.The time of day is typically expressed in English in terms of hours. Whole hours on a 12-hour clock are expressed using the contracted phrase o'clock, from the older of clock.{{sfn|OED|loc=clock, n.¹, & o'clock, adv. (and n.)}} (10 am and 10 pm are both read as "ten o'clock".) Hours on a 24-hour clock ("military time") are expressed as "hundred" or "hundred hours".{{sfn|OED|loc=hundred, n. and adj.}} (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"{{sfn|OED|loc=quarter, n}} 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.{{sfn|OED|loc=quarter, n}} (9:45 may be read "nine forty-five" or "a quarter till ten".)

History

{{Further|History of timekeeping devices}}

Egypt

{{Further|Egyptian calendar|Decans|History of timekeeping devices in Egypt}}The ancient Egyptians began dividing the night into at some time before the compilation of the Dynasty V Pyramid Texts{{harvp|Clagett|1995|p=49}} in the 24th{{nbsp}}century{{nbsp}}{{sc|bc}}.{{harvp|Clagett|1995|p=50}} By 2150{{nbsp}}{{sc|bc}} (Dynasty IX), diagrams of stars inside Egyptian coffin lids—variously known as "diagonal calendars" or "star clocks"—attest that there were exactly 12 of these. Clagett writes that it is "certain" this duodecimal division of the night followed the adoption of the Egyptian civil calendar, usually placed {{c.|2800}}{{nbsp}}{{sc|bc}} on the basis of analyses of the Sothic cycle, but a lunar calendar presumably long predated this{{sfnp|Parker|1950|pp=30-2}} and also would have had twelve months in each of its years. The coffin diagrams show that the Egyptians took note of the heliacal risings of 36 stars or constellations (now known as "decans"), one for each of the ten-day "weeks" of their civil calendar.{{harvp|Clagett|1995|p=50–1}} (12 sets of alternate "triangle decans" were used for the 5 epagomenal days between years.){{harvp|Clagett|1995|p=218}} Each night, the rising of eleven of these decans were noted, separating the night into twelve divisions whose middle terms would have lasted about 40{{nbsp}}minutes each. (Another seven stars were noted by the Egyptians during the twilight and predawn periods,{{citation needed|date=March 2017}} 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 of {{nowrap|Amenhotep III}} ({{c.|1350}}{{nbsp}}{{sc|bc}}), the priests at Karnak were using water 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 twelve gauges, one for each month of the year.{{sfnp|Parker|1950|p=40}} During the New 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 by sundials 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.{{citation needed|date=March 2017}}The Egyptian hours were closely connected both with the priesthood of the gods and with their divine services. By the New Kingdom, each hour was conceived as a specific region of the sky or underworld through which Ra's solar barge travelled.{{sfnp|Wilkinson|2003|p=83}} Protective deities were assigned to each and were used as the names of the hours.{{sfnp|Wilkinson|2003|p=83}} As the protectors and resurrectors of the sun, the goddesses of the night hours were considered to hold power over all lifespans{{sfnp|Wilkinson|2003|p=83}} and thus became part of Egyptian funerary rituals. Two fire-spitting cobras were said to guard the gates of each hour of the underworld, and Wadjet and the rearing cobra (uraeus) were also sometimes referenced as ' from their role protecting the dead through these gates. The Egyptian for astronomer, used as a synonym for priest, was ', "One of the Hours" or "Hour-Watcher".{{efn|' is written variously as E34:N35-G43-X1:Z4-N14,{{sfnp|Vygus|2015|p=400}} E34:N35-W24*X1-D4, E34:N35-W24*X1-N14,{{sfnp|Vygus|2015|p=408}} E34:N35-W24:X1-N14-A24-A1:Z2, E34:N35-W24:X1-N14-N5:D4,{{sfnp|Vygus|2015|p=409}} E34:N35-W24:X1-Z4-A1, E34:N35-W24:X1-Z4-N11:N14-D6, E34:N35-W24:X1-Z4-N14, E34:N35-W24:X1-Z4-N14-A1, E34:N35-W24:X1-Z4-N2-A24,{{sfnp|Vygus|2015|p=410}} E34:N35-X1:Z4-N14:N5,{{sfnp|Vygus|2015|p=412}} N14,{{sfnp|Vygus|2015|p=1235}} N14:W24-X1:Z4,{{sfnp|Vygus|2015|p=1239}} and N14:X1*Z4.{{sfnp|Vygus|2015|p=1240}}}} The earliest forms of ' include one or three stars, with the later solar hours including the determinative hieroglyph for "sun".

East Asia

File:Clock Tower from Su Song's Book.JPG|thumb|right|200px|A Chinese diagram from Su Song's {{sc|ad}}{{nbsp}}1092 Xinyi Xiangfa Yao illustrating his clocktower at KaifengKaifengFile:Beijing_2006_1-14.jpg|thumb|right|200px|A reconstruction of another kind of Chinese clepsydra in Beijing's Drum Tower ]]Ancient China divided its day into 100 "marks"{{sfnp|Stephenson|1997}}{{sfnp|Steele|2000}} {{nowrap|(Chinese: }},}} {{nowrap|oc *kʰək,{{sfnp|Baxter & al.|2014}}}} {{nowrap|p kè)}} running from midnight to midnight.{{sfnp|Sōma & al.|2004|p=887}} The system is said to have been used since remote antiquity,{{sfnp|Sōma & al.|2004|p=887}} credited to the legendary Yellow Emperor,{{sfnp|Petersen|1992|p=129}} but is first attested in Han-era water clocks{{sfnp|Petersen|1992|p=125}} and in the 2nd-century history of that dynasty.{{sfnp|Sōma & al.|2004|p=889}} It was measured with sundials{{sfnp|Stephenson & al.|2002|pp=15–16}} and water clocks.{{efn|According to the 2nd-century Shuowen 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".}} Into the Eastern Han, the Chinese measured their day schematically, adding the 20-ke difference between the solstices evenly throughout the year, one every nine days.{{sfnp|Petersen|1992|p=125}} During the night, time was more commonly reckoned during the night by the "watches" {{nowrap|(Chinese: }},}} {{nowrap|oc *kæŋ,{{sfnp|Baxter & al.|2014}}}} {{nowrap|p gēng)}} of the guard, which were reckoned as a fifth of the time from sunset to sunrise.{{sfnp|Stephenson|1997}}{{sfnp|Sōma & al.|2004|p=888}}Imperial China continued to use ke and geng but also began to divide the day into 12 "double hours" {{nowrap|(t }},}} {{nowrap|s }},}} {{nowrap|oc *də,{{sfnp|Baxter & al.|2014}}}} {{nowrap|p shí,}} {{nowrap|{{abbr|lit.|literally}} "time[s]")}} named after the earthly branches and sometimes also known by the name of the corresponding animal of the Chinese zodiac.{{sfnp|Sōma & al.|2004|p=904}} The first shi originally ran from 11{{nbsp}}pm to 1{{nbsp}}am but was reckoned as starting at midnight by the time of the History of Song, compiled during the early Yuan.{{sfnp|Sōma & al.|2004|p=896}} These apparently began to be used during the Eastern Han that preceded the Three Kingdoms era, but the sections that would have covered them are missing from their official histories; they first appear in official use in the Tang-era Book of Sui.{{sfnp|Sōma & al.|2004|p=889}} Variations of all these units were subsequently adopted by Japan{{sfnp|Sōma & al.|2004|p=888}} and the other countries of the Sinosphere.The 12 shi supposedly began to be divided into 24 hours under the Tang,{{sfnp|Sōma & al.|2004|p=888}} although they are first attested in the Ming-era Book of Yuan.{{sfnp|Sōma & al.|2004|p=887}} In that work, the hours were known by the same earthly branches as the shi, with the first half noted as its "starting" and the second as "completed" or "proper" shi.{{sfnp|Sōma & al.|2004|p=887}} In modern China, these are instead simply numbered and described as "little shi". The modern ke 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 twelve shi has been credited to the example set by the rough number of lunar cycles in a solar year,BOOK,weblink A Timeless Legacy: the Calendars of Ancient Egypt, Canhão, Telo Ferreira, 2013, Edições Afrontamento, 9789892609669, Porto, 10.14195/978-989-26-0966-9_20, Alexandrea ad Aegyptvm: The legacy of multiculturalismo in antiquity, 283–301, although the 12-year Jovian orbital cycle was more important to traditional Chinese{{citation |last=Zai |first=J. |title=Taoism and Science |publisher=Ultravisum |date=2015 }} and Babylonian reckoning of the zodiac.{{sfnp|Rogers|1998|pp=9–28}}{{efn|The late classical Indians also began to reckon years based on the Jovian cycle, but this was much later than their lunar calendar and initially named after it.{{sfnp|Sewell|1924|p=xii}}}}

Southeast Asia

In Thailand, Laos, and Cambodia, the traditional system of noting hours is the six-hour clock. This reckons each of a day's 24 hours apart from noon as part of a fourth of the day. 7 am was the first hour of the first half of daytime; 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 the Ayutthaya Kingdom, deriving its current phrasing from the practice of publicly announcing the daytime hours with a gong and the nighttime hours with a drum.{{sfnp|Thongprasert|1985|pp=229–237}} It was abolished in Laos and Cambodia during their French occupation and is uncommon there now. The Thai system remains in informal use in the form codified in 1901 by King Chulalongkorn.{{citation |contribution=ประกาศใช้ทุ่มโมงยาม |title=Royal Gazette, No. 17 |date=29 July 1901 |url=http://www.ratchakitcha.soc.go.th/DATA/PDF/2443/018/206.PDF |p=206 }}. {{th icon}}

Other ancient cultures

File:Drei Horen.jpg|thumb|200px|right|Two of the deified Hours of the Greeks and Romans]]The Vedas and Puranas employed units of time based on the sidereal day (nakṣatra ahorātram). This was variously divided into 30 muhūtras of 48 minutes each or 60 dandas{{fact|date=March 2017}} or nadís of 24 minutes each.{{harvp|Dershowitz & al.|2008|p=207}} The solar day was later similarly divided into 60 ghaṭikás of about the same duration, each divided in turn into 60 vinadis. The Sinhalese followed a similar system but called their sixtieth of a day a peya.The ancient Greeks and Romans originally divided the day into 12 hours and the night into 3 or 4 night watches. They were notionally deified as the Horae, although sometimes only as a triad. The Greek astronomer Andronicus of Cyrrhus oversaw the construction of a horologion called the Tower of the Winds in Athens during the first century{{nbsp}}{{sc|bc}}. This structure tracked a 24-hour day using both sundials and mechanical hour indicators.WEB, National Institute of Standards and Technology, A Walk Through Time,weblink 2 April 2014, The night was eventually also divided into 12 hours.

Middle Ages

missing image!
- Bishopstone sundial.jpg -
A 7th-century Saxon tide dial on the porch at Bishopstone in Sussex, with larger crosses marking the canonical hours.{{sfnp|Wall|1912|p=67}}
During Europe's Middle Ages, the Roman hours continued to be marked on sundials but the more important units of time were the canonical hours of the Orthodox and Catholic Church. During daylight, these followed the pattern set by the three-hour bells of the Roman markets, which were succeeded by the bells of local churches. They rang prime at about 6{{nbsp}}am, terce at about 9{{nbsp}}am, sext at noon, nones at about 3{{nbsp}}pm, and vespers at either 6{{nbsp}}pm or sunset. Matins and lauds precede these irregularly in the morning hours; compline follows them irregularly before sleep; and the midnight office follows that. Vatican II ordered their reformation for the Catholic Church in 1963,{{citation |author=Paul VI |date=4 December 1963 |title=Constitution on the Sacred Liturgy |url=http://www.vatican.va/archive/hist_councils/ii_vatican_council/documents/vat-ii_const_19631204_sacrosanctum-concilium_en.html |location=Vatican City |at=§89(d) }} though they continue to be observed in the Orthodox churches.When mechanical clocks 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 their pendula). 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.

Modernity

The minor irregularities of the apparent solar day were smoothed by measuring time using the mean solar day, using the Sun's movement along the celestial equator rather than along the ecliptic. 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 of tidal deceleration of the Earth by the Moon, which gradually lengthens the Earth's days. During the French Revolution, a general decimalization of measures was enacted, including decimal time between 1793 and 1795. Under its provisions, the French hour (}}) was {{frac|10}} of the day and divided formally into 100 decimal minutes () and informally into 10 tenths (}}). This hour was only briefly in official use, being repealed by the same 1795 legislation that first established the metric system.The metric system bases its measurements of time upon the second, defined since 1952 in terms of the Earth's rotation in {{sc|ad}}{{nbsp}}1900. Its hours are a secondary unit computed as precisely 3,600 seconds.{{citation |url=http://www.bipm.org/en/publications/si-brochure/ |title=The International System of Units (SI), 8th ed. |contribution-url=http://www.bipm.org/en/publications/si-brochure/table6.html |contribution=Non-SI Units Accepted for Use with the SI, and Units Based on Fundamental Constants (contd.) |publisher=International Bureau of Weights and Measures |date=2014 |location=Paris }} However, an hour of Coordinated 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 of universal time, which is based on measurements of the mean solar day at 0° longitude. The addition of these seconds accommodates the very gradual slowing of the rotation of the Earth.In modern life, the ubiquity of clocks and other timekeeping devices means that segmentation of days according to their hours is commonplace. Most forms of employment, whether wage or salaried labour, involves compensation based upon measured or expected hours worked. The fight for an eight-hour day was a part of labour movements around the world. Informal rush hours and happy hours cover the times of day when commuting slows down due to congestion or alcoholic drinks are available at discounted prices. The hour record for the greatest distance travelled by a cyclist within the span of an hour is one of cycling's greatest honours.

Counting hours

(File:Equatorial sundial topview.gif|thumb|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 is {{frac|5|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.)(File:Planispheric astrolabe.png|thumb|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.Astrolabes, sundials, and astronomical clocks sometimes show the hour length and count using some of these older definitions and counting methods.

Counting from dawn

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.{{sfnp|Landes|1983|p=76}}"Babylonian hours", as used on modern sundials, divide the day and night into 24 equal hours, reckoned from the time of sunrise.WEB,weblink Different Classification of Hours, Math.nus.edu.sg, 2018-09-20, 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 (called kaspu or "double hours") or into 60 equal parts.{{sfnp|Holford-Strevens|2005}}

Unequal hours

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.{{sfnp|Landes|1983|p=76}}This is also the system used in Jewish 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 term hora was used to indicate an hour. The time was measured based on the length of the shadow at day time. A hora translated to 2.5 pe. There are 60 pe per day, 60 minutes per pe and 60 kshana (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 one pe. Kings usually had an officer in charge of this clock.

Counting from sunset

In so-called "Italian time", "Italian hours", or "old Czech time", the first hour started with the sunset Angelus 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 work without artificial light. It was already widely used in Italy 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.{{efn|There is a trace of that system, for instance, in Verdi's operas where in Rigoletto or in Un ballo in maschera midnight is announced by the bell striking 6 times, not 12. But in his last opera, Falstaff, strangely, he abandoned that style, perhaps under influence of contemporary trends at end of 19th century when he composed it, and the midnight bell strikes 12 times.}}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. The St Mark's Clock in Venice, and the Orloj in Prague are famous examples. It was also used in Poland and Bohemia until the 17th century.The Islamic 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.WEB,weblink Saudi Aramco World : Dinner At When?, archive.aramcoworld.com,

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 the Julian 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 modern 12-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 to time zones and daylight saving time), differing according to the equation of time by as much as fifteen minutes either way. At the equinoxes sunrise is around 6 a.m. (, before noon), and sunset around 6 p.m. (, after noon).In the modern 24-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.

Derived measures

See also

Notes

{{noteslist}}

References

Citations

{{Reflist|30em}}

Bibliography

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Further reading

  • BOOK, Gerhard Dohrn-van Rossum, History of the Hour: Clocks and Modern Temporal Orders, University of Chicago Press, 978-0-226-15510-4, 1996,
  • Christopher Walker (ed.), Astronomy before the Telescope. London: British Museum Press, 1996.

External links

{{Wiktionary|hour|stound}}{{Commons|Hour}} {{Time topics}}{{Time measurement and standards}}{{SI units}}{{Authority control}}

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