Babylonian Hour
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An hour (
symbol A symbol is a mark, Sign (semiotics), sign, or word that indicates, signifies, or is understood as representing an idea, physical object, object, or wikt:relationship, relationship. Symbols allow people to go beyond what is known or seen by cr ...
: h; also abbreviated hr) is a
unit Unit may refer to: General measurement * Unit of measurement, a definite magnitude of a physical quantity, defined and adopted by convention or by law **International System of Units (SI), modern form of the metric system **English units, histo ...
of
time Time is the continuous progression of existence that occurs in an apparently irreversible process, irreversible succession from the past, through the present, and into the future. It is a component quantity of various measurements used to sequ ...
historically reckoned as of a
day A day is the time rotation period, period of a full Earth's rotation, rotation of the Earth with respect to the Sun. On average, this is 24 hours (86,400 seconds). As a day passes at a given location it experiences morning, afternoon, evening, ...
and defined contemporarily as exactly 3,600
second The second (symbol: s) is a unit of time derived from the division of the day first into 24 hours, then to 60 minutes, and finally to 60 seconds each (24 × 60 × 60 = 86400). The current and formal definition in the International System of U ...
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 The ancient Near East was home to many cradles of civilization, spanning Mesopotamia, Egypt, Iran (or Persia), Anatolia and the Armenian highlands, the Levant, and the Arabian Peninsula. As such, the fields of ancient Near East studies and Nea ...
as a variable measure of of the
night Night, or nighttime, is the period of darkness when the Sun is below the horizon. Sunlight illuminates one side of the Earth, leaving the other in darkness. The opposite of nighttime is daytime. Earth's rotation causes the appearance of ...
or
daytime Daytime or day as observed on Earth is the period of the day during which a given location experiences Daylight, natural illumination from direct sunlight. Daytime occurs when the Sun appears above the local horizon, that is, anywhere on the ...
. Such seasonal hours, also known as temporal hours or
unequal hours Unequal hours are the division of the daytime and the nighttime into 12 sections each, whatever the season. They are also called temporal hours, seasonal hours, biblical or Jewish hours, as well as ancient or Roman hours (). They are ''unequal du ...
, varied by
season A season is a division of the year based on changes in weather, ecology, and the number of daylight hours in a given region. On Earth, seasons are the result of the axial parallelism of Earth's axial tilt, tilted orbit around the Sun. In temperat ...
and
latitude In geography, latitude is a geographic coordinate system, geographic coordinate that specifies the north-south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from −90° at t ...
. 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 Solar time is a calculation of the passage of time based on the position of the Sun in the sky. The fundamental unit of solar time is the day, based on the synodic rotation period. Traditionally, there are three types of time reckoning based ...
. 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 The metric system is a system of measurement that standardization, standardizes a set of base units and a nomenclature for describing relatively large and small quantities via decimal-based multiplicative unit prefixes. Though the rules gover ...
, one hour is defined as 3,600 atomic seconds. However, on rare occasions an hour may incorporate a positive or negative
leap second A leap second is a one-second adjustment that is occasionally applied to Coordinated Universal Time (UTC), to accommodate the difference between precise time (International Atomic Time (TAI), as measured by atomic clocks) and imprecise solar tim ...
, effectively making it appear to last 3,599 or 3,601 seconds, in order to keep
UTC Coordinated Universal Time (UTC) is the primary time standard globally used to regulate clocks and time. It establishes a reference for the current time, forming the basis for civil time and time zones. UTC facilitates international communica ...
within 0.9 seconds of UT1, the latter of which is based on measurements of the
mean solar day Solar time is a calculation of the passage of time based on the position of the Sun in the sky. The fundamental unit of solar time is the day, based on the synodic rotation period. Traditionally, there are three types of time reckoning based ...
.


Etymology

''Hour'' is a development of the Anglo-Norman ' and
Middle English Middle English (abbreviated to ME) is a form of the English language that was spoken after the Norman Conquest of 1066, until the late 15th century. The English language underwent distinct variations and developments following the Old English pe ...
', first attested in the 13th century. It was a borrowing of
Old French Old French (, , ; ) was the language spoken in most of the northern half of France approximately between the late 8th -4; we might wonder whether there's a point at which it's appropriate to talk of the beginnings of French, that is, when it wa ...
', a variant of ', which derived from Latin ' and Ancient Greek">Greek Greek may refer to: Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group *Greek language, a branch of the Indo-European language family **Proto-Greek language, the assumed last common ancestor of all kno ...
''hṓrā'' () originating in Proto-Indo-European root (linguistics), root ' ("year, summer"), making ''hour'' distantly cognate with ''
year A year is a unit of time based on how long it takes the Earth to orbit the Sun. In scientific use, the tropical year (approximately 365 Synodic day, solar days, 5 hours, 48 minutes, 45 seconds) and the sidereal year (about 20 minutes longer) ...
''; the Greek word ''hṓrā'' was originally a vaguer word for any span of time, including
season A season is a division of the year based on changes in weather, ecology, and the number of daylight hours in a given region. On Earth, seasons are the result of the axial parallelism of Earth's axial tilt, tilted orbit around the Sun. In temperat ...
s and
year A year is a unit of time based on how long it takes the Earth to orbit the Sun. In scientific use, the tropical year (approximately 365 Synodic day, solar days, 5 hours, 48 minutes, 45 seconds) and the sidereal year (about 20 minutes longer) ...
s. The Anglo-Norman word ''hour'' displaced older native words like tide ('' tīd'') and stound ('' stund'', ''span of time''). The time of day is typically expressed in English in terms of hours. Whole hours on a
12-hour clock The 12-hour clock is a time convention in which the 24 hours of the day are divided into two periods: a.m. (from Latin , translating to "before midday") and p.m. (from Latin , translating to "after midday"). Each period consists of 12&nb ...
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 The modern 24-hour clock is the convention of timekeeping in which the day runs from midnight to midnight and is divided into 24 hours. This is indicated by the hours (and minutes) passed since midnight, from to , with as an option to indicate ...
("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".)


History


Antiquity


Ancient Egypt

In ancient Egypt the
flooding of the Nile The flooding of the Nile (commonly referred to as ''the Inundation'') and its silt Deposition (geology), deposition was a natural cycle first attested in Ancient Egypt. It was of singular importance in the history and culture of Egypt. Governments ...
was, and still is, an important annual event, crucial for agriculture. It was accompanied by the rise of
Sirius Sirius is the brightest star in the night sky. Its name is derived from the Greek word (Latin script: ), meaning 'glowing' or 'scorching'. The star is designated  Canis Majoris, Latinized to Alpha Canis Majoris, and abbr ...
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. 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.


Ancient Greece

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". Initially, only the day was divided into 12 seasonal hours and the night into three or four night watches. By the
Hellenistic period In classical antiquity, the Hellenistic period covers the time in Greek history after Classical Greece, between the death of Alexander the Great in 323 BC and the death of Cleopatra VII in 30 BC, which was followed by the ascendancy of the R ...
the night was also divided into 12 hours. The day-and-night () was probably first divided into 24 hours by
Hipparchus of Nicaea Hipparchus (; , ;  BC) was a Greek astronomer, geographer, and mathematician. He is considered the founder of trigonometry, but is most famous for his incidental discovery of the precession of the equinoxes. Hipparchus was born in Nicaea, ...
. The Greek astronomer Andronicus of Cyrrhus oversaw the construction of a horologion called the
Tower of the Winds The Tower of the Winds, known as the in Greek, and by #Names, other names, is an octagonal Pentelic marble tower in the Roman Agora in Athens, named after the eight large reliefs of wind gods around its top. Its date is uncertain, but was compl ...
in Athens during the first century BCE. This structure tracked a 24-hour day using both sundials and mechanical hour indicators. The
canonical hours In the practice of Christianity, canonical hours mark the divisions of the day in terms of Fixed prayer times#Christianity, fixed times of prayer at regular intervals. A book of hours, chiefly a breviary, normally contains a version of, or sel ...
were inherited into
early Christianity Early Christianity, otherwise called the Early Church or Paleo-Christianity, describes the History of Christianity, historical era of the Christianity, Christian religion up to the First Council of Nicaea in 325. Spread of Christianity, Christian ...
from
Second Temple Judaism Second Temple Judaism is the Judaism, Jewish religion as it developed during the Second Temple period, which began with the construction of the Second Temple around 516 BCE and ended with the Siege of Jerusalem (70 CE), destruction of Jerusalem in ...
. By AD 60, the ''
Didache The ''Didache'' (; ), also known as ''The Lord's Teaching Through the Twelve Apostles to the Nations'' (), is a brief anonymous early Christian treatise ( ancient church order) written in Koine Greek, dated by modern scholars to the first or (l ...
'' recommends disciples to pray the
Lord's Prayer The Lord's Prayer, also known by its incipit Our Father (, ), is a central Christian prayer attributed to Jesus. It contains petitions to God focused on God’s holiness, will, and kingdom, as well as human needs, with variations across manusc ...
three times a day; this practice found its way into the canonical hours as well. By the second and third centuries, such
Church Fathers The Church Fathers, Early Church Fathers, Christian Fathers, or Fathers of the Church were ancient and influential Christian theologians and writers who established the intellectual and doctrinal foundations of Christianity. The historical peri ...
as
Clement of Alexandria Titus Flavius Clemens, also known as Clement of Alexandria (; – ), was a Christian theology, Christian theologian and philosopher who taught at the Catechetical School of Alexandria. Among his pupils were Origen and Alexander of Jerusalem. A ...
,
Origen Origen of Alexandria (), also known as Origen Adamantius, was an Early Christianity, early Christian scholar, Asceticism#Christianity, ascetic, and Christian theology, theologian who was born and spent the first half of his career in Early cent ...
, and
Tertullian Tertullian (; ; 155 – 220 AD) was a prolific Early Christianity, early Christian author from Roman Carthage, Carthage in the Africa (Roman province), Roman province of Africa. He was the first Christian author to produce an extensive co ...
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 A vigil, from the Latin meaning 'wakefulness' ( Greek: , or ), is a period of purposeful sleeplessness, an occasion for devotional watching, or an observance. The Italian word has become generalized in this sense and means 'eve' (as in "on t ...
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 In Greek mythology, the Horae (), Horai () or Hours (, ) were the goddesses of the seasons and the natural portions of time. Etymology The term ''hora'' comes from the Proto-Indo-European ("year"). Function The Horae were originally the ...
'' 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 Late antiquity marks the period that comes after the end of classical antiquity and stretches into the onset of the Early Middle Ages. Late antiquity as a period was popularized by Peter Brown (historian), Peter Brown in 1971, and this periodiza ...
, by
Nonnus Nonnus of Panopolis (, ''Nónnos ho Panopolítēs'', 5th century AD) was the most notable Greek epic poet of the Imperial Roman era. He was a native of Panopolis (Akhmim) in the Egyptian Thebaid and probably lived in the 5th century AD. He i ...
. 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

Medieval astronomers such as
al-Biruni Abu Rayhan Muhammad ibn Ahmad al-Biruni (; ; 973after 1050), known as al-Biruni, was a Khwarazmian Iranian scholar and polymath during the Islamic Golden Age. He has been called variously "Father of Comparative Religion", "Father of modern ...
and Sacrobosco, divided the hour into 60
minute A minute is a unit of time defined as equal to 60 seconds. It is not a unit in the International System of Units (SI), but is accepted for use with SI. The SI symbol for minutes is min (without a dot). The prime symbol is also sometimes used i ...
s, each of 60
second The second (symbol: s) is a unit of time derived from the division of the day first into 24 hours, then to 60 minutes, and finally to 60 seconds each (24 × 60 × 60 = 86400). The current and formal definition in the International System of U ...
s; this derives from
Babylonian astronomy Babylonian astronomy was the study or recording of celestial objects during the early history of Mesopotamia. The numeral system used, sexagesimal, was based on 60, as opposed to ten in the modern decimal system. This system simplified the ca ...
, where the corresponding terms denoted the time required for the Sun's apparent motion through the
ecliptic The ecliptic or ecliptic plane is the orbital plane of Earth's orbit, Earth around the Sun. It was a central concept in a number of ancient sciences, providing the framework for key measurements in astronomy, astrology and calendar-making. Fr ...
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. In medieval Europe, the Roman hours continued to be marked on
sundial A sundial is a horology, horological device that tells the time of day (referred to as civil time in modern usage) when direct sunlight shines by the position of the Sun, apparent position of the Sun in the sky. In the narrowest sense of the ...
s but the more important units of time were the
canonical hours In the practice of Christianity, canonical hours mark the divisions of the day in terms of Fixed prayer times#Christianity, fixed times of prayer at regular intervals. A book of hours, chiefly a breviary, normally contains a version of, or sel ...
of the Orthodox and
Catholic Church The Catholic Church (), also known as the Roman Catholic Church, is the List of Christian denominations by number of members, largest Christian church, with 1.27 to 1.41 billion baptized Catholics Catholic Church by country, worldwid ...
. 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 A prime number (or a prime) is a natural number greater than 1 that is not a product of two smaller natural numbers. A natural number greater than 1 that is not prime is called a composite number. For example, 5 is prime because the only ways ...
at about 6am,
terce Terce is a canonical hour of the Divine Office. It consists mainly of psalms and is held around 9 a.m. Its name comes from Latin and refers to the third hour of the day after dawn. Along with Prime, Sext, None, and Compline, Terce belongs ...
at about 9am,
sext Sext is a canonical hour of the Divine Office in the liturgies of many Christian denominations. It consists mainly of psalms and is held around noon. Its name comes from Latin and refers to the sixth hour of the day after dawn. With Terce, None ...
at noon, nones at about 3pm, and
vespers Vespers /ˈvɛspərz/ () is a Christian liturgy, liturgy of evening prayer, one of the canonical hours in Catholic (both Latin liturgical rites, Latin and Eastern Catholic liturgy, Eastern Catholic liturgical rites), Eastern Orthodox, Oriental O ...
at either 6pm or
sunset Sunset (or sundown) is the disappearance of the Sun at the end of the Sun path, below the horizon of the Earth (or any other astronomical object in the Solar System) due to its Earth's rotation, rotation. As viewed from everywhere on Earth, it ...
.
Matins Matins (also Mattins) is a canonical hour in Christian liturgy, originally sung during the darkness of early morning (between midnight and dawn). The earliest use of the term was in reference to the canonical hour, also called the vigil, which w ...
and
lauds Lauds is a canonical hour of the Divine office. In the Roman Rite Liturgy of the Hours it is one of the major hours, usually held after Matins, in the early morning hours (between 3:00:00 and 5:59:59). Name The name is derived from the three la ...
precede these irregularly in the morning hours;
compline Compline ( ), also known as Complin, Night Prayer, or the Prayers at the End of the Day, is the final prayer liturgy (or office) of the day in the Christian tradition of canonical hours, which are prayed at fixed prayer times. The English wor ...
follows them irregularly before sleep; and the midnight office follows that.
Vatican II The Second Ecumenical Council of the Vatican, commonly known as the or , was the 21st and most recent Catholic ecumenical councils, ecumenical council of the Catholic Church. The council met each autumn from 1962 to 1965 in St. Peter's Basilic ...
ordered their reformation for the Catholic Church in 1963, though they continue to be observed in the Orthodox churches. When mechanical
clock A clock or chronometer is a device that measures and displays time. The clock is one of the oldest Invention, human inventions, meeting the need to measure intervals of time shorter than the natural units such as the day, the lunar month, a ...
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

The minor irregularities of the apparent solar day were smoothed by measuring time using the
mean solar day Solar time is a calculation of the passage of time based on the position of the Sun in the sky. The fundamental unit of solar time is the day, based on the synodic rotation period. Traditionally, there are three types of time reckoning based ...
, using the Sun's movement along the
celestial equator The celestial equator is the great circle of the imaginary celestial sphere on the same plane as the equator of Earth. By extension, it is also a plane of reference in the equatorial coordinate system. Due to Earth's axial tilt, the celestial ...
rather than along the
ecliptic The ecliptic or ecliptic plane is the orbital plane of Earth's orbit, Earth around the Sun. It was a central concept in a number of ancient sciences, providing the framework for key measurements in astronomy, astrology and calendar-making. Fr ...
. 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 Tidal acceleration is an effect of the tidal forces between an orbiting natural satellite (e.g. the Moon) and the primary planet that it orbits (e.g. Earth). The acceleration causes a gradual recession of a satellite in a prograde orbit (satel ...
of the
Earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...
by the
Moon The Moon is Earth's only natural satellite. It Orbit of the Moon, orbits around Earth at Lunar distance, an average distance of (; about 30 times Earth diameter, Earth's diameter). The Moon rotation, rotates, with a rotation period (lunar ...
, which gradually lengthens the Earth's days. During the French Revolution, a general decimalisation of measures was enacted, including
decimal time 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, France from 1794 to 1800, during the Fre ...
between 1794 and 1800. Under its provisions, the French hour () was of the day and divided formally into 100 decimal minutes (') 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 The metric system is a system of measurement that standardization, standardizes a set of base units and a nomenclature for describing relatively large and small quantities via decimal-based multiplicative unit prefixes. Though the rules gover ...
bases its measurements of time upon the
second The second (symbol: s) is a unit of time derived from the division of the day first into 24 hours, then to 60 minutes, and finally to 60 seconds each (24 × 60 × 60 = 86400). The current and formal definition in the International System of U ...
, 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 Coordinated Universal Time (UTC) is the primary time standard globally used to regulate clocks and time. It establishes a reference for the current time, forming the basis for civil time and time zones. UTC facilitates international communicat ...
(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 Universal Time (UT or UT1) is a time standard based on Earth's rotation. While originally it was mean solar time at 0° longitude, precise measurements of the Sun are difficult. Therefore, UT1 is computed from a measure of the Earth's angle wi ...
, which is based on measurements of the
mean solar day Solar time is a calculation of the passage of time based on the position of the Sun in the sky. The fundamental unit of solar time is the day, based on the synodic rotation period. Traditionally, there are three types of time reckoning based ...
at 0° longitude. The addition of these seconds accommodates the very gradual slowing of the
rotation Rotation or rotational/rotary motion is the circular movement of an object around a central line, known as an ''axis of rotation''. A plane figure can rotate in either a clockwise or counterclockwise sense around a perpendicular axis intersect ...
of the
Earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...
. 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 Employment is a relationship between two party (law), parties Regulation, regulating the provision of paid Labour (human activity), labour services. Usually based on a employment contract, contract, one party, the employer, which might be a cor ...
, whether
wage A wage is payment made by an employer to an employee for work (human activity), work done in a specific period of time. Some examples of wage payments include wiktionary:compensatory, compensatory payments such as ''minimum wage'', ''prevailin ...
or salaried labour, involve compensation based upon measured or expected hours worked. The fight for an
eight-hour day The eight-hour day movement (also known as the 40-hour week movement or the short-time movement) was a social movement to regulate the length of a working day, preventing excesses and abuses of working time. The modern movement originated i ...
was a part of
labour movement The labour movement is the collective organisation of working people to further their shared political and economic interests. It consists of the trade union or labour union movement, as well as political parties of labour. It can be considere ...
s around the world. Informal
rush hour A rush hour (American English, British English) or peak hour (Australian English, Indian English) is a part of the day during which traffic congestion on roads and crowding on public transport is at its highest. Normally, this happens twice e ...
s and
happy hour Happy hour is a marketing term for a time when a venue such as a restaurant or bar offers reduced prices on alcoholic drinks. Discounted menu items like appetizers are often served during happy hour. This is a way for bars and restaurants to ...
s cover the times of day when commuting slows down due to congestion or alcoholic drinks being available at discounted prices. The
hour record The hour record is the record for the longest distance cycled in one hour on a bicycle from a stationary start. Cyclists attempt this record alone on the track without other competitors present. It is considered one of the most prestigious record ...
for the greatest distance travelled by a cyclist within the span of an hour is one of
cycling Cycling, also known as bicycling or biking, is the activity of riding a bicycle or other types of pedal-driven human-powered vehicles such as balance bikes, unicycles, tricycles, and quadricycles. Cycling is practised around the world fo ...
's greatest honours.


Counting hours

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 An astrolabe (; ; ) is an astronomy, astronomical list of astronomical instruments, instrument dating to ancient times. It serves as a star chart and Model#Physical model, physical model of the visible celestial sphere, half-dome of the sky. It ...
s,
sundial A sundial is a horology, horological device that tells the time of day (referred to as civil time in modern usage) when direct sunlight shines by the position of the Sun, apparent position of the Sun in the sky. In the narrowest sense of the ...
s, and
astronomical clock An astronomical clock, horologium, or orloj is a clock with special mechanisms and dials to display astronomical information, such as the relative positions of the Sun, Moon, zodiacal constellations, and sometimes major planets. Definition ...
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. "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.


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. This is also the system used in
Jewish law ''Halakha'' ( ; , ), also transliterated as ''halacha'', ''halakhah'', and ''halocho'' ( ), is the collective body of Jewish religious laws that are derived from the Written and Oral Torah. ''Halakha'' is based on biblical commandments ('' mit ...
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 Italian(s) may refer to: * Anything of, from, or related to the people of Italy over the centuries ** Italians, a Romance ethnic group related to or simply a citizen of the Italian Republic or Italian Kingdom ** Italian language, a Romance languag ...
time", "Italian hours", or "old Czech time", the first hour started with the sunset
Angelus FIle:Jean-François Millet (II) 001.jpg, ''The Angelus (painting), The Angelus'' (1857–1859) by Jean-François Millet The Angelus (; Latin for "angel") is a Catholic devotion commemorating the Incarnation (Christianity), Incarnation of Jesus ...
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 Italy, officially the Italian Republic, is a country in Southern Europe, Southern and Western Europe, Western Europe. It consists of Italian Peninsula, a peninsula that extends into the Mediterranean Sea, with the Alps on its northern land b ...
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. 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 Poland, officially the Republic of Poland, is a country in Central Europe. It extends from the Baltic Sea in the north to the Sudetes and Carpathian Mountains in the south, bordered by Lithuania and Russia to the northeast, Belarus and Ukrai ...
,
Silesia Silesia (see names #Etymology, below) is a historical region of Central Europe that lies mostly within Poland, with small parts in the Czech Silesia, Czech Republic and Germany. Its area is approximately , and the population is estimated at 8, ...
, and
Bohemia Bohemia ( ; ; ) is the westernmost and largest historical region of the Czech Republic. In a narrow, geographic sense, it roughly encompasses the territories of present-day Czechia that fall within the Elbe River's drainage basin, but historic ...
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 Islam is an Abrahamic religions, Abrahamic monotheistic religion based on the Quran, and the teachings of Muhammad. Adherents of Islam are called Muslims, who are estimated to number Islam by country, 2 billion worldwide and are the world ...
day begins at sunset. The first prayer of the day (
maghrib Maghrib () is one of the five mandatory salah (Islamic prayers), and contains three cycles (''rak'a''). If counted from midnight, it is the fourth one. According to Shia and Sunni Muslims, the period for Maghrib prayer starts just after suns ...
) 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.


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 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 (e.g., food production date and sell by date). Th ...
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 The 12-hour clock is a time convention in which the 24 hours of the day are divided into two periods: a.m. (from Latin , translating to "before midday") and p.m. (from Latin , translating to "after midday"). Each period consists of 12&nb ...
, counting the hours starts at midnight and restarts at noon. Hours are numbered 12, 1, 2, ..., 11.
Solar noon Noon (also known as noontime or midday) is 12 o'clock in the daytime. It is written as 12 noon, 12:00 m. (for '' meridiem'', literally 12:00 midday), 12 p.m. (for ''post meridiem'', literally "after midday"), 12 pm, or 12:00 (using a 24-hour clo ...
is always close to 12 noon (ignoring artificial adjustments due to
time zone A time zone is an area which observes a uniform standard time for legal, Commerce, commercial and social purposes. Time zones tend to follow the boundaries between Country, countries and their Administrative division, subdivisions instead of ...
s and
daylight saving time Daylight saving time (DST), also referred to as daylight savings time, daylight time (Daylight saving time in the United States, United States and Daylight saving time in Canada, Canada), or summer time (British Summer Time, United Kingdom, ...
), differing according to the
equation of time The equation of time describes the discrepancy between two kinds of solar time. The two times that differ are the apparent solar time, which directly tracks the diurnal motion of the Sun, and mean solar time, which tracks a theoretical mean Sun ...
by as much as fifteen minutes either way. At the
equinox A solar equinox is a moment in time when the Sun appears directly above the equator, rather than to its north or south. On the day of the equinox, the Sun appears to rise directly east and set directly west. This occurs twice each year, arou ...
es sunrise is around 6 a.m. (, before noon), and sunset around 6 p.m. (, after noon). In the modern
24-hour clock The modern 24-hour clock is the convention of timekeeping in which the day runs from midnight to midnight and is divided into 24 hours. This is indicated by the hours (and minutes) passed since midnight, from to , with as an option to indicate ...
, 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


Egypt

The
ancient Egypt Ancient Egypt () was a cradle of civilization concentrated along the lower reaches of the Nile River in Northeast Africa. It emerged from prehistoric Egypt around 3150BC (according to conventional Egyptian chronology), when Upper and Lower E ...
ians began dividing the night into ' at some time before the compilation of the Dynasty V
Pyramid Texts The Pyramid Texts are the oldest ancient Egyptian funerary texts, dating to the late Old Kingdom. They are the earliest known corpus of ancient Egyptian religious texts. Written in Old Egyptian, the pyramid texts were carved onto the subterranea ...
in the 24thcenturyBC. By 2150BC ( Dynasty IX), diagrams of stars inside Egyptian
coffin A coffin or casket is a funerary box used for viewing or keeping a corpse, for burial, entombment or cremation. Coffins are sometimes referred to as caskets, particularly in American English. A distinction is commonly drawn between "coffins" a ...
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 BC on the basis of analyses of the
Sothic cycle The Sothic cycle or Canicular period is a period of 1,461  Egyptian civil years of 365 days each or 1,460  Julian years averaging  days each. During a Sothic cycle, the 365-day year loses enough time that the start of its year ...
, but a
lunar calendar A lunar calendar is a calendar based on the monthly cycles of the Moon's phases ( synodic months, lunations), in contrast to solar calendars, whose annual cycles are based on the solar year, and lunisolar calendars, whose lunar months are br ...
presumably long predated this and also would have had 12
month A month is a unit of time, used with calendars, that is approximately as long as a natural phase cycle of the Moon; the words ''month'' and ''Moon'' are cognates. The traditional concept of months arose with the cycle of Moon phases; such lunar mo ...
s in each of its years. The coffin diagrams show that the Egyptians took note of the
heliacal rising The heliacal rising ( ) of a star or a planet occurs annually when it becomes visible above the eastern horizon at dawn just before sunrise (thus becoming "the Morning Star (disambiguation)#Astronomy, morning star"). A heliacal rising marks the ti ...
s of 36 stars or
constellations A constellation is an area on the celestial sphere in which a group of visible stars forms a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The first constellations were likely defin ...
(now known as " decans"), one for each of the ten-day "weeks" of their civil calendar. (12 sets of alternate "triangle decans" were used for the 5 epagomenal days between years.) 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
minute A minute is a unit of time defined as equal to 60 seconds. It is not a unit in the International System of Units (SI), but is accepted for use with SI. The SI symbol for minutes is min (without a dot). The prime symbol is also sometimes used i ...
s each. (Another seven stars were noted by the Egyptians during the twilight and predawn periods, 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 (BC), the priests at
Karnak The Karnak Temple Complex, commonly known as Karnak (), comprises a vast mix of temples, pylons, chapels, and other buildings near Luxor, Egypt. Construction at the complex began during the reign of Senusret I (reigned 1971–1926 BC) in the ...
were using
water clock A water clock, or clepsydra (; ; ), is a timepiece by which time is measured by the regulated flow of liquid into (inflow type) or out from (outflow type) a vessel, and where the amount of liquid can then be measured. Water clocks are some of ...
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. During the
New Kingdom New or NEW may refer to: Music * New, singer of K-pop group The Boyz * ''New'' (album), by Paul McCartney, 2013 ** "New" (Paul McCartney song), 2013 * ''New'' (EP), by Regurgitator, 1995 * "New" (Daya song), 2017 * "New" (No Doubt song), 1 ...
, 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 A sundial is a horology, horological device that tells the time of day (referred to as civil time in modern usage) when direct sunlight shines by the position of the Sun, apparent position of the Sun in the sky. In the narrowest sense of the ...
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. The Egyptian hours were closely connected both with the priesthood of the gods and with their divine services. By the
New Kingdom New or NEW may refer to: Music * New, singer of K-pop group The Boyz * ''New'' (album), by Paul McCartney, 2013 ** "New" (Paul McCartney song), 2013 * ''New'' (EP), by Regurgitator, 1995 * "New" (Daya song), 2017 * "New" (No Doubt song), 1 ...
, each hour was conceived as a specific region of the sky or
underworld The underworld, also known as the netherworld or hell, is the supernatural world of the dead in various religious traditions and myths, located below the world of the living. Chthonic is the technical adjective for things of the 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 Wadjet (; "Green One"), known to the Greek world as Uto (; ) or Buto (; ) among other renderings including Wedjat, Uadjet, and Udjo, was originally the ancient Egyptian Tutelary deity, local goddess of the city of Dep or Buto in Lower Egypt, ...
and the rearing
cobra COBRA or Cobra, often stylized as CoBrA, was a European avant-garde art group active from 1948 to 1951. The name was coined in 1948 by Christian Dotremont from the initials of the members' home countries' capital cities: Copenhagen (Co), Brussels ...
(
uraeus drawing of a Uraeus The Uraeus () or Ouraeus (Ancient Greek: , ; Egyptian: ', "rearing cobra", plural: ''Uraei'') is the stylized, upright form of an Egyptian cobra, used as a symbol of sovereignty, royalty, deity and divine authority in anci ...
) were also sometimes referenced as ' from their role protecting the dead through these gates. The Egyptian word for
astronomer An astronomer is a scientist in the field of astronomy who focuses on a specific question or field outside the scope of Earth. Astronomers observe astronomical objects, such as stars, planets, natural satellite, moons, comets and galaxy, galax ...
, used as a synonym for priest, was ', "one of the ''wnwt''", as it were "one of the hours". The earliest forms of ' include one or three stars, with the later solar hours including the
determinative A determinative, also known as a taxogram or semagram, is an ideogram used to mark semantic categories of words in logographic scripts which helps to disambiguate interpretation. They have no direct counterpart in spoken language, though they ...
hieroglyph Ancient Egyptian hieroglyphs ( ) were the formal writing system used in Ancient Egypt for writing the Egyptian language. Hieroglyphs combined ideographic, logographic, syllabic and alphabetic elements, with more than 1,000 distinct characters. ...
for "sun".


East Asia

Ancient China The history of China spans several millennia across a wide geographical area. Each region now considered part of the Chinese world has experienced periods of unity, fracture, prosperity, and strife. Chinese civilization first emerged in the Y ...
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 The Yellow Emperor, also known as the Yellow Thearch, or Huangdi ( zh, t=黃帝, s=黄帝, first=t) in Chinese, is a mythical Chinese sovereign and culture hero included among the legendary Three Sovereigns and Five Emperors. He is revered as ...
, but is first attested in Han-era
water clock A water clock, or clepsydra (; ; ), is a timepiece by which time is measured by the regulated flow of liquid into (inflow type) or out from (outflow type) a vessel, and where the amount of liquid can then be measured. Water clocks are some of ...
s and in the 2nd-century history of that
dynasty A dynasty is a sequence of rulers from the same family, usually in the context of a monarchy, monarchical system, but sometimes also appearing in republics. A dynasty may also be referred to as a "house", "family" or "clan", among others. H ...
. It was measured with sundials and
water clock A water clock, or clepsydra (; ; ), is a timepiece by which time is measured by the regulated flow of liquid into (inflow type) or out from (outflow type) a vessel, and where the amount of liquid can then be measured. Water clocks are some of ...
s. Into the
Eastern Han The Han dynasty was an Dynasties of China, imperial dynasty of China (202 BC9 AD, 25–220 AD) established by Liu Bang and ruled by the House of Liu. The dynasty was preceded by the short-lived Qin dynasty (221–206 BC ...
, 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 Sunset (or sundown) is the disappearance of the Sun at the end of the Sun path, below the horizon of the Earth (or any other astronomical object in the Solar System) due to its Earth's rotation, rotation. As viewed from everywhere on Earth, it ...
to
sunrise Sunrise (or sunup) is the moment when the upper rim of the Sun appears on the horizon in the morning, at the start of the Sun path. The term can also refer to the entire process of the solar disk crossing the horizon. Terminology Although the S ...
.
Imperial China The history of China spans several millennia across a wide geographical area. Each region now considered part of the Chinese world has experienced periods of unity, fracture, prosperity, and strife. Chinese civilization first emerged in the Y ...
continued to use ''ke'' and ''geng'' but also began to divide the day into 12 "double hours" named after the
earthly branches The Earthly Branches (also called the Terrestrial Branches or the 12-cycle) are a system of twelve ordered symbols used throughout East Asia. They are indigenous to China, and are themselves Chinese characters, corresponding to words with no co ...
and sometimes also known by the name of the corresponding animal of the
Chinese zodiac The Chinese zodiac is a traditional classification scheme based on the Chinese calendar that assigns an animal and its reputed attributes to each year in a repeating twelve-year (or duodenary) cycle. The zodiac is very important in traditional ...
. 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. These apparently began to be used during the
Eastern Han The Han dynasty was an Dynasties of China, imperial dynasty of China (202 BC9 AD, 25–220 AD) established by Liu Bang and ruled by the House of Liu. The dynasty was preceded by the short-lived Qin dynasty (221–206 BC ...
that preceded the
Three Kingdoms The Three Kingdoms of Cao Wei, Shu Han, and Eastern Wu dominated China from AD 220 to 280 following the end of the Han dynasty. This period was preceded by the Eastern Han dynasty and followed by the Jin dynasty (266–420), Western Jin dyna ...
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 The ''Book of Sui'' () is the official history of the Sui dynasty, which ruled China in the years AD 581–618. It ranks among the official Twenty-Four Histories of imperial China. It was written by Yan Shigu, Kong Yingda, and Zhangsun Wuji, ...
. Variations of all these units were subsequently adopted by
Japan Japan is an island country in East Asia. Located in the Pacific Ocean off the northeast coast of the Asia, Asian mainland, it is bordered on the west by the Sea of Japan and extends from the Sea of Okhotsk in the north to the East China Sea ...
and the other countries of the
Sinosphere The Sinosphere, also known as the Chinese cultural sphere, East Asian cultural sphere, or the Sinic world, encompasses multiple countries in East Asia and Southeast Asia that were historically heavily influenced by Chinese culture. The Sinosph ...
. The 12 ''shi'' supposedly began to be divided into 24 hours under the Tang, although they are first attested in the Ming-era Book of Yuan. In that work, the hours were known by the same
earthly branches The Earthly Branches (also called the Terrestrial Branches or the 12-cycle) are a system of twelve ordered symbols used throughout East Asia. They are indigenous to China, and are themselves Chinese characters, corresponding to words with no co ...
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
Jovian Jovian is the adjectival form of Jupiter and may refer to: * Jovian (emperor) (Flavius Iovianus Augustus), Roman emperor (363–364 AD) * Jovians and Herculians, Roman imperial guard corps * Jovian (lemur), a Coquerel's sifaka known for ''Zobooma ...
orbital cycle was more important to
traditional Chinese A tradition is a system of beliefs or behaviors (folk custom) passed down within a group of people or society with symbolic meaning or special significance with origins in the past. A component of cultural expressions and folklore, common examp ...
and Babylonian reckoning of the zodiac.


Southeast Asia

In
Thailand Thailand, officially the Kingdom of Thailand and historically known as Siam (the official name until 1939), is a country in Southeast Asia on the Mainland Southeast Asia, Indochinese Peninsula. With a population of almost 66 million, it spa ...
,
Laos Laos, officially the Lao People's Democratic Republic (LPDR), is the only landlocked country in Southeast Asia. It is bordered by Myanmar and China to the northwest, Vietnam to the east, Cambodia to the southeast, and Thailand to the west and ...
, and
Cambodia Cambodia, officially the Kingdom of Cambodia, is a country in Southeast Asia on the Mainland Southeast Asia, Indochinese Peninsula. It is bordered by Thailand to the northwest, Laos to the north, and Vietnam to the east, and has a coastline ...
, the traditional system of noting hours is the six-hour clock. This reckons each of a day's 24 hours apart from
noon Noon (also known as noontime or midday) is 12 o'clock in the daytime. It is written as 12 noon, 12:00 m. (for '' meridiem'', literally 12:00 midday), 12 p.m. (for ''post meridiem'', literally "after midday"), 12 pm, or 12:00 (using a 24-hour cl ...
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 The Ayutthaya Kingdom or the Empire of Ayutthaya was a Thai people, Thai kingdom that existed in Southeast Asia from 1351 to 1767, centered around the city of Phra Nakhon Si Ayutthaya (city), Ayutthaya, in Siam, or present-day Thailand. Europe ...
, deriving its current phrasing from the practice of publicly announcing the daytime hours with a
gong A gongFrom Indonesian language, Indonesian and ; ; zh, c=鑼, p=luó; ; ; ; ; is a percussion instrument originating from Southeast Asia, and used widely in Southeast Asian and East Asian musical traditions. Gongs are made of metal and ...
and the nighttime hours with a
drum The drum is a member of the percussion group of musical instruments. In the Hornbostel–Sachs classification system, it is a membranophone. Drums consist of at least one membrane, called a drumhead or drum skin, that is stretched over a ...
. 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.


India

The
Vedas FIle:Atharva-Veda samhita page 471 illustration.png, upright=1.2, The Vedas are ancient Sanskrit texts of Hinduism. Above: A page from the ''Atharvaveda''. The Vedas ( or ; ), sometimes collectively called the Veda, are a large body of relig ...
and
Puranas Puranas (Merriam-Webster's Encyclopedia of Literature
(1995 Editio ...
employed
units of time A unit of time is any particular time interval, used as a standard way of measuring or expressing duration. The SI base unit, base unit of time in the International System of Units (SI), and by extension most of the Western world, is the second, ...
based on the
sidereal day Sidereal time ("sidereal" pronounced ) is a system of timekeeping used especially by astronomers. Using sidereal time and the celestial coordinate system, it is easy to locate the positions of celestial objects in the night sky. Sidereal t ...
(''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 A synodic day (or synodic rotation period or solar day) is the period for a celestial object to rotate once in relation to the star it is orbiting, and is the basis of solar time. The synodic day is distinguished from the sidereal day, which is ...
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''.


Derived measures

*
air changes per hour Air changes per hour, abbreviated ACPH or ACH, or air change rate is the number of times that the total air volume in a room or space is completely removed and replaced in an hour. If the air in the space is either uniform or perfectly mixed, air c ...
(ACH), a measure of the replacements of air within a defined space used for
indoor air quality Indoor air quality (IAQ) is the air quality within buildings and Nonbuilding structure, structures. Poor indoor air quality due to indoor air pollution is known to affect the health, comfort, and well-being of building occupants. It has also be ...
*
ampere hour An ampere-hour or amp-hour (symbol: A⋅h or A h; often simplified as Ah) is a unit of electric charge, having dimensions of electric current multiplied by time, equal to the charge transferred by a steady current of one ampere flowing for ...
(Ah), a measure of
electrical charge Electricity is the set of physical phenomena associated with the presence and motion of matter possessing an electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described by Maxwel ...
used in
electrochemistry Electrochemistry is the branch of physical chemistry concerned with the relationship between Electric potential, electrical potential difference and identifiable chemical change. These reactions involve Electron, electrons moving via an electronic ...
*
BTU The British thermal unit (Btu) is a measure of heat, which is a form of energy. It was originally defined as the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit. It is also part of the United Stat ...
-hour, a measure of power used in the power industry and for air conditioners and heaters *
credit hour A course credit is a measure of the size of an educational course, often used to determine whether the requirements for an award have been met, to facilitate transfer between institutions, or to enhance intercomparability of qualifications. Credit ...
, a measure of an academic course's contracted instructional time per week for a semester * horsepower-hour (hph), a measure of energy used in the railroad industry *
hour angle In astronomy and celestial navigation, the hour angle is the dihedral angle between the '' meridian plane'' (containing Earth's axis and the zenith) and the '' hour circle'' (containing Earth's axis and a given point of interest). It may be ...
, a measure of the angle between the meridian plane and the
hour circle In astronomy, the hour circle is the great circle through a given object and the two celestial poles. Together with declination and distance (from the planet's centre of mass), it determines the location of any celestial object. As such, it is ...
passing through a certain point used in the
equatorial coordinate system The equatorial coordinate system is a celestial coordinate system widely used to specify the positions of astronomical object, celestial objects. It may be implemented in spherical coordinate system, spherical or Cartesian coordinate system, rect ...
*
kilometres per hour The kilometre per hour (International System of Units, SI symbol: km/h; non-SI abbreviations: kph, kmph, km/hr) is a Units of measurement, unit of speed, expressing the number of kilometres travelled in one hour. History Although the metre was f ...
(km/h), a measure of land speed *
kilowatt-hour A kilowatt-hour ( unit symbol: kW⋅h or kW h; commonly written as kWh) is a non-SI unit of energy equal to 3.6 megajoules (MJ) in SI units, which is the energy delivered by one kilowatt of power for one hour. Kilowatt-hours are a comm ...
(kWh), a measure of energy commonly used as an electrical billing unit *
knot A knot is an intentional complication in Rope, cordage which may be practical or decorative, or both. Practical knots are classified by function, including List of hitch knots, hitches, List of bend knots, bends, List of loop knots, loop knots, ...
(kn), a measure of
nautical mile A nautical mile is a unit of length used in air, marine, and space navigation, and for the definition of territorial waters. Historically, it was defined as the meridian arc length corresponding to one minute ( of a degree) of latitude at t ...
s per hour, used for maritime and aerial speed *
man-hour A man-hour or human-hour is the amount of work performed by the average worker in one hour. It is used for estimation of the total amount of uninterrupted labor required to perform a task. For example, researching and writing a college paper ...
, 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 Miles per hour (mph, m.p.h., MPH, or mi/h) is a British imperial and United States customary unit of speed expressing the number of miles travelled in one hour. It is used in the United Kingdom, the United States, and a number of smaller count ...
(mph), a measure of land speed * passengers per hour per direction (p/h/d), a measure of the capacity of
public transport Public transport (also known as public transit, mass transit, or simply transit) are forms of transport available to the general public. It typically uses a fixed schedule, route and charges a fixed fare. There is no rigid definition of whic ...
ation systems * pound per hour (PPH), a measure of
mass flow rate In physics and engineering, mass flow rate is the Temporal rate, rate at which mass of a substance changes over time. Its unit of measurement, unit is kilogram per second (kg/s) in SI units, and Slug (unit), slug per second or pound (mass), pou ...
used for engines' fuel flow * work or working hour, a measure of working time used in various regulations, such as those distinguishing part- and full-time employment and those limiting
truck driver A truck driver (commonly referred to as a trucker, teamster or driver in the United States and Canada; a truckie in Australia and New Zealand; an HGV driver in the United Kingdom, Ireland and the European Union, a lorry driver, or driver in ...
s'
working hours Working time or laboring time is the period of time that a person spends at paid labor. Unpaid labor such as personal housework or caring for children or pets is not considered part of the working week. Many countries regulate the work wee ...
or
hours of service Hours of service (HOS) regulations are issued by the Federal Motor Carrier Safety Administration (FMCSA) and govern the working hours of anyone operating a commercial motor vehicle (CMV) in the United States. These regulations apply to truck d ...


See also

* Danna * Decimal hour or deciday, a French Revolutionary unit lasting 2h 24min * Equinoctial hours * Golden Hour & Blue Hour in photography * Hexadecimal hour, a proposed unit lasting 1h 30min *
Horae In Greek mythology, the Horae (), Horai () or Hours (, ) were the goddesses of the seasons and the natural portions of time. Etymology The term ''hora'' comes from the Proto-Indo-European ("year"). Function The Horae were originally the ...
, the deified hours of ancient Greece and Rome *
Horology Chronometry or horology () is the science studying the measurement of time and timekeeping. Chronometry enables the establishment of standard measurements of time, which have applications in a broad range of social and scientific areas. ''Hor ...
*
Julian day 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 (e.g., food production date and sell by date). Th ...
*
Liturgy of the Hours The Liturgy of the Hours (), Divine Office (), or ''Opus Dei'' ("Work of God") are a set of Catholic prayers comprising the canonical hours, often also referred to as the breviary, of the Latin Church. The Liturgy of the Hours forms the official ...
*
Metric time Metric time is the measure of time intervals using the metric system. The modern SI system defines the second as the base unit of time, and forms multiples and submultiples with metric prefixes such as kiloseconds and milliseconds. Other units ...
*
Six-hour day The six-hour day is a schedule by which the employees or other members of an institution (which may also be, for example, a school) spend six hours contributing. This is in contrast to the widespread eight-hour day, or any other time arrangemen ...
* Temporal hours


Explanatory notes


Citations


General and cited references

* * * * * * * * * * * * * * * * . * *


Further reading

* * Christopher Walker (ed.), ''Astronomy before the Telescope''. London:
British Museum Press The British Museum is a public museum dedicated to human history, art and culture located in the Bloomsbury area of London. Its permanent collection of eight million works is the largest in the world. It documents the story of human culture f ...
, 1996.


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{{Authority control Orders of magnitude (time) Units of time