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An hour (metric symbol: h; also abbreviated hr) is a unit of time historically reckoned as of a day and defined contemporarily as exactly 3,600 (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 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 of the day as measured from noon to noon; the minor seasonal variations of this unit were eventually smoothed by making it 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.
It is a non-SI unit that is accepted for use with SI. 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, 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.
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. (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". (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" 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. (9:45 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 |
By the Hellenistic period the night was also divided into 12 hours. The day-and-night (νυχθήμερον) was probably first divided into 24 hours by Hipparchus of Nicaea. 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.
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.
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. The first and twelfth of the Horae were added to the original set of ten:
In medieval Europe, the Roman hours continued to be marked on 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 church bell of local churches. They rang prime at about 6am, terce at about 9am, sext at noon, nones at about 3pm, and vespers at either 6pm 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, though they continue to be observed in the Orthodox churches.
When mechanical 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.
During the French Revolution, a general decimalisation of measures was enacted, including decimal time between 1794 and 1800. Under its provisions, the French hour () was of the day and divided formally into 100 decimal minutes ( minute décimale) and informally into 10 tenths ( ). 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 AD1900. Its hours are a secondary unit computed as precisely 3,600 seconds. 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 labor or salaried labour, involve compensation based upon measured or expected hours worked. The fight for an eight-hour day was a part of around the world. Informal and 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's greatest honours.
, , and astronomical clocks sometimes show the hour length and count using some of these older definitions and counting methods.
"Babylonian hours" divide the day and night into 24 equal hours, reckoned from the time of sunrise. 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.
This is also the system used in Jewish law and frequently called "Relative 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.
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. 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."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.
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 twice twelve (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.
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.
The later division of the day into 12 hours was accomplished by 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.
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. Protective deities were assigned to each and were used as the names of the hours. As the protectors and resurrectors of the sun, the goddesses of the night hours were considered to hold power over all lifespans 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 wnwt from their role protecting the dead through these gates. The Egyptian word for astronomer, used as a synonym for priest, was wnwty, "one of the wnwt", as it were "one of the hours". The earliest forms of wnwt include one or three stars, with the later solar hours including the determinative hieroglyph for "sun".
Imperial China continued to use ke and geng but also began to divide the day into 12 "double hours" named after the earthly branches and sometimes also known by the name of the corresponding animal of the Chinese zodiac. The first shi originally ran from 11pm to 1am but was reckoned as starting at midnight by the time of the History of Song, compiled during the early Yuan Dynasty. 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-era Book of Sui. Variations of all these units were subsequently adopted by ancient Japan and the other countries of the Sinosphere.
The 12 shi supposedly began to be divided into 24 hours under the Tang dynasty, although they are first attested in the Ming-era Book of Yuan. 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. 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, although the 12-year Jupiter orbital cycle was more important to Chinese zodiac and Babylonian reckoning of the zodiac.
East Asia
Ancient China divided its day into 100 "marks" running from midnight to midnight. The system is said to have been used since remote antiquity, credited to the legendary Yellow Emperor, but is first attested in Western Han-era and in the 2nd-century history of that dynasty. It was measured with sundials and . 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. During the night, time was more commonly reckoned during the night by the "watches" of the guard, which were reckoned as a fifth of the time from sunset to sunrise.
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. 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. It was abolished in Laos and Cambodia during their French Indochina and is uncommon there now. The Thai system remains in informal use in the form codified in 1901 by King Chulalongkorn..
India
The Vedas and Puranas employed units of time based on the sidereal day ( nakṣatra ahorātra). This was variously divided into 30 muhūrta-s of 48 minutes each or 60 dandas or nadī-s of 24 minutes each. The solar day was later similarly divided into 60 ghaṭikás of about the same duration, each divided in turn into 60 vinadis. The Sinhalese people followed a similar system but called their sixtieth of a day a peya.
Derived measures
See also
Explanatory notes
Citations
General and cited references
Robert Sewell (2025). 9788120603646, Government of India Central Publication Branch. . ISBN 9788120603646
(2025). 9780198507666, Oxford University Press. ISBN 9780198507666
Further reading
External links
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