View source for Hour

{{Short description|Unit of time equal to 60 minutes}}
{{other uses}}
{{EngvarB|date=September 2020}}
{{Infobox unit
| name     = hour<!-- Please do not capitalize: all unit infoboxes help to familiarize correct usage. -->
| image    = AnalogClockAnimation1 2hands 1h in 6sec.gif
| caption  = [[Midnight]] (or [[noon]]) to 1 on a 12-hour clock with an analogue face
| symbol   = h
| symbol2  = hr
| standard = [[Non-SI units mentioned in the SI|Non-SI units accepted for use with SI]]
| quantity = [[time]]
| units1   = [[SI units]]
| inunits1 = {{val|3600|ul=s}}
| units2   = Non-SI units
| inunits2 = {{val|60|ul=min}}
}}
[[File:DigitalClock 1hour.gif|thumb|[[Midnight]] to 1 a.m. on a 24-hour clock with a digital face]]

An '''hour''' ([[metric symbol|symbol]]: '''h''';<ref>{{citation |title=Resolution 7 |date=October 1948 |url=https://www.bipm.org/en/committees/cg/cgpm/9-1948/resolution-7 |work=Resolutions of the CGPM: 9th Meeting |publisher=International Bureau of Weights and Measures |location=Paris }}</ref> also abbreviated '''hr''') is a [[unit of measurement|unit]] of [[time]] historically reckoned as {{frac|24}} of a [[day]] and defined contemporarily as exactly 3,600 [[second]]s ([[SI]]). There are 60 minutes in an hour, and 24 hours in a day.

The hour was initially established in the [[ancient Near East]] as a variable measure of {{frac|12}} of the [[night]] or [[daytime]]. Such '''seasonal hours''', also known as '''temporal hours''' or [[unequal hours]], varied by [[season]] and [[latitude]].

'''Equal hours''' 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 [[ΔT (timekeeping)|long term variations]] in the Earth's rotation, the hour was finally separated from the Earth's rotation and defined in terms of the [[Second#"Atomic" second|atomic or physical second]].

It is a [[Non-SI units mentioned in the SI|non-SI unit that is accepted for use with SI]].<ref>{{cite web|url=https://web.archive.org/web/20200316121104/https://www.bipm.org/utils/common/pdf/si-brochure/SI-Brochure-9-EN.pdf|work=Bureau International de Poids et Mesures|title=Non-SI units accepted for use with the SI, and units based on fundamental constants|pages=145-146}}</ref> In the modern [[metric system]], one hour is 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 second]]s added to UTC have all been additions.}} effectively making it appear to last 3,599 or 3,601 seconds, in order to keep [[UTC]] within 0.9 seconds of [[UT1]], the latter of which is based on measurements of the [[mean solar day]].

==Etymology<span class="anchor" id="Etymology"></span><span class="anchor" id="Names"></span><span class="anchor" id="Time of Day"></span><span class="anchor" id="Time of day"></span>==
''Hour'' is a development of the [[Anglo-Norman language|Anglo-Norman]] ''{{lang|xno|houre}}'' and [[Middle English]] ''{{lang|enm|ure}}'', first attested in the 13th century.<ref name=hoed>{{harvnb|''OED''|loc=hour, ''n.''}}</ref>{{efn|From the {{c.|1250}} [[sermon]] for [[Sexagesima Sunday]]: {{lang|ang|...Þos laste on ure habbeþ i-travailed...}}<ref>{{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) |page=[https://archive.org/stream/anoldenglishmis00sullgoog#page/n55/mode/2up 34] }}</ref>}} It was a [[borrowing (linguistics)|borrowing]] of [[Old French]] ''{{lang|fro|ure}}'', a variant of ''{{lang|fro|ore}}'', which derived from [[Latin]] ''{{lang|la|hōra}}'' and [[Ancient Greek|Greek]] ''hṓrā'' ({{lang|grc|{{linktext|ὥρα}}}}) originating in [[Proto-Indo-European]] [[root (linguistics)|root]] ''{{PIE|[[wikt:Reconstruction:Proto-Indo-European/yeh₁-|*yeh₁-]]}}'' ("year, [[summer]]"), making ''hour'' distantly [[cognate]] with ''[[year]]''; the Greek word ''hṓrā'' was originally a vaguer word for any span of time, including [[season]]s and [[year]]s. The Anglo-Norman word ''hour'' displaced [[Old English|older]] native words like '''tide''' (''[[wikt:tid#Old English|tīd]]''){{sfn|''OED''|loc=tide, ''n''}} and '''stound''' (''[[wikt:stund#Old English|stund]]'', ''span of time'').{{sfn|''OED''|loc=stound, ''n.¹''}} 

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 the 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==

=== Antiquity ===

==== Ancient Egypt ====
In ancient Egypt the [[flooding of the Nile]] was, and still is, an important annual event, crucial for agriculture. It was accompanied by the rise of [[Sirius]] 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.<ref>{{Cite web |last=Andrewes |first=William J. H. |date=February 1, 2006 |title=A Chronicle Of Timekeeping |url=https://www.scientificamerican.com/article/a-chronicle-of-timekeeping-2006-02/ |access-date=2024-08-20 |website=Scientific American |language=en}}</ref> These were [[Unequal hours|seasonal 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.

==== Ancient Greece ====
{{Further|Horae}}
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".<ref>{{cite book |first=James |last=Evans |year=1998 |title=The History and Practice of Ancient Astronomy |publisher=Oxford University Press |page=95 |isbn=978-0-19-509539-5 |url=https://books.google.com/books?id=nS51_7qbEWsC&pg=PA95 |via=Google Books}}</ref> Initially, only the day was divided into 12&nbsp;seasonal hours and the night into three or four night watches.<ref>
{{cite book
 | last1 = Liddell | first1 = Henry George | author-link1 = Henry Liddell
 | last2 = Scott   | first2 = Robert       | author-link2 = Robert Scott (philologist)
 | year = 1883
 | title = A Lexicon Abridged from Liddell & Scott's Greek-English Lexicon
 | edition = 20
 | publisher = Harper & Brothers
 | orig-year = 1883
 | page = 469
 | url = https://books.google.com/books?id=LeFFAQAAMAAJ&pg=PA469
 | access-date = 12 April 2021
 | quote = [...] from Homer downwards, the Greeks divided the night into three watches.
}}
</ref>

By the [[Hellenistic period]] the night was also divided into 12 hours.<ref>{{cite book |author=Polybius |title=Histories, Book 9 |chapter=15 Mode of Calculating Time |chapter-url=https://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.01.0234%3Abook%3D9%3Achapter%3D15}}</ref> The day-and-night ({{lang|grc|νυχθήμερον}}) was probably first divided into 24 hours by [[Hipparchus of Nicaea]].<ref>{{cite book |last1=Liddell |first1=Henry George |last2=Scott |first2=Robert |last3=Jones |first3=Henry Stuart  |section=ὥρα |title=A Greek-English Lexicon |at=Α.ΙΙ.2 |section-url=https://lsj.gr/wiki/%E1%BD%A5%CF%81%CE%B1}}</ref> The Greek astronomer [[Andronicus of Cyrrhus]] oversaw the construction of a [[horologion]] called the [[Tower of the Winds]] in Athens during the first century BCE. This structure tracked a 24-hour day using both sundials and mechanical hour indicators.<ref name="nist">{{cite web |title=Early Clocks |date= 12 August 2009 |series=A Walk Through Time |publisher=[[National Institute of Standards and Technology]] |url=https://www.nist.gov/pml/time-and-frequency-division/popular-links/walk-through-time/walk-through-time-early-clocks |access-date= 13 October 2022}}</ref>

The [[canonical hours]] were inherited into [[early Christianity]]  from [[Second Temple Judaism]].
By AD 60, the ''[[Didache]]'' recommends disciples to pray the [[Lord's Prayer]] three times a day; this practice found its way into the canonical hours as well. By the second and third centuries, such [[Church Fathers]] as [[Clement of Alexandria]], [[Origen]], and [[Tertullian]] 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, a [[vigil]] was kept. The word "Vigils", at first applied to the Night Office, comes from a Latin source, namely the ''Vigiliae'' 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.<ref>{{CE1913|last=Cabrol |first=Fernand |wstitle=Matins |volume=10}}</ref>

The ''[[Horae]]'' were originally personifications of seasonal aspects of nature, not of the time of day.
The list of 12 ''Horae'' representing the 12 hours of the day is recorded only in [[Late Antiquity]], by [[Nonnus]].<ref>{{cite book |author=[[Nonnus]] |title=Dionysiaca |at=41.263}}</ref> The first and twelfth of the ''Horae'' were added to the original set of ten:

# ''Auge'' (first light)
# ''Anatole'' (sunrise)
# ''Mousike'' (morning hour of music and study)
# ''Gymnastike'' (morning hour of exercise)
# ''Nymphe'' (morning hour of ablutions)
# ''Mesembria'' (noon)
# ''Sponde'' (libations poured after lunch)
# ''Elete'' (prayer)
# ''Akte'' (eating and pleasure)
# ''Hesperis'' (start of evening)
# ''Dysis'' (sunset)
# ''Arktos'' (night sky)

===Middle Ages===
{{main|Canonical hours}}
[[Image:Bishopstone sundial.jpg|right|thumb|200px|A 7th-century Saxon [[tide dial]] on the porch at [[Bishopstone, East Sussex|Bishopstone]] in [[Sussex]], with larger crosses marking the [[canonical hours]].{{sfnp|Wall|1912|p=67}}]]

Medieval astronomers such as [[al-Biruni]]<ref>{{ cite book | author=Al-Biruni | year=1879 | orig-year=1000 | title=The Chronology of Ancient Nations | url=https://books.google.com/books?id=pFIEAAAAIAAJ&pg=PA147 | pages=147–149 | translator-last=Sachau | translator-first=C. Edward | author-link=Al-Biruni}}</ref> and [[Sacrobosco]],<ref>{{ Citation | last = Nothaft | first = C. Philipp E. | date = 2018 | title = Scandalous Error: Calendar Reform and Calendrical Astronomy in Medieval Europe | publisher = Oxford University Press | place = Oxford | page = 126 | isbn = 9780198799559}}</ref>   divided the hour into 60 [[minute]]s, each of 60 [[second]]s; this derives from [[Babylonian astronomy]], where the corresponding terms{{clarify|date=April 2020}} denoted the time required for the Sun's apparent motion through the [[ecliptic]] 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.<ref>{{cite journal |last=Correll |first=Malcolm |journal=The Physics Teacher |volume=15 |pages=476–479 |issue=8 |date=November 1977 |title=Early Time Measurements|doi=10.1119/1.2339739 |bibcode=1977PhTea..15..476C }}</ref><ref>{{cite journal|journal=Journal for the History of Astronomy|author1=F. Richard Stephenson|author-link=F. Richard Stephenson|author2=Louay J. Fatoohi|date=May 1994|doi=10.1177/002182869402500203|title=The Babylonian Unit of Time|volume=25|issue=2|pages=99–110|bibcode=1994JHA....25...99S|s2cid=117951139}}</ref>

In medieval Europe, the Roman hours continued to be marked on [[sundial]]s but the more important units of time were the [[canonical hours]] of the [[Greek Orthodox Church|Orthodox]] and [[Catholic Church]]. During daylight, these followed the pattern set by the three-hour bells of the [[forum (market)|Roman markets]], which were succeeded by the [[church bell|bells]] of local churches. They rang [[prime (liturgy)|prime]] at about 6{{nbsp}}am, [[terce]] at about 9{{nbsp}}am, [[sext]] at noon, [[nones (liturgy)|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,<ref>{{citation |author=Paul&nbsp;VI |author-link=Pope Paul VI |date=4 December 1963 |title=Constitution on<!--sic--> the Sacred Liturgy |url=https://www.vatican.va/archive/hist_councils/ii_vatican_council/documents/vat-ii_const_19631204_sacrosanctum-concilium_en.html |location=Vatican City |at=§89(d) }}</ref> though they continue to be observed in the Orthodox churches.

When mechanical [[clock]]s 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===
{{main|Decimal time|Metric system}}
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 [[French Revolutionary units|general decimalisation of measures]] was enacted, including [[decimal time]] between 1794 and 1800. Under its provisions, the French hour ({{langx|fr|{{linktext|heure}}}}) was {{frac|10}} of the day and divided formally into 100 decimal minutes (''{{lang|fr|minute décimale}}'') and informally into 10 tenths (''{{lang|fr|{{linktext|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.

The [[metric system]] bases its measurements of time upon the [[second]], defined since 1952 in terms of the Earth's rotation in AD{{nbsp}}1900. Its hours are a secondary unit computed as precisely 3,600 seconds.<ref name=sportsillustrated>{{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 }}</ref> 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 [[UT1|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 [[Earth's rotation|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 labor|wage]] or [[salaried]] labour, involve compensation based upon measured or expected hours worked. The fight for an [[eight-hour day]] was a part of [[labour movement]]s around the world. Informal [[rush hour]]s and [[happy hour]]s cover the times of day when commuting slows down due to congestion or alcoholic drinks being available at discounted prices. The [[hour record]] for the greatest distance travelled by a cyclist within the span of an hour is one of [[cycle sport|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&nbsp;a.m. to 9&nbsp;p.m. on mid-summer's day, when the Sun is at its highest declination (roughly 23.5°). Sunrise and sunset occur at 3&nbsp;a.m. and 9&nbsp;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.

[[Astrolabe]]s, [[sundial]]s, and [[astronomical clock]]s 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" divide the day and night into 24 equal hours, reckoned from the time of sunrise.<ref>{{cite web|url=http://www.math.nus.edu.sg/aslaksen/projects/sundials/dial_furniture_hours.html |title=Different Classification of Hours |publisher=Math.nus.edu.sg |access-date=2018-09-20}}</ref>  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 ====
{{Main|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 "[[Relative hour|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 ===
[[File:AstiCattedrale.jpg|thumb|Sundial with Italian hours in [[Asti]]]]

In so-called "[[Italy|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 [[daylight|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 [[opera]]s where in ''[[Rigoletto]]'' or in ''[[Un ballo in maschera]]'' midnight is announced by the bell striking six times, not 12. But in his last opera, ''[[Falstaff (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.}}<ref>"Nach langem stillen Stauen trennten wir uns, da es fernher 7 schlug, nach unserer Uhr 12 Uhr Mitternacht." ([[Carl Oesterley senior|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.&nbsp;33.</ref>

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]], [[Silesia]], and [[Bohemia]] until the 17th century.

Its replacement by the more practical division into [[12-hour clock|twice twelve]] ([[Equinoctial hours|equinoctial]]) hours (also called small clock or civic hours) began as early as the 16th 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.<ref>{{cite web|url=http://archive.aramcoworld.com/issue/196902/dinner.at.when.htm|title=Saudi Aramco World : Dinner At When?|website=archive.aramcoworld.com}}</ref>

=== 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 time|Solar noon]] is always close to 12 noon (ignoring artificial adjustments due to [[time zone]]s and [[daylight saving time]]), differing according to the [[equation of time]] by as much as fifteen minutes either way. At the [[equinox]]es sunrise is around 6&nbsp;a.m. ({{langx|la|ante meridiem}}, before noon), and sunset around 6&nbsp;p.m. ({{langx|la|post meridiem}}, 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.

==History of timekeeping in other cultures==
{{Further|History of timekeeping devices}}
{{off topic|date=April 2020}}

===Egypt===
{{Further|Egyptian calendar|Decans|History of timekeeping devices in Egypt}}

The [[ancient Egypt]]ians began dividing the night into ''{{lang|egy|wnwt}}'' at some time before the compilation of the [[Dynasty V]] [[Pyramid Texts]]<ref name=comeandlisten>{{harvp|Clagett|1995|p=[https://archive.org/details/bub_gb_8c10QYoGa4UC/page/n57 49]}}</ref> in the 24th{{nbsp}}century{{nbsp}}BC.<ref name=toastory>{{harvp|Clagett|1995|p=[https://archive.org/details/bub_gb_8c10QYoGa4UC/page/n58 50]}}</ref> By 2150{{nbsp}}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.<ref name=toastory/> [[Marshall Clagett|Clagett]] writes that it is "certain" this duodecimal division of the night followed the adoption of the [[Egyptian civil calendar]],<ref name=comeandlisten/> usually placed {{c.|2800}}{{nbsp}}BC on the basis of analyses of the [[Sothic cycle]], but a [[Egyptian lunar calendar|lunar calendar]] presumably long predated this{{sfnp|Parker|1950|pp=30-2}} and also would have had 12 [[month]]s in each of its years. The coffin diagrams show that the Egyptians took note of the [[heliacal rising]]s of 36 stars or [[Asterism (astronomy)|constellations]] (now known as "[[decans]]"), one for each of the ten-day "weeks" of their civil calendar.<ref name=amannamedjed>{{harvp|Clagett|1995|p=[https://archive.org/details/bub_gb_8c10QYoGa4UC/page/n58 50–1]}}</ref>  (12 sets of alternate "triangle decans" were used for the 5 [[Egyptian intercalary month|epagomenal days]] between years.)<ref name=poormountaineer>{{harvp|Clagett|1995|p=[https://archive.org/details/bub_gb_8c10QYoGa4UC/page/n226 218]}}</ref> Each night, the rising of eleven of these decans were noted, separating the night into 12 divisions whose middle terms would have lasted about 40{{nbsp}}[[minute]]s 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}}BC), the priests at [[Karnak]] were using [[water clock]]s 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.{{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 [[sundial]]s 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.<ref>{{Cite web|title=Why is a minute divided into 60 seconds, an hour into 60 minutes, yet there are only 24 hours in a day?|url=https://www.scientificamerican.com/article/experts-time-division-days-hours-minutes/|access-date=2022-01-24|website=Scientific American|language=en}}</ref>

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 [[Egyptian underworld|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 ''{{lang|egy|wnwt}}'' from their role protecting the dead through these gates. The Egyptian word for [[astronomer]], used as a synonym for priest, was ''{{lang|egy|wnwty}}'', "one of the ''wnwt''", as it were "one of the hours".{{efn|''{{lang|egy|Wnwty}}'' is written variously as <hiero>E34:N35-G43-X1:Z4-N14</hiero>,{{sfnp|Vygus|2015|p=400}} <hiero>E34:N35-W24*X1-D4</hiero>, <hiero>E34:N35-W24*X1-N14</hiero>,{{sfnp|Vygus|2015|p=408}} <hiero>E34:N35-W24:X1-N14-A24-A1:Z2</hiero>, <hiero>E34:N35-W24:X1-N14-N5:D4</hiero>,{{sfnp|Vygus|2015|p=409}} <hiero>E34:N35-W24:X1-Z4-A1</hiero>, <hiero>E34:N35-W24:X1-Z4-N11:N14-D6</hiero>, <hiero>E34:N35-W24:X1-Z4-N14</hiero>, <hiero>E34:N35-W24:X1-Z4-N14-A1</hiero>, <hiero>E34:N35-W24:X1-Z4-N2-A24</hiero>,{{sfnp|Vygus|2015|p=410}} <hiero>E34:N35-X1:Z4-N14:N5</hiero>,{{sfnp|Vygus|2015|p=412}} <hiero>N14</hiero>,{{sfnp|Vygus|2015|p=1235}} <hiero>N14:W24-X1:Z4</hiero>,{{sfnp|Vygus|2015|p=1239}} and <hiero>N14:X1*Z4</hiero>.{{sfnp|Vygus|2015|p=1240}}}} The earliest forms of ''{{lang|egy|wnwt}}'' include one or three stars, with the later solar hours including the [[determinative#Egyptian|determinative]] [[hieroglyph]] for "sun".<ref name=comeandlisten/>

===East Asia===
{{main|Traditional Chinese timekeeping}}
[[File:Clock Tower from Su Song's Book.JPG|thumb|right|200px|A Chinese diagram from [[Su Song]]'s AD{{nbsp}}1092 ''Xinyi Xiangfa Yao'' illustrating his clocktower at [[Kaifeng]].]]
[[File:Beijing_2006_1-14.jpg|thumb|right|200px|A reconstruction of another kind of Chinese [[Water clock|clepsydra]] in [[Beijing]]'s [[Beijing Drum Tower|Drum Tower]]]]

[[Ancient China]] divided its day into 100 "marks"{{sfnp|Stephenson|1997}}{{sfnp|Steele|2000}} {{nowrap|([[Chinese language|Chinese]]: {{lang|zh|{{linktext|刻}}}},}} {{nowrap|<small>[[Old Chinese|oc]]</small> [[linguistic reconstruction|*]]''kʰək'',{{sfnp|Baxter & al.|2014}}}} {{nowrap|<small>[[pinyin|p]]</small> ''kè'')}} running from midnight to midnight.{{sfnp|Sōma & al.|2004|p=887}} The system is said to have been used since [[prehistoric China|remote antiquity]],{{sfnp|Sōma & al.|2004|p=887}} credited to the legendary [[Yellow Emperor]],{{sfnp|Petersen|1992|p=129}} but is first attested in [[Western Han|Han]]-era [[water clock]]s{{sfnp|Petersen|1992|p=125}} and in the [[Book of Han|2nd-century history]] of that [[list of Chinese dynasties|dynasty]].{{sfnp|Sōma & al.|2004|p=889}} It was measured with sundials{{sfnp|Stephenson & al.|2002|pp=15–16}} and [[water clock]]s.{{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 language|Chinese]]: {{lang|zh|{{linktext|更}}}},}} {{nowrap|<small>[[Old Chinese|oc]]</small> [[linguistic reconstruction|*]]''kæŋ'',{{sfnp|Baxter & al.|2014}}}} {{nowrap|<small>[[pinyin|p]]</small> ''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|(<small>[[traditional characters|t]]</small> {{lang|zh|{{linktext|時}}}},}} {{nowrap|<small>[[simplified characters|s]]</small> {{lang|zh|{{linktext|时}}}},}} {{nowrap|<small>[[Old Chinese|oc]]</small> [[linguistic reconstruction|*]]''də'',{{sfnp|Baxter & al.|2014}}}} {{nowrap|<small>[[pinyin|p]]</small> ''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 (Yuan dynasty)|History of Song]], compiled during the early [[Yuan Dynasty|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 dynasty|Tang]]-era [[Book of Sui]].{{sfnp|Sōma & al.|2004|p=889}} Variations of all these units were subsequently adopted by [[ancient Japan|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 dynasty|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 12 ''shi'' has been credited to the example set by the rough number of lunar cycles in a solar year,<ref>{{Cite book|chapter-url=https://digitalis.uc.pt/pt-pt/livro/timeless_legacy_calendars_ancient_egypt|chapter=A Timeless Legacy: the Calendars of Ancient Egypt|last=Canhão|first=Telo Ferreira|date=2013|publisher=Edições Afrontamento|isbn=9789892609669|location=Porto|doi=10.14195/978-989-26-0966-9_20|title=Alexandrea ad Aegyptvm: The legacy of multiculturalismo in antiquity|pages=283–301}}</ref> although the 12-year [[Jupiter|Jovian]] orbital cycle was more important to [[Chinese zodiac|traditional Chinese]]<ref>{{citation |last=Zai |first=J. |title=Taoism and Science |publisher=Ultravisum |date=2015 }}</ref> and Babylonian reckoning of the zodiac.{{sfnp|Rogers|1998|pp=9–28}}{{efn|The late classical Indians also began to reckon [[Samvatsara|years based on the Jovian cycle]], but this was much later than their lunar calendar and initially named after it.{{sfnp|Sewell|1924|p=[https://books.google.com/books?id=LQffS160KxEC&pg=PR12 xii]}}}}

===Southeast Asia===
{{main|Thai six-hour clock}}

In [[Thailand]], [[Laos]], and [[Cambodia]], the traditional system of noting hours is the [[Thai six-hour clock|six-hour clock]]. This reckons each of a day's 24 hours apart from [[noon]] 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 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 Indochina|French occupation]] and is uncommon there now. The Thai system remains in informal use in the form codified in 1901 by [[King Chulalongkorn]].<ref>{{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 |page=206 |access-date=8 March 2017 |archive-url=https://web.archive.org/web/20120609235147/http://www.ratchakitcha.soc.go.th/DATA/PDF/2443/018/206.PDF |archive-date=9 June 2012 |url-status=dead }}. {{in lang|th}}</ref>

===India===
{{main|Hindu units of time}}
[[File:Drei Horen.jpg|thumb|200px|right|Two of the [[Horae|deified Hours]] of the Greeks and Romans]]

The [[Vedas]] and [[Puranas]] employed [[Hindu units of time|units of time]] based on the [[sidereal day]] (''nakṣatra ahorātra''). This was variously divided into 30 ''muhūrta-s'' of 48 minutes each<ref>{{Cite book|last=Srinivasan|first=Saradha|url=https://archive.org/details/Mensuration_in_Ancient_India|title=Mensuration in Ancient India|publisher=Ajanta Publications|year=1979|location=Delhi, India|pages=119–122}}</ref> or 60 ''dandas''{{citation needed|date=March 2017}} or ''nadī-s'' of 24 minutes each.<ref name=dersh>{{harvp|Dershowitz & al.|2008|p=[https://books.google.com/books?id=DPbx0-qgXu0C&pg=PA207 207]}}</ref> The [[solar day]] was later similarly divided into 60 ''ghaṭikás'' of about the same duration, each divided in turn into 60 ''vinadis''.<ref name=dersh/>  The [[Sinhalese people|Sinhalese]] followed a similar system but called their sixtieth of a day a ''peya''.

== Derived measures==
* [[air changes per hour]] (ACH), a measure of the replacements of air within a defined space used for [[indoor air quality]]
* [[ampere hour]] (Ah), a measure of [[electrical charge]] used in [[electrochemistry]]
* [[BTU]]-hour, a measure of power used in the [[power industry]] and for air conditioners and heaters
* [[credit hour]], a measure of an academic [[course (education)|course]]'s contracted instructional time per week for a semester
* [[horsepower-hour]] (hph), a measure of energy used in the railroad industry
* [[hour angle]], a measure of the angle between the [[meridian plane]] and the [[hour circle]] passing through a certain point used in the [[equatorial coordinate system]]
* [[kilometres per hour]] (km/h), a measure of land speed
* [[kilowatt-hour]] (kWh), a measure of energy commonly used as an electrical billing unit
* [[knot (unit)|knot]] (kn), a measure of [[nautical mile]]s per hour, used for maritime and aerial speed
* [[man-hour]], the amount of work performed by the average worker in one hour, used in productivity analysis
* [[metre per hour]] (m/h), a measure of slow speeds
* [[mile per hour]] (mph), a measure of land speed
* [[passengers per hour per direction]] (p/h/d), a measure of the capacity of [[public transport]]ation systems
* [[pound per hour]] (PPH), a measure of [[mass flow rate]] used for engines' fuel flow
* [[working hours|work or working hour]], a measure of working time used in various regulations, such as those distinguishing [[Part-time job|part-]] and [[Full-time job|full-time]] employment and those limiting [[truck driver]]s' [[Drivers' working hours|working hours]] or [[hours of service]]

==See also==
* [[Danna (Mesopotamian)|Danna]]
* [[Decimal time|Decimal hour]] or deciday, a French Revolutionary unit lasting 2{{nbsp}}h 24{{nbsp}}min
* [[Equinoctial hours]]
* [[Golden Hour (photography)|Golden Hour]] & [[Blue hour|Blue Hour]] in photography
* [[Hexadecimal time|Hexadecimal hour]], a proposed unit lasting 1{{nbsp}}h 30{{nbsp}}min
* [[Horae]], the deified hours of ancient Greece and Rome
* [[Horology]]
* [[Julian day]]
* [[Liturgy of the Hours]]
* [[Metric time]]
* [[Six-hour day]]
* [[Temporal hours]]

== Explanatory notes ==
{{notelist}}

== Citations ==
{{Reflist|25em}}

== General and cited references ==
{{refbegin |colwidth=25em |small=yes}} 
* {{cite book |title=The Oxford English Dictionary |title-link=Oxford English Dictionary |location=Oxford, UK |publisher=Oxford University Press |ref={{harvid|''OED''}} }} 
* {{cite book |last=Baxter |first=William H. |author2=Laurent Sagart |year=2014 |title=Old Chinese: A New Reconstruction |location=Oxford, UK |publisher=Oxford University Press  |ref={{harvid|Baxter & al.|2014}} }} {{cite book |section=parts available here |title=The Baxter-Sagart reconstruction of Old Chinese |id=version&nbsp;1.1 |date=20 September 2014 |publisher=University of Michigan |section-url=http://ocbaxtersagart.lsait.lsa.umich.edu }}
* {{cite book |last=Clagett |first=Marshall |author-link=Marshall Clagett |year=1995 |title=Ancient Egyptian Science |section=Volume&nbsp;II: Calendars, clocks, and astronomy |publisher=American Philosophical Society |location=Philadelphia, PA |series=Memoirs of the APS |volume=214 |isbn=9780871692146 |url=https://books.google.com/books?id=xKKPUpDOTKAC }} 
* {{cite book |last=Dershowitz |first=Nachum |author2=Edward M. Reingold |year=2008 |title=Calendrical Calculations |title-link=Calendrical Calculations |publisher=Cambridge University Press |location=[[Cambridge, UK]] |isbn=9780521885409 |ref={{harvid|Dershowitz & al.|2008}} }} 
* {{cite book |last=Holford-Strevens |first=Leofranc |year=2005 |title=The History of Time: A Very Short Introduction |publisher=Oxford University Press |location=Oxford, UK |series=Very Short Introductions |volume=133 |isbn=9780192804990 |url-access=registration |url=https://archive.org/details/historyoftime00holf }} 
* {{cite book |last=Landes |first=David S. |year=1983 |title=Revolution in Time |location=[[Cambridge, MA]] |publisher=Harvard University Press}} 
* {{cite book |last=Parker |first=Richard Anthony |author-link=Richard Anthony Parker |year=1950 |title=The Calendars of Ancient Egypt |series=Studies in Ancient Oriental Civilization |volume=26 |location=Chicago, IL |publisher=University of Chicago Press |url=https://oi.uchicago.edu/sites/oi.uchicago.edu/files/uploads/shared/docs/saoc26.pdf }} 
* {{cite journal |last=Petersen |first=Jens Østergård |year=1992 |title=The Taiping Jing and the A.D.&nbsp;102 clepsydra reform |journal=Acta Orientalia |volume=53 |pages=122–158 |location=Copenhagen, DK |url=https://www.academia.edu/12945035 }} 
* {{cite journal |last=Rogers |first=J.H. |year=1998 |title=Origins of the ancient constellations |journal=Journal of the British Astronomical Association |volume=108 |location=London, UK }}
* {{cite book |last=Sewell |first=Robert |title=The Siddhantas and the Indian Calendar |location=Kolkata, IN |publisher=Government of India Central Publication Branch |url=https://books.google.com/books?id=LQffS160KxEC |year=1924 |isbn=9788120603646 }} 
* {{cite journal |author1=Sōma, Mitsuru |author2=Kinaki Kawabata |author3=Kiyotaka Tanikawa |date=25 October 2004 |title=Units of Time in Ancient China and Japan |journal=Publications of the Astronomical Society of Japan |volume=56 |issue=5 |pages=887–904 |location=Tokyo, JP |doi=10.1093/pasj/56.5.887  |url=https://academic.oup.com/pasj/article/56/5/887/2948928/Units-of-Time-in-Ancient-China-and-Japan |ref={{harvid|Sōma & al.|2004}} |url-access=subscription }}
* {{cite book |last=Steele |first=J.M. |year=2000 |title=Observations and Predications of Eclipse Times by Early Astronomers |location=Amsterdam, NL |publisher=Kluwer Academic Publications }} 
* {{cite book |last=Stephenson |first=F. Richard |author-link=F. Richard Stephenson |year=1997 |title=Historical Eclipses and Earth's Rotation |location=Cambridge, UK |publisher=Cambridge University Press }} 
* {{cite book |last1=Stephenson |first1=F. Richard |author1-link=F. Richard Stephenson |last2=Green |first2=David A. |year=2002 |title=Historical Supernovae and their Remnants |location=Oxford, UK |publisher=Oxford University Press |isbn=978-0-19-850766-6 |ref={{harvid|Stephenson & al.|2002}} }} 
* {{cite book |last=Thongprasert |first=Chamnong |year=1985 |section=ทุ่ม-โมง-นาฬิกา [Thum-Mong-Nalika] |title=ภาษาไทยไขขาน |language=th |trans-title=Thai Unlocked |publisher=Prae Pitaya Press |place=Bangkok, TH |section-url=http://www.royin.go.th/th/knowledge/detail.php?ID=1273}}
* {{citation |last=Vygus |first=Mark |date=2015 |title=Middle Egyptian Dictionary |url=http://www.pyramidtextsonline.com/documents/VygusDictionaryApril2015.pdf }}.
* {{cite book |last=Wall |first=J. Charles |year=1912 |title=Porches & Fonts |location=London, UK |publisher=Wells, Gardner, Darton & Co. }}
* {{cite book |last=Wilkinson |first=Richard H. |year=2003 |title=The Complete Gods and Goddesses of Ancient Egypt |location=Singapore |publisher=Tien Wah Press / Thames & Hudson }}
{{refend}}

==Further reading==
* {{cite book|author=Gerhard Dohrn-van Rossum|title=History of the Hour: Clocks and Modern Temporal Orders|publisher=[[University of Chicago Press]]|isbn=978-0-226-15510-4|year=1996}}
* Christopher Walker (ed.), ''Astronomy before the Telescope''. London: [[British Museum|British Museum Press]], 1996.

== External links ==
{{Wiktionary|hour|stound}}
{{Commons}}
* [http://www.worldtimezone.net/index24.php World time zones]
* [http://www.centralx.com/time/index.en.html Accurate time vs. PC Clock Difference]

{{Time topics}}
{{Time measurement and standards}}
{{SI units}}
{{Authority control}}

[[Category:Orders of magnitude (time)]]
[[Category:Units of time]]