Midnight to 1 a.m. on a 24-hour clock with a digital face
An hour (symbol: h;[1] also abbreviated hr) is a unit of time historically reckoned as 1⁄24 of a day and defined contemporarily as exactly 3,600 seconds (SI). There are 60 minutes in an hour, and 24 hours in a day.
Equal hours or equinoctial hours were taken as 1⁄24 of the day as measured from noon to noon; the minor seasonal variations of this unit were eventually smoothed by making it 1⁄24 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,[a] 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.
It displaced tidetīd, 'time'[4] and stoundstund, span of time.[5] The Anglo-Norman term was a borrowing of Old Frenchure, a variant of ore, which derived from Latinhōra and Greekhṓrā (ὥρα).
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.[6] (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".[7] (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"[8] 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.[8] (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.[9] These were 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.
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".[10] Initially, only the day was divided into 12seasonal hours and the night into three or four night watches.[11]
By the Hellenistic period the night was also divided into 12 hours.[12] The day-and-night (νυχθήμερον) was probably first divided into 24 hours by Hipparchus of Nicaea.[13] 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.[14]
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.[15]
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.[16] The first and twelfth of the Horae were added to the original set of ten:
Medieval astronomers such as al-Biruni[18] and Sacrobosco,[19] divided the hour into 60 minutes, each of 60 seconds; this derives from Babylonian astronomy, where the corresponding terms[clarification needed] 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.[20][21]
In medieval Europe, 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 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,[22] 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.
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 decimalisation of measures was enacted, including decimal time between 1794 and 1800. Under its provisions, the French hour (French: heure) was 1⁄10 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.
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.[23] 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, involve 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 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 cycling'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 is 5⁄12 the outer radius of the dial. This animation depicts the motion of the shadow from 3a.m. to 9p.m. on mid-summer's day, when the Sun is at its highest declination (roughly 23.5°). Sunrise and sunset occur at 3a.m. and 9p.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.[24]
"Babylonian hours" divide the day and night into 24 equal hours, reckoned from the time of sunrise.[25] 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.[26]
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.[24]
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.
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.[c][27]
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.[28]
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 6a.m. (Latin: ante meridiem, before noon), and sunset around 6p.m. (Latin: 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.
The ancient Egyptians began dividing the night into wnwt at some time before the compilation of the Dynasty VPyramid Texts[29] in the 24thcenturyBC.[30] By 2150BC (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.[30]Clagett writes that it is "certain" this duodecimal division of the night followed the adoption of the Egyptian civil calendar,[29] usually placed c.2800BC on the basis of analyses of the Sothic cycle, but a lunar calendar presumably long predated this[31] and also would have had 12 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.[32] (12 sets of alternate "triangle decans" were used for the 5 epagomenal days between years.)[33] 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 of Amenhotep III (c.1350BC), 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 12 gauges, one for each month of the year.[34] 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.[35]
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.[36] Protective deities were assigned to each and were used as the names of the hours.[36] As the protectors and resurrectors of the sun, the goddesses of the night hours were considered to hold power over all lifespans[36] 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".[d] The earliest forms of wnwt include one or three stars, with the later solar hours including the determinativehieroglyph for "sun".[29]
A Chinese diagram from Su Song's AD1092 Xinyi Xiangfa Yao illustrating his clocktower at Kaifeng.A reconstruction of another kind of Chinese clepsydra in Beijing's Drum Tower
Imperial China continued to use ke and geng but also began to divide the day into 12 "double hours" ( t時,s时,oc*də,[47]pshí,lit. "time[s]") named after the earthly branches and sometimes also known by the name of the corresponding animal of the Chinese zodiac.[54] 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.[55] 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.[51] Variations of all these units were subsequently adopted by Japan[53] and the other countries of the Sinosphere.
The 12 shi supposedly began to be divided into 24 hours under the Tang,[53] although they are first attested in the Ming-era Book of Yuan.[48] 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.[48] 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,[56] although the 12-year Jovian orbital cycle was more important to traditional Chinese[57] and Babylonian reckoning of the zodiac.[58][f]
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.[60] 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.[61]
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[62] or 60 dandas[citation needed] or nadī-s of 24 minutes each.[63] The solar day was later similarly divided into 60 ghaṭikás of about the same duration, each divided in turn into 60 vinadis.[63] The Sinhalese followed a similar system but called their sixtieth of a day a peya.
↑ 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 six 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.
↑ 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".
↑ 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.[59]
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 the public 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.
↑ "ประกาศใช้ทุ่มโมงยาม", Royal Gazette, No. 17(PDF), 29 July 1901, p.206, archived from the original(PDF) on 9 June 2012, retrieved 8 March 2017. (in Thai)
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. version1.1.
A day is the time period of a full rotation of the Earth with respect to the Sun. On average, this is 24 hours. As a day passes at a given location it experiences morning, noon, afternoon, evening, and night. This daily cycle drives circadian rhythms in many organisms, which are vital to many life processes.
The second is a unit of time, historically defined as 1⁄86400 of a day – this factor derived from the division of the day first into 24 hours, then to 60 minutes and finally to 60 seconds each.
The Julian day is a continuous count of days from the beginning of the Julian period; it is used primarily by astronomers, and in software for easily calculating elapsed days between two events.
Sopdet is the ancient Egyptian name of the star Sirius and its personification as an Egyptian goddess. Known to the Greeks as Sothis, she was conflated with Isis as a goddess and Anubis as a god.
The Season of the Inundation or Flood was the first season of the lunar and civil Egyptian calendars. It fell after the intercalary month of Days over the Year and before the Season of the Emergence. In the Coptic and Egyptian calendars this season begins at the start of the month of Thout, continues through the months of Paopi and Hathor, before concluding at the end of Koiak.
The Season of the Emergence was the second season of the lunar and civil Egyptian calendars. It fell after the Season of the Inundation and before the Season of the Harvest. In the Coptic and Egyptian calendars this season begins at the start of the month of Tobi, continues through the months of Meshir and Paremhat, before concluding at the end of Parmouti.
The Season of the Harvest or Low Water was the third and final season of the lunar and civil Egyptian calendars. It fell after the Season of the Emergence and before the spiritually dangerous intercalary month, after which the New Year's festivities began the Season of the Inundation (Ꜣḫt). In the Coptic and Egyptian calendars this season begins at the start of the month of Pashons, continues through the months of Paoni and Epip, before concluding at the end of Mesori.
The ancient Egyptian calendar – a civil calendar – was a solar calendar with a 365-day year. The year consisted of three seasons of 120 days each, plus an intercalary month of five epagomenal days treated as outside of the year proper. Each season was divided into four months of 30 days. These twelve months were initially numbered within each season but came to also be known by the names of their principal festivals. Each month was divided into three 10-day periods known as decans or decades. It has been suggested that during the Nineteenth Dynasty and the Twentieth Dynasty the last two days of each decan were usually treated as a kind of weekend for the royal craftsmen, with royal artisans free from work.
Noon is 12 o'clock in the daytime. It is written as 12 noon, 12:00 m., 12 p.m., 12 pm, or 12:00 or 1200 . Solar noon is the time when the Sun appears to contact the local celestial meridian. This is when the Sun reaches its apparent highest point in the sky, at 12 noon apparent solar time and can be observed using a sundial. The local or clock time of solar noon depends on the date, longitude, and time zone, with Daylight Saving Time tending to place solar noon closer to 1:00pm.
Midnight is the transition time from one day to the next – the moment when the date changes, on the local official clock time for any particular jurisdiction. By clock time, midnight is the opposite of noon, differing from it by 12 hours.
In modern usage, civil time refers to statutory time as designated by civilian authorities. Modern civil time is generally national standard time in a time zone at a fixed offset from Coordinated Universal Time (UTC), possibly adjusted by daylight saving time during part of the year. UTC is calculated by reference to atomic clocks and was adopted in 1972. Older systems use telescope observations.
The ancient Egyptian units of measurement are those used by the dynasties of ancient Egypt prior to its incorporation in the Roman Empire and general adoption of Roman, Greek, and Byzantine units of measurement. The units of length seem to have originally been anthropic, based on various parts of the human body, although these were standardized using cubit rods, strands of rope, and official measures maintained at some temples.
The units of measurement of ancient Rome were generally consistent and well documented.
Decimal time is the representation of the time of day using units which are decimally related. This term is often used specifically to refer to the French Republican calendar time system used in France from 1794 to 1800, during the French Revolution, which divided the day into 10 decimal hours, each decimal hour into 100 decimal minutes and each decimal minute into 100 decimal seconds, as opposed to the more familiar standard time, which divides the day into 24 hours, each hour into 60 minutes and each minute into 60 seconds.
The decans are 36 groups of stars used in the ancient Egyptian astronomy to conveniently divide the 360 degree ecliptic into 36 parts of 10 degrees each, both for theurgical and heliacal horological purposes. The decans each appeared, geocentrically, to rise consecutively on the horizon throughout each daily Earth rotation. The rising of each decan marked the beginning of a new decanal "hour" of the night for the ancient Egyptians, and they were used as a sidereal star clock beginning by at least the 9th or 10th Dynasty.
Traditional Chinese timekeeping refers to the time standards for divisions of the day used in China until the introduction of the Shixian calendar in 1628 at the beginning of the Qing dynasty.
Clocks and watches with a 24-hour analog dial have an hour hand that makes one complete revolution, 360°, in a day. The more familiar 12-hour analog dial has an hour hand that makes two complete revolutions in a day.
Choghadiya refers to an auspicious period of ninety-eight minutes in Indian astrology.
In Roman timekeeping, a day was divided into periods according to the available technology. Initially, the day was divided into two parts: the ante meridiem and the post meridiem. With the introduction of the Greek sundial to Rome from the Samnites circa 293 BC, the period of the natural day from sunrise to sunset was divided into twelve hours.
Relative hour, sometimes called halachic hour, temporal hour, seasonal hour and variable hour, is a term used in rabbinic Jewish law that assigns 12 hours to each day and 12 hours to each night, all throughout the year. A relative hour has no fixed length in absolute time, but changes with the length of daylight each day - depending on summer, and in winter. Even so, in all seasons a day is always divided into 12 hours, and a night is always divided into 12 hours, which invariably makes for a longer hour or a shorter hour. At Mediterranean latitude, one hour can be about 45 minutes at the winter solstice, and 75 minutes at summer solstice. All of the hours mentioned by the Sages in either the Mishnah or Talmud, or in other rabbinic writings, refer strictly to relative hours.
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