Day

# Day

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A day is a unit of time
Time
Time is a part of the measuring system used to sequence events, to compare the durations of events and the intervals between them, and to quantify rates of change such as the motions of objects....

, commonly defined as an interval equal to 24 hours. It also can mean that portion of the full day during which a location is illuminated by the light of the sun. The period of time measured from local noon to the following local noon is called a solar day
Solar time
Solar time is a reckoning of the passage of time based on the Sun's position in the sky. The fundamental unit of solar time is the day. Two types of solar time are apparent solar time and mean solar time .-Introduction:...

.

Several definitions of this universal human concept are used according to context, need and convenience. In 1967, the second
Second
The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock....

was redefined in terms of the wavelength of light, and it became the SI base unit
SI base unit
The International System of Units defines seven units of measure as a basic set from which all other SI units are derived. These SI base units and their physical quantities are:* metre for length...

of time
Time
Time is a part of the measuring system used to sequence events, to compare the durations of events and the intervals between them, and to quantify rates of change such as the motions of objects....

. The unit of measurement
Units of measurement
A unit of measurement is a definite magnitude of a physical quantity, defined and adopted by convention and/or by law, that is used as a standard for measurement of the same physical quantity. Any other value of the physical quantity can be expressed as a simple multiple of the unit of...

for time called "day", redefined in 1967 as 86,400 SI seconds and symbolized d, is not an SI
International System of Units
The International System of Units is the modern form of the metric system and is generally a system of units of measurement devised around seven base units and the convenience of the number ten. The older metric system included several groups of units...

unit, but it is accepted for use with SI. A civil day is usually also 86,400 seconds, plus or minus a possible leap second
Leap second
A leap second is a positive or negative one-second adjustment to the Coordinated Universal Time time scale that keeps it close to mean solar time. UTC, which is used as the basis for official time-of-day radio broadcasts for civil time, is maintained using extremely precise atomic clocks...

in Coordinated Universal Time UTC, and, in some locations, occasionally plus or minus an hour when changing from or to daylight saving time
Daylight saving time
Daylight saving time —also summer time in several countries including in British English and European official terminology —is the practice of temporarily advancing clocks during the summertime so that afternoons have more daylight and mornings have less...

. The word day may also refer to a day of the week
Week
A week is a time unit equal to seven days.The English word week continues an Old English wice, ultimately from a Common Germanic , from a root "turn, move, change"...

or to a calendar date
Calendar date
A date in a calendar is a reference to a particular day represented within a calendar system. The calendar date allows the specific day to be identified. The number of days between two dates may be calculated. For example, "24 " is ten days after "14 " in the Gregorian calendar. The date of a...

, as in answer to the question "On which day?" Day also refers to the part of the day that is not night — also known as 'daytime
Daytime (astronomy)
On Earth, daytime is roughly the period on any given point of the planet's surface during which it experiences natural illumination from indirect or direct sunlight....

'. The life patterns of humans and many other species are related to Earth's solar day and the cycle of day and night (see circadian rhythm
A circadian rhythm, popularly referred to as body clock, is an endogenously driven , roughly 24-hour cycle in biochemical, physiological, or behavioural processes. Circadian rhythms have been widely observed in plants, animals, fungi and cyanobacteria...

s).

The average length of a solar day on Earth is about 86,400 seconds (24 hours) and there are about 365.2422 solar days in one mean tropical year
Tropical year
A tropical year , for general purposes, is the length of time that the Sun takes to return to the same position in the cycle of seasons, as seen from Earth; for example, the time from vernal equinox to vernal equinox, or from summer solstice to summer solstice...

. Because celestial orbits are not perfectly circular, and thus objects travel at different speeds at various positions in their orbit, a solar day is not the same length of time throughout the orbital year. A day, understood as the span of time it takes for the Earth to make one entire rotation
Rotation
A rotation is a circular movement of an object around a center of rotation. A three-dimensional object rotates always around an imaginary line called a rotation axis. If the axis is within the body, and passes through its center of mass the body is said to rotate upon itself, or spin. A rotation...

with respect to the celestial background or a distant star (assumed, to be fixed), is called stellar day
Sidereal time
Sidereal time is a time-keeping system astronomers use to keep track of the direction to point their telescopes to view a given star in the night sky...

. This period of rotation is about 4 minutes less than 24 hours and there are about 366.2422 in one mean tropical year (one more stellar day than the number of solar days). Mainly due to tidal effects
Tidal acceleration
Tidal acceleration is an effect of the tidal forces between an orbiting natural satellite , and the primary planet that it orbits . The "acceleration" is usually negative, as it causes a gradual slowing and recession of a satellite in a prograde orbit away from the primary, and a corresponding...

, the Earth's rotational period is not constant, resulting in further minor variations for both solar days and stellar 'days'. Other planets and moons also have stellar and solar days.

## Introduction

Besides the day of 24 hours (86,400 seconds), the word day is used for several different spans of time based on the rotation of the Earth around its axis. An important one is the solar day, defined as the time it takes for the sun to return to the zenith
Zenith
The zenith is an imaginary point directly "above" a particular location, on the imaginary celestial sphere. "Above" means in the vertical direction opposite to the apparent gravitational force at that location. The opposite direction, i.e...

(its highest point in the sky). Because the Earth orbits the Sun elliptically as the Earth spins on an inclined axis, this period can be up to 7.9 seconds more than (or less than) 24 hours. On average over the year this day is equivalent to 24 hours (86,400 seconds).

A day, in the sense of daytime
DAYTIME
The Daytime Protocol is a service in the Internet Protocol Suite, defined in 1983 in RFC 867. It is intended for testing and measurement purposes in computer networks....

that is distinguished from night-time, is commonly defined as the period during which sunlight
Sunlight
Sunlight, in the broad sense, is the total frequency spectrum of electromagnetic radiation given off by the Sun. On Earth, sunlight is filtered through the Earth's atmosphere, and solar radiation is obvious as daylight when the Sun is above the horizon.When the direct solar radiation is not blocked...

directly reaches the ground, assuming that there are no local obstacles. The length of daytime averages slightly more than half of the 24-hour day. Two effects make daytime on average longer than nights. The Sun is not a point, but has an apparent size of about 32 minutes of arc
Minute of arc
A minute of arc, arcminute, or minute of angle , is a unit of angular measurement equal to one sixtieth of one degree. In turn, a second of arc or arcsecond is one sixtieth of one minute of arc....

. Additionally, the atmosphere
Earth's atmosphere
The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...

refracts
Refraction
Refraction is the change in direction of a wave due to a change in its speed. It is essentially a surface phenomenon . The phenomenon is mainly in governance to the law of conservation of energy. The proper explanation would be that due to change of medium, the phase velocity of the wave is changed...

sunlight in such a way that some of it reaches the ground even when the Sun is below the horizon by about 34 minutes of arc. So the first light reaches the ground when the centre of the Sun is still below the horizon by about 50 minutes of arc. The difference in time depends on the angle at which the Sun rises and sets (itself a function of latitude
Latitude
In geography, the latitude of a location on the Earth is the angular distance of that location south or north of the Equator. The latitude is an angle, and is usually measured in degrees . The equator has a latitude of 0°, the North pole has a latitude of 90° north , and the South pole has a...

), but can amount to around seven minutes.

Ancient custom has a new day start at either the rising or setting of the Sun on the local horizon (Italian reckoning, for example) The exact moment of, and the interval between, two sunrises or two sunsets depends on the geographical position (longitude
Longitude
Longitude is a geographic coordinate that specifies the east-west position of a point on the Earth's surface. It is an angular measurement, usually expressed in degrees, minutes and seconds, and denoted by the Greek letter lambda ....

as well as latitude), and the time of year
Year
A year is the orbital period of the Earth moving around the Sun. For an observer on Earth, this corresponds to the period it takes the Sun to complete one course throughout the zodiac along the ecliptic....

. This is the time as indicated by ancient hemispherical sundial
Sundial
A sundial is a device that measures time by the position of the Sun. In common designs such as the horizontal sundial, the sun casts a shadow from its style onto a surface marked with lines indicating the hours of the day. The style is the time-telling edge of the gnomon, often a thin rod or a...

s.

A more constant day can be defined by the Sun passing through the local meridian
Meridian (geography)
A meridian is an imaginary line on the Earth's surface from the North Pole to the South Pole that connects all locations along it with a given longitude. The position of a point along the meridian is given by its latitude. Each meridian is perpendicular to all circles of latitude...

, which happens at local noon
Noon
Noon is usually defined as 12 o'clock in the daytime. The word noon is also used informally to mean midday regarding the location of the sun not the middle of a persons day. Although this is a time around the middle of the day when people in many countries take a lunch break...

(upper culmination
Culmination
In astronomy, the culmination of a planet, star, constellation, etc. is the altitude reached when the object transits over an observer's meridian....

) or midnight
Midnight
Midnight is the transition time period from one day to the next: the moment when the date changes. In the Roman time system, midnight was halfway between sunset and sunrise, varying according to the seasons....

(lower culmination). The exact moment is dependent on the geographical longitude, and to a lesser extent on the time of the year. The length of such a day is nearly constant (24 hours ± 30 seconds). This is the time as indicated by modern sundials.

A further improvement defines a fictitious mean Sun that moves with constant speed along the celestial equator
Celestial equator
The celestial equator is a great circle on the imaginary celestial sphere, in the same plane as the Earth's equator. In other words, it is a projection of the terrestrial equator out into space...

; the speed is the same as the average speed of the real Sun, but this removes the variation over a year as the Earth moves along its orbit around the Sun (due to both its velocity and its axial tilt).

The Earth's day has increased in length over time. This phenomenon is due to tide
Tide
Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the moon and the sun and the rotation of the Earth....

s raised by the Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

which slow Earth's rotation. Because of the way the second is defined, the mean length of a day is now about 86,400.002 seconds, and is increasing by about 1.7 milliseconds per century (an average over the last 2,700 years). See tidal acceleration
Tidal acceleration
Tidal acceleration is an effect of the tidal forces between an orbiting natural satellite , and the primary planet that it orbits . The "acceleration" is usually negative, as it causes a gradual slowing and recession of a satellite in a prograde orbit away from the primary, and a corresponding...

for details. The length of one day was about 21.9 hours 620 million years ago as recorded by rhythmites (alternating layers in sandstone
Sandstone
Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...

); when the Earth was new about 4.5 billion years ago, it was probably around six hours as determined by computer simulations. The length of day for the Earth or Proto-Earth before the event which created our moon by an impact is yet unknown.

## Etymology

The term comes from the Old English dæg, with its cognates such as Tag in German
German language
German is a West Germanic language, related to and classified alongside English and Dutch. With an estimated 90 – 98 million native speakers, German is one of the world's major languages and is the most widely-spoken first language in the European Union....

, and dag in Norwegian
Norwegian language
Norwegian is a North Germanic language spoken primarily in Norway, where it is the official language. Together with Swedish and Danish, Norwegian forms a continuum of more or less mutually intelligible local and regional variants .These Scandinavian languages together with the Faroese language...

, Danish
Danish language
Danish is a North Germanic language spoken by around six million people, principally in the country of Denmark. It is also spoken by 50,000 Germans of Danish ethnicity in the northern parts of Schleswig-Holstein, Germany, where it holds the status of minority language...

, Swedish
Swedish language
Swedish is a North Germanic language, spoken by approximately 10 million people, predominantly in Sweden and parts of Finland, especially along its coast and on the Åland islands. It is largely mutually intelligible with Norwegian and Danish...

and Dutch
Dutch language
Dutch is a West Germanic language and the native language of the majority of the population of the Netherlands, Belgium, and Suriname, the three member states of the Dutch Language Union. Most speakers live in the European Union, where it is a first language for about 23 million and a second...

.

## International System of Units (SI)

A day is defined as 86,400 seconds.

A day on the UTC time scale
Time scale
A time scale specifies divisions of time.*Time standard, a specification of either the rate at which time passes, or points in time, or both*Duration , a quantity of time...

can include a negative or positive leap second
Leap second
A leap second is a positive or negative one-second adjustment to the Coordinated Universal Time time scale that keeps it close to mean solar time. UTC, which is used as the basis for official time-of-day radio broadcasts for civil time, is maintained using extremely precise atomic clocks...

, and can therefore have a length of 86,399 or 86,401 seconds.

The International Bureau of Weights and Measures
International Bureau of Weights and Measures
The International Bureau of Weights and Measures , is an international standards organisation, one of three such organisations established to maintain the International System of Units under the terms of the Metre Convention...

(BIPM) currently defines a second as
… the duration of 9 192 631 770 periods of the radiation
Radioactive decay is the process by which an atomic nucleus of an unstable atom loses energy by emitting ionizing particles . The emission is spontaneous, in that the atom decays without any physical interaction with another particle from outside the atom...

corresponding to the transition between two hyperfine
Hyperfine structure
The term hyperfine structure refers to a collection of different effects leading to small shifts and splittings in the energy levels of atoms, molecules and ions. The name is a reference to the fine structure which results from the interaction between the magnetic moments associated with electron...

levels
Energy level
A quantum mechanical system or particle that is bound -- that is, confined spatially—can only take on certain discrete values of energy. This contrasts with classical particles, which can have any energy. These discrete values are called energy levels...

of the ground state
Stationary state
In quantum mechanics, a stationary state is an eigenvector of the Hamiltonian, implying the probability density associated with the wavefunction is independent of time . This corresponds to a quantum state with a single definite energy...

of the caesium
Caesium
Caesium or cesium is the chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-gold alkali metal with a melting point of 28 °C , which makes it one of only five elemental metals that are liquid at room temperature...

133 atom
Atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...

.

This makes the SI-based day last exactly 794,243,384,928,000 of those periods.

### Decimal and metric time

In the 19th century it had also been suggested to make a decimal fraction ( or ) of an astronomic day the base unit of time. This was an afterglow of decimal time
Decimal time
Decimal time is the representation of the time of day using units which are decimally related. This term is often used to refer specifically to French Revolutionary Time, which divides the day into 10 decimal hours, each decimal hour into 100 decimal minutes and each decimal minute into 100 decimal...

and calendar
French Republican Calendar
The French Republican Calendar or French Revolutionary Calendar was a calendar created and implemented during the French Revolution, and used by the French government for about 12 years from late 1793 to 1805, and for 18 days by the Paris Commune in 1871...

, which had been given up already for its difficulty to comply with familiar units. The still most successful candidate is the centiday = 14.4 minutes, as a shorter quarter
Quarter
Quarter is one fourth, ¼, or 25%, and may refer to:*Quarter , a section or area, usually of a town*Quarter , valued at one-fourth of a U.S...

of an hour and also close to the SI target kilosecond
Kilosecond
A kilosecond is 1000 seconds , so there are 86.4 kiloseconds in a 24 hour day, and 604.8 kiloseconds in a week. The second is the International System of Units base unit of time, combine with the prefix kilo- which means 1000, results in the definition of a kilosecond...

and old Chinese ke
Ke (unit)
The ke is a traditional Chinese unit of decimal time lasting approximately a quarter of a western hour. Traditionally the ke divides a day into 100 equal intervals of 14.4 minutes . The ke is equivalent to the centiday , a non-SI prefixed unit...

.

## Astronomy

A day of exactly 86,400 SI seconds is the astronomical unit of time (the second is not preferred in astronomy).

For a given planet, there are three types of day defined in astronomy
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...

:
• stellar day - an entire rotation of a planet with respect to the distant star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...

s
• sidereal day
Sidereal time
Sidereal time is a time-keeping system astronomers use to keep track of the direction to point their telescopes to view a given star in the night sky...

- a single rotation of a planet with respect to the vernal equinox
• mean solar day
Solar time
Solar time is a reckoning of the passage of time based on the Sun's position in the sky. The fundamental unit of solar time is the day. Two types of solar time are apparent solar time and mean solar time .-Introduction:...

- average time of a single rotation of a planet with respect to the sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

as the central star

For Earth, the stellar day and the sidereal day are nearly of the same length and about 3 minutes 56 seconds shorter than the solar day. In fact, the Earth spins 366 times about its axis during a 365-day year, because the Earth's revolution about the Sun removes one apparent turn of the Sun about the Earth.

## Colloquial

The word refers to various relatedly defined ideas, including the following:
• 24 hours (exactly)
• the period of light when the Sun is above the local horizon
Horizon
The horizon is the apparent line that separates earth from sky, the line that divides all visible directions into two categories: those that intersect the Earth's surface, and those that do not. At many locations, the true horizon is obscured by trees, buildings, mountains, etc., and the resulting...

(that is, the time period from sunrise
Sunrise
Sunrise is the instant at which the upper edge of the Sun appears above the horizon in the east. Sunrise should not be confused with dawn, which is the point at which the sky begins to lighten, some time before the sun itself appears, ending twilight...

to sunset
Sunset
Sunset or sundown is the daily disappearance of the Sun below the horizon in the west as a result of Earth's rotation.The time of sunset is defined in astronomy as the moment the trailing edge of the Sun's disk disappears below the horizon in the west...

);
• the full day covering a dark and a light period, beginning from the beginning of the dark period or from a point near the middle of the dark period;
• a full dark and light period, sometimes called a nychthemeron
Nychthemeron
Nychthemeron , occasionally nycthemeron or nuchthemeron is a period of 24 consecutive hours...

in English
English language
English is a West Germanic language that arose in the Anglo-Saxon kingdoms of England and spread into what was to become south-east Scotland under the influence of the Anglian medieval kingdom of Northumbria...

, from the Greek
Greek language
Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...

for night-day;
• the time period from 6:00 AM
12-hour clock
The 12-hour clock is a time conversion convention in which the 24 hours of the day are divided into two periods called ante meridiem and post meridiem...

to 6:00 PM or 9:00 PM or some other fixed clock period overlapping or set off from other time periods such as "morning
Morning
The word morning originally referred to the sunrise. Morning precedes midday, afternoon, and night in the sequence of a day.Morning is the part of the day usually reckoned from dawn to noon...

", "evening
Evening
Evening is the period between the late afternoon and night when daylight is decreasing, around dinner time at 6pm. Though the term is subjective, evening is typically understood to begin before sunset, during the close of the standard business day and extend until nightfall, the beginning of night...

", or "night
Night
Night or nighttime is the period of time when the sun is below the horizon. This occurs after dusk. The opposite of night is day...

".

## Civil day

For civil purposes a common clock time has been defined for an entire region based on the mean local solar time at some central meridian. Such time zone
Time zone
A time zone is a region on Earth that has a uniform standard time for legal, commercial, and social purposes. In order for the same clock time to always correspond to the same portion of the day as the Earth rotates , different places on the Earth need to have different clock times...

s began to be adopted about the middle of the 19th century when railroad
Rail transport
Rail transport is a means of conveyance of passengers and goods by way of wheeled vehicles running on rail tracks. In contrast to road transport, where vehicles merely run on a prepared surface, rail vehicles are also directionally guided by the tracks they run on...

s with regular schedules came into use, with most major countries having adopted them by 1929. For the whole world, 40 such time zones are now in use. The main one is "world time" or Coordinated Universal Time
Coordinated Universal Time
Coordinated Universal Time is the primary time standard by which the world regulates clocks and time. It is one of several closely related successors to Greenwich Mean Time. Computer servers, online services and other entities that rely on having a universally accepted time use UTC for that purpose...

(UTC).

The present common convention has the civil day starting at midnight, which is near the time of the lower culmination of the mean Sun on the central meridian of the time zone. A day is commonly divided into 24 hours of 60 minutes of 60 seconds each.

## Leap seconds

To keep the civil day aligned with the apparent movement of the Sun, positive or negative leap second
Leap second
A leap second is a positive or negative one-second adjustment to the Coordinated Universal Time time scale that keeps it close to mean solar time. UTC, which is used as the basis for official time-of-day radio broadcasts for civil time, is maintained using extremely precise atomic clocks...

s may be inserted.

A civil clock day is typically 86,400 SI seconds long, but will be 86,401 s or 86,399 s long in the event of a leap second.

Leap seconds are announced in advance by the International Earth Rotation and Reference Systems Service
International Earth Rotation and Reference Systems Service
The International Earth Rotation and Reference Systems Service , formerly the International Earth Rotation Service, is the body responsible for maintaining global time and reference frame standards, notably through its Earth Orientation Parameter and International Celestial Reference System ...

which measures the Earth's rotation and determines whether a leap second is necessary. Leap seconds occur only at the end of a UTC month, and have only ever been inserted at the end of June 30 or December 31.

## Boundaries of the day

For most diurnal animals, the day naturally begins at dawn and ends at sunset.Humans, with our cultural norms and scientific knowledge, have supplanted Nature with several different conceptions of the day's boundaries. The Jewish
Judaism
Judaism ) is the "religion, philosophy, and way of life" of the Jewish people...

day begins at either sunset or at nightfall (when three second-magnitude
Magnitude (astronomy)
Magnitude is the logarithmic measure of the brightness of an object, in astronomy, measured in a specific wavelength or passband, usually in optical or near-infrared wavelengths.-Background:...

stars appear). Medieval
Middle Ages
The Middle Ages is a periodization of European history from the 5th century to the 15th century. The Middle Ages follows the fall of the Western Roman Empire in 476 and precedes the Early Modern Era. It is the middle period of a three-period division of Western history: Classic, Medieval and Modern...

Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...

followed this tradition, known as Florentine
Florentine calendar
The Florentine calendar was used in Italy in the Middle Ages. In this system, the new day begins at sunset. When the reference of a birth was, for example, "two hours into the day", this meant two hours after sunset...

reckoning: in this system, a reference like "two hours into the day" meant two hours after sunset and thus times during the evening need to be shifted back one calendar day in modern reckoning. Days such as Christmas Eve
Christmas Eve
Christmas Eve refers to the evening or entire day preceding Christmas Day, a widely celebrated festival commemorating the birth of Jesus of Nazareth that takes place on December 25...

, Halloween
Halloween
Hallowe'en , also known as Halloween or All Hallows' Eve, is a yearly holiday observed around the world on October 31, the night before All Saints' Day...

, and the Eve of Saint Agnes are the remnants of the older pattern when holidays began the evening before. Present common convention is for the civil day to begin at midnight, that is 00:00 (inclusive), and last a full 24 hours until 24:00 (exclusive).

In ancient Egypt
Ancient Egypt
Ancient Egypt was an ancient civilization of Northeastern Africa, concentrated along the lower reaches of the Nile River in what is now the modern country of Egypt. Egyptian civilization coalesced around 3150 BC with the political unification of Upper and Lower Egypt under the first pharaoh...

, the day was reckoned from sunrise to sunrise. Muslim
Muslim
A Muslim, also spelled Moslem, is an adherent of Islam, a monotheistic, Abrahamic religion based on the Quran, which Muslims consider the verbatim word of God as revealed to prophet Muhammad. "Muslim" is the Arabic term for "submitter" .Muslims believe that God is one and incomparable...

s fast from daybreak to sunset each day of the month of Ramadan
Ramadan is the ninth month of the Islamic calendar, and the month in which the Quran was revealed.Ramadan is the holiest of months in the Islamic calendar, and fasting in this month is one of the Five Pillars of Islam. The month is spent by Muslims fasting during the daylight hours from dawn to...

. The "Damascus Document
Damascus Document
The Damascus Document or Damascus Rule is one of the most interesting texts of the Dead Sea Scrolls because it is the only Qumran sectarian work that was known before the discovery of the Dead Sea Scrolls....

", copies of which were also found among the Dead Sea scrolls
The Dead Sea Scrolls are a collection of 972 texts from the Hebrew Bible and extra-biblical documents found between 1947 and 1956 on the northwest shore of the Dead Sea, from which they derive their name...

, states regarding Sabbath
Shabbat
Shabbat is the seventh day of the Jewish week and a day of rest in Judaism. Shabbat is observed from a few minutes before sunset on Friday evening until a few minutes after when one would expect to be able to see three stars in the sky on Saturday night. The exact times, therefore, differ from...

observance that "No one is to do any work on Friday from the moment that the sun's disk stands distant from the horizon by the length of its own diameter," presumably indicating that the monastic community responsible for producing this work counted the day as ending shortly before the sun had begun to set.

In the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

, nights are named after the previous day, e.g. "Friday night" usually means the entire night between Friday and Saturday. This is the opposite of the Jewish pattern. This difference from the civil day often leads to confusion. Events starting at midnight are often announced as occurring the day before. TV-guides tend to list nightly programs at the previous day, although programming a VCR
Videocassette recorder
The videocassette recorder , is a type of electro-mechanical device that uses removable videocassettes that contain magnetic tape for recording analog audio and analog video from broadcast television so that the images and sound can be played back at a more convenient time...

requires the strict logic of starting the new day at 00:00 (to further confuse the issue, VCRs set to the 12-hour clock notation will label this "12:00 AM"). Expressions like "today", "yesterday" and "tomorrow" become ambiguous during the night.

Validity of ticket
A ticket is a voucher that indicates that one has paid for admission to an event or establishment such as a theatre, movie theater, amusement park, zoo, museum, concert, or other attraction, or permission to travel on a vehicle such as an airliner, train, bus, or boat, typically because one has...

s, passes, etc., for a day or a number of days may end at midnight, or closing time, when that is earlier. However, if a service (e.g. public transport
Public transport
Public transport is a shared passenger transportation service which is available for use by the general public, as distinct from modes such as taxicab, car pooling or hired buses which are not shared by strangers without private arrangement.Public transport modes include buses, trolleybuses, trams...

) operates from for example, 6:00 to 1:00 the next day (which may be noted as 25:00), the last hour may well count as being part of the previous day (also for the arrangement of the timetable
Public transport timetable
A public transport timetable is a representation of public transport information to assist a passenger with planning a trip using public transport. A timetable details when vehicle will arrive and depart specified locations and may be organised for by route or for a particular stop...

). For services depending on the day ("closed on Sundays", "does not run on Fridays", and so on) there is a risk of ambiguity. As an example, for the Nederlandse Spoorwegen
Nederlandse Spoorwegen
Nederlandse Spoorwegen , or NS, is the principal passenger railway operator in the Netherlands.Its trains operate over the tracks of the Dutch national railinfrastructure, operated by ProRail, which was split off from NS in 2003...

(Dutch Railways), a day ticket is valid 28 hours, from 0:00 to 28:00 (that is, 4:00 the next day). To give another example, the validity of a pass on London Regional Transport
London Regional Transport
London Regional Transport was the organisation responsible for the public transport network in Greater London, UK from 1984-2000. In common with all London transport authorities from 1933 to 2000, the public name and operational brand of the organisation was London Transport.The organisation was...

services is until the end of the "transport day" -- that is to say, until 4:30 am on the day after the "expiry" date stamped on the pass.

## 24 hours vs daytime

To distinguish between a full day and daytime, the word nychthemeron
Nychthemeron
Nychthemeron , occasionally nycthemeron or nuchthemeron is a period of 24 consecutive hours...

(from Greek for a night and a day) may be used in English for the former, or more colloquially the term . In other languages, the latter is also often used. Other languages also have a separate word for a full day, such as יממה in Hebrew, dygn in Swedish, etmaal in Dutch, doba in Polish, сутки (sutki) in Russian, суткі (sutki) in Belarusian, døgn in Danish and доба́ (doba) in Ukrainian. In Italian, giorno is used to indicate a full day, while means daytime. In Spanish, singladura is used, but only as a marine unit of length, being the distance covered in 24 hours.

• 10
{{Other uses}}
{{pp-move-indef|small=yes}}

A day is a unit of time
Time
Time is a part of the measuring system used to sequence events, to compare the durations of events and the intervals between them, and to quantify rates of change such as the motions of objects....

, commonly defined as an interval equal to 24 hours. It also can mean that portion of the full day during which a location is illuminated by the light of the sun. The period of time measured from local noon to the following local noon is called a solar day
Solar time
Solar time is a reckoning of the passage of time based on the Sun's position in the sky. The fundamental unit of solar time is the day. Two types of solar time are apparent solar time and mean solar time .-Introduction:...

.

Several definitions of this universal human concept are used according to context, need and convenience. In 1967, the second
Second
The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock....

was redefined in terms of the wavelength of light, and it became the SI base unit
SI base unit
The International System of Units defines seven units of measure as a basic set from which all other SI units are derived. These SI base units and their physical quantities are:* metre for length...

of time
Time
Time is a part of the measuring system used to sequence events, to compare the durations of events and the intervals between them, and to quantify rates of change such as the motions of objects....

. The unit of measurement
Units of measurement
A unit of measurement is a definite magnitude of a physical quantity, defined and adopted by convention and/or by law, that is used as a standard for measurement of the same physical quantity. Any other value of the physical quantity can be expressed as a simple multiple of the unit of...

for time called "day", redefined in 1967 as 86,400 SI seconds and symbolized d, is not an SI
International System of Units
The International System of Units is the modern form of the metric system and is generally a system of units of measurement devised around seven base units and the convenience of the number ten. The older metric system included several groups of units...

unit, but it is accepted for use with SI. A civil day is usually also 86,400 seconds, plus or minus a possible leap second
Leap second
A leap second is a positive or negative one-second adjustment to the Coordinated Universal Time time scale that keeps it close to mean solar time. UTC, which is used as the basis for official time-of-day radio broadcasts for civil time, is maintained using extremely precise atomic clocks...

in Coordinated Universal Time UTC, and, in some locations, occasionally plus or minus an hour when changing from or to daylight saving time
Daylight saving time
Daylight saving time —also summer time in several countries including in British English and European official terminology —is the practice of temporarily advancing clocks during the summertime so that afternoons have more daylight and mornings have less...

. The word day may also refer to a day of the week
Week
A week is a time unit equal to seven days.The English word week continues an Old English wice, ultimately from a Common Germanic , from a root "turn, move, change"...

or to a calendar date
Calendar date
A date in a calendar is a reference to a particular day represented within a calendar system. The calendar date allows the specific day to be identified. The number of days between two dates may be calculated. For example, "24 " is ten days after "14 " in the Gregorian calendar. The date of a...

, as in answer to the question "On which day?" Day also refers to the part of the day that is not night — also known as 'daytime
Daytime (astronomy)
On Earth, daytime is roughly the period on any given point of the planet's surface during which it experiences natural illumination from indirect or direct sunlight....

'. The life patterns of humans and many other species are related to Earth's solar day and the cycle of day and night (see circadian rhythm
A circadian rhythm, popularly referred to as body clock, is an endogenously driven , roughly 24-hour cycle in biochemical, physiological, or behavioural processes. Circadian rhythms have been widely observed in plants, animals, fungi and cyanobacteria...

s).

The average length of a solar day on Earth is about 86,400 seconds (24 hours) and there are about 365.2422 solar days in one mean tropical year
Tropical year
A tropical year , for general purposes, is the length of time that the Sun takes to return to the same position in the cycle of seasons, as seen from Earth; for example, the time from vernal equinox to vernal equinox, or from summer solstice to summer solstice...

. Because celestial orbits are not perfectly circular, and thus objects travel at different speeds at various positions in their orbit, a solar day is not the same length of time throughout the orbital year. A day, understood as the span of time it takes for the Earth to make one entire rotation
Rotation
A rotation is a circular movement of an object around a center of rotation. A three-dimensional object rotates always around an imaginary line called a rotation axis. If the axis is within the body, and passes through its center of mass the body is said to rotate upon itself, or spin. A rotation...

with respect to the celestial background or a distant star (assumed, to be fixed), is called stellar day
Sidereal time
Sidereal time is a time-keeping system astronomers use to keep track of the direction to point their telescopes to view a given star in the night sky...

. This period of rotation is about 4 minutes less than 24 hours and there are about 366.2422 in one mean tropical year (one more stellar day than the number of solar days). Mainly due to tidal effects
Tidal acceleration
Tidal acceleration is an effect of the tidal forces between an orbiting natural satellite , and the primary planet that it orbits . The "acceleration" is usually negative, as it causes a gradual slowing and recession of a satellite in a prograde orbit away from the primary, and a corresponding...

, the Earth's rotational period is not constant, resulting in further minor variations for both solar days and stellar 'days'. Other planets and moons also have stellar and solar days.

## Introduction

Besides the day of 24 hours (86,400 seconds), the word day is used for several different spans of time based on the rotation of the Earth around its axis. An important one is the solar day, defined as the time it takes for the sun to return to the zenith
Zenith
The zenith is an imaginary point directly "above" a particular location, on the imaginary celestial sphere. "Above" means in the vertical direction opposite to the apparent gravitational force at that location. The opposite direction, i.e...

(its highest point in the sky). Because the Earth orbits the Sun elliptically as the Earth spins on an inclined axis, this period can be up to 7.9 seconds more than (or less than) 24 hours. On average over the year this day is equivalent to 24 hours (86,400 seconds).

A day, in the sense of daytime
DAYTIME
The Daytime Protocol is a service in the Internet Protocol Suite, defined in 1983 in RFC 867. It is intended for testing and measurement purposes in computer networks....

that is distinguished from night-time, is commonly defined as the period during which sunlight
Sunlight
Sunlight, in the broad sense, is the total frequency spectrum of electromagnetic radiation given off by the Sun. On Earth, sunlight is filtered through the Earth's atmosphere, and solar radiation is obvious as daylight when the Sun is above the horizon.When the direct solar radiation is not blocked...

directly reaches the ground, assuming that there are no local obstacles. The length of daytime averages slightly more than half of the 24-hour day. Two effects make daytime on average longer than nights. The Sun is not a point, but has an apparent size of about 32 minutes of arc
Minute of arc
A minute of arc, arcminute, or minute of angle , is a unit of angular measurement equal to one sixtieth of one degree. In turn, a second of arc or arcsecond is one sixtieth of one minute of arc....

. Additionally, the atmosphere
Earth's atmosphere
The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...

refracts
Refraction
Refraction is the change in direction of a wave due to a change in its speed. It is essentially a surface phenomenon . The phenomenon is mainly in governance to the law of conservation of energy. The proper explanation would be that due to change of medium, the phase velocity of the wave is changed...

sunlight in such a way that some of it reaches the ground even when the Sun is below the horizon by about 34 minutes of arc. So the first light reaches the ground when the centre of the Sun is still below the horizon by about 50 minutes of arc. The difference in time depends on the angle at which the Sun rises and sets (itself a function of latitude
Latitude
In geography, the latitude of a location on the Earth is the angular distance of that location south or north of the Equator. The latitude is an angle, and is usually measured in degrees . The equator has a latitude of 0°, the North pole has a latitude of 90° north , and the South pole has a...

), but can amount to around seven minutes.

Ancient custom has a new day start at either the rising or setting of the Sun on the local horizon (Italian reckoning, for example) The exact moment of, and the interval between, two sunrises or two sunsets depends on the geographical position (longitude
Longitude
Longitude is a geographic coordinate that specifies the east-west position of a point on the Earth's surface. It is an angular measurement, usually expressed in degrees, minutes and seconds, and denoted by the Greek letter lambda ....

as well as latitude), and the time of year
Year
A year is the orbital period of the Earth moving around the Sun. For an observer on Earth, this corresponds to the period it takes the Sun to complete one course throughout the zodiac along the ecliptic....

. This is the time as indicated by ancient hemispherical sundial
Sundial
A sundial is a device that measures time by the position of the Sun. In common designs such as the horizontal sundial, the sun casts a shadow from its style onto a surface marked with lines indicating the hours of the day. The style is the time-telling edge of the gnomon, often a thin rod or a...

s.

A more constant day can be defined by the Sun passing through the local meridian
Meridian (geography)
A meridian is an imaginary line on the Earth's surface from the North Pole to the South Pole that connects all locations along it with a given longitude. The position of a point along the meridian is given by its latitude. Each meridian is perpendicular to all circles of latitude...

, which happens at local noon
Noon
Noon is usually defined as 12 o'clock in the daytime. The word noon is also used informally to mean midday regarding the location of the sun not the middle of a persons day. Although this is a time around the middle of the day when people in many countries take a lunch break...

(upper culmination
Culmination
In astronomy, the culmination of a planet, star, constellation, etc. is the altitude reached when the object transits over an observer's meridian....

) or midnight
Midnight
Midnight is the transition time period from one day to the next: the moment when the date changes. In the Roman time system, midnight was halfway between sunset and sunrise, varying according to the seasons....

(lower culmination). The exact moment is dependent on the geographical longitude, and to a lesser extent on the time of the year. The length of such a day is nearly constant (24 hours ± 30 seconds). This is the time as indicated by modern sundials.

A further improvement defines a fictitious mean Sun that moves with constant speed along the celestial equator
Celestial equator
The celestial equator is a great circle on the imaginary celestial sphere, in the same plane as the Earth's equator. In other words, it is a projection of the terrestrial equator out into space...

; the speed is the same as the average speed of the real Sun, but this removes the variation over a year as the Earth moves along its orbit around the Sun (due to both its velocity and its axial tilt).

The Earth's day has increased in length over time. This phenomenon is due to tide
Tide
Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the moon and the sun and the rotation of the Earth....

s raised by the Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

which slow Earth's rotation. Because of the way the second is defined, the mean length of a day is now about 86,400.002 seconds, and is increasing by about 1.7 milliseconds per century (an average over the last 2,700 years). See tidal acceleration
Tidal acceleration
Tidal acceleration is an effect of the tidal forces between an orbiting natural satellite , and the primary planet that it orbits . The "acceleration" is usually negative, as it causes a gradual slowing and recession of a satellite in a prograde orbit away from the primary, and a corresponding...

for details. The length of one day was about 21.9 hours 620 million years ago as recorded by rhythmites (alternating layers in sandstone
Sandstone
Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...

); when the Earth was new about 4.5 billion years ago, it was probably around six hours as determined by computer simulations.{{citation needed|date=May 2011}} The length of day for the Earth or Proto-Earth before the event which created our moon by an impact is yet unknown.

## Etymology

The term comes from the Old English dæg, with its cognates such as Tag in German
German language
German is a West Germanic language, related to and classified alongside English and Dutch. With an estimated 90 – 98 million native speakers, German is one of the world's major languages and is the most widely-spoken first language in the European Union....

, and dag in Norwegian
Norwegian language
Norwegian is a North Germanic language spoken primarily in Norway, where it is the official language. Together with Swedish and Danish, Norwegian forms a continuum of more or less mutually intelligible local and regional variants .These Scandinavian languages together with the Faroese language...

, Danish
Danish language
Danish is a North Germanic language spoken by around six million people, principally in the country of Denmark. It is also spoken by 50,000 Germans of Danish ethnicity in the northern parts of Schleswig-Holstein, Germany, where it holds the status of minority language...

, Swedish
Swedish language
Swedish is a North Germanic language, spoken by approximately 10 million people, predominantly in Sweden and parts of Finland, especially along its coast and on the Åland islands. It is largely mutually intelligible with Norwegian and Danish...

and Dutch
Dutch language
Dutch is a West Germanic language and the native language of the majority of the population of the Netherlands, Belgium, and Suriname, the three member states of the Dutch Language Union. Most speakers live in the European Union, where it is a first language for about 23 million and a second...

.

## International System of Units (SI)

A day is defined as 86,400 seconds.

A day on the UTC time scale
Time scale
A time scale specifies divisions of time.*Time standard, a specification of either the rate at which time passes, or points in time, or both*Duration , a quantity of time...

can include a negative or positive leap second
Leap second
A leap second is a positive or negative one-second adjustment to the Coordinated Universal Time time scale that keeps it close to mean solar time. UTC, which is used as the basis for official time-of-day radio broadcasts for civil time, is maintained using extremely precise atomic clocks...

, and can therefore have a length of 86,399 or 86,401 seconds.

The International Bureau of Weights and Measures
International Bureau of Weights and Measures
The International Bureau of Weights and Measures , is an international standards organisation, one of three such organisations established to maintain the International System of Units under the terms of the Metre Convention...

(BIPM) currently defines a second as
… the duration of 9 192 631 770 periods of the radiation
Radioactive decay is the process by which an atomic nucleus of an unstable atom loses energy by emitting ionizing particles . The emission is spontaneous, in that the atom decays without any physical interaction with another particle from outside the atom...

corresponding to the transition between two hyperfine
Hyperfine structure
The term hyperfine structure refers to a collection of different effects leading to small shifts and splittings in the energy levels of atoms, molecules and ions. The name is a reference to the fine structure which results from the interaction between the magnetic moments associated with electron...

levels
Energy level
A quantum mechanical system or particle that is bound -- that is, confined spatially—can only take on certain discrete values of energy. This contrasts with classical particles, which can have any energy. These discrete values are called energy levels...

of the ground state
Stationary state
In quantum mechanics, a stationary state is an eigenvector of the Hamiltonian, implying the probability density associated with the wavefunction is independent of time . This corresponds to a quantum state with a single definite energy...

of the caesium
Caesium
Caesium or cesium is the chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-gold alkali metal with a melting point of 28 °C , which makes it one of only five elemental metals that are liquid at room temperature...

133 atom
Atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...

.

This makes the SI-based day last exactly 794,243,384,928,000 of those periods.

### Decimal and metric time

{{Main|Metric time}}
In the 19th century it had also been suggested to make a decimal fraction ({{frac|1|10,000}} or {{frac|1|100,000}}) of an astronomic day the base unit of time. This was an afterglow of decimal time
Decimal time
Decimal time is the representation of the time of day using units which are decimally related. This term is often used to refer specifically to French Revolutionary Time, which divides the day into 10 decimal hours, each decimal hour into 100 decimal minutes and each decimal minute into 100 decimal...

and calendar
French Republican Calendar
The French Republican Calendar or French Revolutionary Calendar was a calendar created and implemented during the French Revolution, and used by the French government for about 12 years from late 1793 to 1805, and for 18 days by the Paris Commune in 1871...

, which had been given up already for its difficulty to comply with familiar units. The still most successful candidate is the centiday = 14.4 minutes, as a shorter quarter
Quarter
Quarter is one fourth, ¼, or 25%, and may refer to:*Quarter , a section or area, usually of a town*Quarter , valued at one-fourth of a U.S...

of an hour and also close to the SI target kilosecond
Kilosecond
A kilosecond is 1000 seconds , so there are 86.4 kiloseconds in a 24 hour day, and 604.8 kiloseconds in a week. The second is the International System of Units base unit of time, combine with the prefix kilo- which means 1000, results in the definition of a kilosecond...

and old Chinese ke
Ke (unit)
The ke is a traditional Chinese unit of decimal time lasting approximately a quarter of a western hour. Traditionally the ke divides a day into 100 equal intervals of 14.4 minutes . The ke is equivalent to the centiday , a non-SI prefixed unit...

.

## Astronomy

A day of exactly 86,400 SI seconds is the astronomical unit of time (the second is not preferred in astronomy).

For a given planet, there are three types of day defined in astronomy
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...

:
• stellar day - an entire rotation of a planet with respect to the distant star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...

s
• sidereal day
Sidereal time
Sidereal time is a time-keeping system astronomers use to keep track of the direction to point their telescopes to view a given star in the night sky...

- a single rotation of a planet with respect to the vernal equinox
• mean solar day
Solar time
Solar time is a reckoning of the passage of time based on the Sun's position in the sky. The fundamental unit of solar time is the day. Two types of solar time are apparent solar time and mean solar time .-Introduction:...

- average time of a single rotation of a planet with respect to the sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

as the central star

For Earth, the stellar day and the sidereal day are nearly of the same length and about 3 minutes 56 seconds shorter than the solar day. In fact, the Earth spins 366 times about its axis during a 365-day year, because the Earth's revolution about the Sun removes one apparent turn of the Sun about the Earth.

## Colloquial

The word refers to various relatedly defined ideas, including the following:
• 24 hours (exactly)
• the period of light when the Sun is above the local horizon
Horizon
The horizon is the apparent line that separates earth from sky, the line that divides all visible directions into two categories: those that intersect the Earth's surface, and those that do not. At many locations, the true horizon is obscured by trees, buildings, mountains, etc., and the resulting...

(that is, the time period from sunrise
Sunrise
Sunrise is the instant at which the upper edge of the Sun appears above the horizon in the east. Sunrise should not be confused with dawn, which is the point at which the sky begins to lighten, some time before the sun itself appears, ending twilight...

to sunset
Sunset
Sunset or sundown is the daily disappearance of the Sun below the horizon in the west as a result of Earth's rotation.The time of sunset is defined in astronomy as the moment the trailing edge of the Sun's disk disappears below the horizon in the west...

);
• the full day covering a dark and a light period, beginning from the beginning of the dark period or from a point near the middle of the dark period;
• a full dark and light period, sometimes called a nychthemeron
Nychthemeron
Nychthemeron , occasionally nycthemeron or nuchthemeron is a period of 24 consecutive hours...

in English
English language
English is a West Germanic language that arose in the Anglo-Saxon kingdoms of England and spread into what was to become south-east Scotland under the influence of the Anglian medieval kingdom of Northumbria...

, from the Greek
Greek language
Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...

for night-day;
• the time period from 6:00 AM
12-hour clock
The 12-hour clock is a time conversion convention in which the 24 hours of the day are divided into two periods called ante meridiem and post meridiem...

to 6:00 PM or 9:00 PM or some other fixed clock period overlapping or set off from other time periods such as "morning
Morning
The word morning originally referred to the sunrise. Morning precedes midday, afternoon, and night in the sequence of a day.Morning is the part of the day usually reckoned from dawn to noon...

", "evening
Evening
Evening is the period between the late afternoon and night when daylight is decreasing, around dinner time at 6pm. Though the term is subjective, evening is typically understood to begin before sunset, during the close of the standard business day and extend until nightfall, the beginning of night...

", or "night
Night
Night or nighttime is the period of time when the sun is below the horizon. This occurs after dusk. The opposite of night is day...

".

## Civil day

For civil purposes a common clock time has been defined for an entire region based on the mean local solar time at some central meridian. Such time zone
Time zone
A time zone is a region on Earth that has a uniform standard time for legal, commercial, and social purposes. In order for the same clock time to always correspond to the same portion of the day as the Earth rotates , different places on the Earth need to have different clock times...

s began to be adopted about the middle of the 19th century when railroad
Rail transport
Rail transport is a means of conveyance of passengers and goods by way of wheeled vehicles running on rail tracks. In contrast to road transport, where vehicles merely run on a prepared surface, rail vehicles are also directionally guided by the tracks they run on...

s with regular schedules came into use, with most major countries having adopted them by 1929. For the whole world, 40 such time zones are now in use. The main one is "world time" or Coordinated Universal Time
Coordinated Universal Time
Coordinated Universal Time is the primary time standard by which the world regulates clocks and time. It is one of several closely related successors to Greenwich Mean Time. Computer servers, online services and other entities that rely on having a universally accepted time use UTC for that purpose...

(UTC).

The present common convention has the civil day starting at midnight, which is near the time of the lower culmination of the mean Sun on the central meridian of the time zone. A day is commonly divided into 24 hours of 60 minutes of 60 seconds each.

## Leap seconds

To keep the civil day aligned with the apparent movement of the Sun, positive or negative leap second
Leap second
A leap second is a positive or negative one-second adjustment to the Coordinated Universal Time time scale that keeps it close to mean solar time. UTC, which is used as the basis for official time-of-day radio broadcasts for civil time, is maintained using extremely precise atomic clocks...

s may be inserted.

A civil clock day is typically 86,400 SI seconds long, but will be 86,401 s or 86,399 s long in the event of a leap second.

Leap seconds are announced in advance by the International Earth Rotation and Reference Systems Service
International Earth Rotation and Reference Systems Service
The International Earth Rotation and Reference Systems Service , formerly the International Earth Rotation Service, is the body responsible for maintaining global time and reference frame standards, notably through its Earth Orientation Parameter and International Celestial Reference System ...

which measures the Earth's rotation and determines whether a leap second is necessary. Leap seconds occur only at the end of a UTC month, and have only ever been inserted at the end of June 30 or December 31.

## Boundaries of the day

For most diurnal animals, the day naturally begins at dawn and ends at sunset.Humans, with our cultural norms and scientific knowledge, have supplanted Nature with several different conceptions of the day's boundaries. The Jewish
Judaism
Judaism ) is the "religion, philosophy, and way of life" of the Jewish people...

day begins at either sunset or at nightfall (when three second-magnitude
Magnitude (astronomy)
Magnitude is the logarithmic measure of the brightness of an object, in astronomy, measured in a specific wavelength or passband, usually in optical or near-infrared wavelengths.-Background:...

stars appear). Medieval
Middle Ages
The Middle Ages is a periodization of European history from the 5th century to the 15th century. The Middle Ages follows the fall of the Western Roman Empire in 476 and precedes the Early Modern Era. It is the middle period of a three-period division of Western history: Classic, Medieval and Modern...

Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...

followed this tradition, known as Florentine
Florentine calendar
The Florentine calendar was used in Italy in the Middle Ages. In this system, the new day begins at sunset. When the reference of a birth was, for example, "two hours into the day", this meant two hours after sunset...

reckoning: in this system, a reference like "two hours into the day" meant two hours after sunset and thus times during the evening need to be shifted back one calendar day in modern reckoning. Days such as Christmas Eve
Christmas Eve
Christmas Eve refers to the evening or entire day preceding Christmas Day, a widely celebrated festival commemorating the birth of Jesus of Nazareth that takes place on December 25...

, Halloween
Halloween
Hallowe'en , also known as Halloween or All Hallows' Eve, is a yearly holiday observed around the world on October 31, the night before All Saints' Day...

, and the Eve of Saint Agnes are the remnants of the older pattern when holidays began the evening before. Present common convention is for the civil day to begin at midnight, that is 00:00 (inclusive), and last a full 24 hours until 24:00 (exclusive).

In ancient Egypt
Ancient Egypt
Ancient Egypt was an ancient civilization of Northeastern Africa, concentrated along the lower reaches of the Nile River in what is now the modern country of Egypt. Egyptian civilization coalesced around 3150 BC with the political unification of Upper and Lower Egypt under the first pharaoh...

, the day was reckoned from sunrise to sunrise. Muslim
Muslim
A Muslim, also spelled Moslem, is an adherent of Islam, a monotheistic, Abrahamic religion based on the Quran, which Muslims consider the verbatim word of God as revealed to prophet Muhammad. "Muslim" is the Arabic term for "submitter" .Muslims believe that God is one and incomparable...

s fast from daybreak to sunset each day of the month of Ramadan
Ramadan is the ninth month of the Islamic calendar, and the month in which the Quran was revealed.Ramadan is the holiest of months in the Islamic calendar, and fasting in this month is one of the Five Pillars of Islam. The month is spent by Muslims fasting during the daylight hours from dawn to...

. The "Damascus Document
Damascus Document
The Damascus Document or Damascus Rule is one of the most interesting texts of the Dead Sea Scrolls because it is the only Qumran sectarian work that was known before the discovery of the Dead Sea Scrolls....

", copies of which were also found among the Dead Sea scrolls
The Dead Sea Scrolls are a collection of 972 texts from the Hebrew Bible and extra-biblical documents found between 1947 and 1956 on the northwest shore of the Dead Sea, from which they derive their name...

, states regarding Sabbath
Shabbat
Shabbat is the seventh day of the Jewish week and a day of rest in Judaism. Shabbat is observed from a few minutes before sunset on Friday evening until a few minutes after when one would expect to be able to see three stars in the sky on Saturday night. The exact times, therefore, differ from...

observance that "No one is to do any work on Friday from the moment that the sun's disk stands distant from the horizon by the length of its own diameter," presumably indicating that the monastic community responsible for producing this work counted the day as ending shortly before the sun had begun to set.

In the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

, nights are named after the previous day, e.g. "Friday night" usually means the entire night between Friday and Saturday. This is the opposite of the Jewish pattern. This difference from the civil day often leads to confusion. Events starting at midnight are often announced as occurring the day before. TV-guides tend to list nightly programs at the previous day, although programming a VCR
Videocassette recorder
The videocassette recorder , is a type of electro-mechanical device that uses removable videocassettes that contain magnetic tape for recording analog audio and analog video from broadcast television so that the images and sound can be played back at a more convenient time...

requires the strict logic of starting the new day at 00:00 (to further confuse the issue, VCRs set to the 12-hour clock notation will label this "12:00 AM"). Expressions like "today", "yesterday" and "tomorrow" become ambiguous during the night.

Validity of ticket
A ticket is a voucher that indicates that one has paid for admission to an event or establishment such as a theatre, movie theater, amusement park, zoo, museum, concert, or other attraction, or permission to travel on a vehicle such as an airliner, train, bus, or boat, typically because one has...

s, passes, etc., for a day or a number of days may end at midnight, or closing time, when that is earlier. However, if a service (e.g. public transport
Public transport
Public transport is a shared passenger transportation service which is available for use by the general public, as distinct from modes such as taxicab, car pooling or hired buses which are not shared by strangers without private arrangement.Public transport modes include buses, trolleybuses, trams...

) operates from for example, 6:00 to 1:00 the next day (which may be noted as 25:00), the last hour may well count as being part of the previous day (also for the arrangement of the timetable
Public transport timetable
A public transport timetable is a representation of public transport information to assist a passenger with planning a trip using public transport. A timetable details when vehicle will arrive and depart specified locations and may be organised for by route or for a particular stop...

). For services depending on the day ("closed on Sundays", "does not run on Fridays", and so on) there is a risk of ambiguity. As an example, for the Nederlandse Spoorwegen
Nederlandse Spoorwegen
Nederlandse Spoorwegen , or NS, is the principal passenger railway operator in the Netherlands.Its trains operate over the tracks of the Dutch national railinfrastructure, operated by ProRail, which was split off from NS in 2003...

(Dutch Railways), a day ticket is valid 28 hours, from 0:00 to 28:00 (that is, 4:00 the next day). To give another example, the validity of a pass on London Regional Transport
London Regional Transport
London Regional Transport was the organisation responsible for the public transport network in Greater London, UK from 1984-2000. In common with all London transport authorities from 1933 to 2000, the public name and operational brand of the organisation was London Transport.The organisation was...

services is until the end of the "transport day" -- that is to say, until 4:30 am on the day after the "expiry" date stamped on the pass.

## 24 hours vs daytime

To distinguish between a full day and daytime, the word nychthemeron
Nychthemeron
Nychthemeron , occasionally nycthemeron or nuchthemeron is a period of 24 consecutive hours...

(from Greek for a night and a day) may be used in English for the former, or more colloquially the term {{nowrap|24 hours}}. In other languages, the latter is also often used. Other languages also have a separate word for a full day, such as יממה in Hebrew, dygn in Swedish, etmaal in Dutch, doba in Polish, сутки (sutki) in Russian, суткі (sutki) in Belarusian, døgn in Danish and доба́ (doba) in Ukrainian. In Italian, giorno is used to indicate a full day, while means daytime. In Spanish, singladura is used, but only as a marine unit of length, being the distance covered in 24 hours.{{inline relevance|date=June 2011}}

• 10
{{Other uses}}
{{pp-move-indef|small=yes}}

A day is a unit of time
Time
Time is a part of the measuring system used to sequence events, to compare the durations of events and the intervals between them, and to quantify rates of change such as the motions of objects....

, commonly defined as an interval equal to 24 hours. It also can mean that portion of the full day during which a location is illuminated by the light of the sun. The period of time measured from local noon to the following local noon is called a solar day
Solar time
Solar time is a reckoning of the passage of time based on the Sun's position in the sky. The fundamental unit of solar time is the day. Two types of solar time are apparent solar time and mean solar time .-Introduction:...

.

Several definitions of this universal human concept are used according to context, need and convenience. In 1967, the second
Second
The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock....

was redefined in terms of the wavelength of light, and it became the SI base unit
SI base unit
The International System of Units defines seven units of measure as a basic set from which all other SI units are derived. These SI base units and their physical quantities are:* metre for length...

of time
Time
Time is a part of the measuring system used to sequence events, to compare the durations of events and the intervals between them, and to quantify rates of change such as the motions of objects....

. The unit of measurement
Units of measurement
A unit of measurement is a definite magnitude of a physical quantity, defined and adopted by convention and/or by law, that is used as a standard for measurement of the same physical quantity. Any other value of the physical quantity can be expressed as a simple multiple of the unit of...

for time called "day", redefined in 1967 as 86,400 SI seconds and symbolized d, is not an SI
International System of Units
The International System of Units is the modern form of the metric system and is generally a system of units of measurement devised around seven base units and the convenience of the number ten. The older metric system included several groups of units...

unit, but it is accepted for use with SI. A civil day is usually also 86,400 seconds, plus or minus a possible leap second
Leap second
A leap second is a positive or negative one-second adjustment to the Coordinated Universal Time time scale that keeps it close to mean solar time. UTC, which is used as the basis for official time-of-day radio broadcasts for civil time, is maintained using extremely precise atomic clocks...

in Coordinated Universal Time UTC, and, in some locations, occasionally plus or minus an hour when changing from or to daylight saving time
Daylight saving time
Daylight saving time —also summer time in several countries including in British English and European official terminology —is the practice of temporarily advancing clocks during the summertime so that afternoons have more daylight and mornings have less...

. The word day may also refer to a day of the week
Week
A week is a time unit equal to seven days.The English word week continues an Old English wice, ultimately from a Common Germanic , from a root "turn, move, change"...

or to a calendar date
Calendar date
A date in a calendar is a reference to a particular day represented within a calendar system. The calendar date allows the specific day to be identified. The number of days between two dates may be calculated. For example, "24 " is ten days after "14 " in the Gregorian calendar. The date of a...

, as in answer to the question "On which day?" Day also refers to the part of the day that is not night — also known as 'daytime
Daytime (astronomy)
On Earth, daytime is roughly the period on any given point of the planet's surface during which it experiences natural illumination from indirect or direct sunlight....

'. The life patterns of humans and many other species are related to Earth's solar day and the cycle of day and night (see circadian rhythm
A circadian rhythm, popularly referred to as body clock, is an endogenously driven , roughly 24-hour cycle in biochemical, physiological, or behavioural processes. Circadian rhythms have been widely observed in plants, animals, fungi and cyanobacteria...

s).

The average length of a solar day on Earth is about 86,400 seconds (24 hours) and there are about 365.2422 solar days in one mean tropical year
Tropical year
A tropical year , for general purposes, is the length of time that the Sun takes to return to the same position in the cycle of seasons, as seen from Earth; for example, the time from vernal equinox to vernal equinox, or from summer solstice to summer solstice...

. Because celestial orbits are not perfectly circular, and thus objects travel at different speeds at various positions in their orbit, a solar day is not the same length of time throughout the orbital year. A day, understood as the span of time it takes for the Earth to make one entire rotation
Rotation
A rotation is a circular movement of an object around a center of rotation. A three-dimensional object rotates always around an imaginary line called a rotation axis. If the axis is within the body, and passes through its center of mass the body is said to rotate upon itself, or spin. A rotation...

with respect to the celestial background or a distant star (assumed, to be fixed), is called stellar day
Sidereal time
Sidereal time is a time-keeping system astronomers use to keep track of the direction to point their telescopes to view a given star in the night sky...

. This period of rotation is about 4 minutes less than 24 hours and there are about 366.2422 in one mean tropical year (one more stellar day than the number of solar days). Mainly due to tidal effects
Tidal acceleration
Tidal acceleration is an effect of the tidal forces between an orbiting natural satellite , and the primary planet that it orbits . The "acceleration" is usually negative, as it causes a gradual slowing and recession of a satellite in a prograde orbit away from the primary, and a corresponding...

, the Earth's rotational period is not constant, resulting in further minor variations for both solar days and stellar 'days'. Other planets and moons also have stellar and solar days.

## Introduction

Besides the day of 24 hours (86,400 seconds), the word day is used for several different spans of time based on the rotation of the Earth around its axis. An important one is the solar day, defined as the time it takes for the sun to return to the zenith
Zenith
The zenith is an imaginary point directly "above" a particular location, on the imaginary celestial sphere. "Above" means in the vertical direction opposite to the apparent gravitational force at that location. The opposite direction, i.e...

(its highest point in the sky). Because the Earth orbits the Sun elliptically as the Earth spins on an inclined axis, this period can be up to 7.9 seconds more than (or less than) 24 hours. On average over the year this day is equivalent to 24 hours (86,400 seconds).

A day, in the sense of daytime
DAYTIME
The Daytime Protocol is a service in the Internet Protocol Suite, defined in 1983 in RFC 867. It is intended for testing and measurement purposes in computer networks....

that is distinguished from night-time, is commonly defined as the period during which sunlight
Sunlight
Sunlight, in the broad sense, is the total frequency spectrum of electromagnetic radiation given off by the Sun. On Earth, sunlight is filtered through the Earth's atmosphere, and solar radiation is obvious as daylight when the Sun is above the horizon.When the direct solar radiation is not blocked...

directly reaches the ground, assuming that there are no local obstacles. The length of daytime averages slightly more than half of the 24-hour day. Two effects make daytime on average longer than nights. The Sun is not a point, but has an apparent size of about 32 minutes of arc
Minute of arc
A minute of arc, arcminute, or minute of angle , is a unit of angular measurement equal to one sixtieth of one degree. In turn, a second of arc or arcsecond is one sixtieth of one minute of arc....

. Additionally, the atmosphere
Earth's atmosphere
The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...

refracts
Refraction
Refraction is the change in direction of a wave due to a change in its speed. It is essentially a surface phenomenon . The phenomenon is mainly in governance to the law of conservation of energy. The proper explanation would be that due to change of medium, the phase velocity of the wave is changed...

sunlight in such a way that some of it reaches the ground even when the Sun is below the horizon by about 34 minutes of arc. So the first light reaches the ground when the centre of the Sun is still below the horizon by about 50 minutes of arc. The difference in time depends on the angle at which the Sun rises and sets (itself a function of latitude
Latitude
In geography, the latitude of a location on the Earth is the angular distance of that location south or north of the Equator. The latitude is an angle, and is usually measured in degrees . The equator has a latitude of 0°, the North pole has a latitude of 90° north , and the South pole has a...

), but can amount to around seven minutes.

Ancient custom has a new day start at either the rising or setting of the Sun on the local horizon (Italian reckoning, for example) The exact moment of, and the interval between, two sunrises or two sunsets depends on the geographical position (longitude
Longitude
Longitude is a geographic coordinate that specifies the east-west position of a point on the Earth's surface. It is an angular measurement, usually expressed in degrees, minutes and seconds, and denoted by the Greek letter lambda ....

as well as latitude), and the time of year
Year
A year is the orbital period of the Earth moving around the Sun. For an observer on Earth, this corresponds to the period it takes the Sun to complete one course throughout the zodiac along the ecliptic....

. This is the time as indicated by ancient hemispherical sundial
Sundial
A sundial is a device that measures time by the position of the Sun. In common designs such as the horizontal sundial, the sun casts a shadow from its style onto a surface marked with lines indicating the hours of the day. The style is the time-telling edge of the gnomon, often a thin rod or a...

s.

A more constant day can be defined by the Sun passing through the local meridian
Meridian (geography)
A meridian is an imaginary line on the Earth's surface from the North Pole to the South Pole that connects all locations along it with a given longitude. The position of a point along the meridian is given by its latitude. Each meridian is perpendicular to all circles of latitude...

, which happens at local noon
Noon
Noon is usually defined as 12 o'clock in the daytime. The word noon is also used informally to mean midday regarding the location of the sun not the middle of a persons day. Although this is a time around the middle of the day when people in many countries take a lunch break...

(upper culmination
Culmination
In astronomy, the culmination of a planet, star, constellation, etc. is the altitude reached when the object transits over an observer's meridian....

) or midnight
Midnight
Midnight is the transition time period from one day to the next: the moment when the date changes. In the Roman time system, midnight was halfway between sunset and sunrise, varying according to the seasons....

(lower culmination). The exact moment is dependent on the geographical longitude, and to a lesser extent on the time of the year. The length of such a day is nearly constant (24 hours ± 30 seconds). This is the time as indicated by modern sundials.

A further improvement defines a fictitious mean Sun that moves with constant speed along the celestial equator
Celestial equator
The celestial equator is a great circle on the imaginary celestial sphere, in the same plane as the Earth's equator. In other words, it is a projection of the terrestrial equator out into space...

; the speed is the same as the average speed of the real Sun, but this removes the variation over a year as the Earth moves along its orbit around the Sun (due to both its velocity and its axial tilt).

The Earth's day has increased in length over time. This phenomenon is due to tide
Tide
Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the moon and the sun and the rotation of the Earth....

s raised by the Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

which slow Earth's rotation. Because of the way the second is defined, the mean length of a day is now about 86,400.002 seconds, and is increasing by about 1.7 milliseconds per century (an average over the last 2,700 years). See tidal acceleration
Tidal acceleration
Tidal acceleration is an effect of the tidal forces between an orbiting natural satellite , and the primary planet that it orbits . The "acceleration" is usually negative, as it causes a gradual slowing and recession of a satellite in a prograde orbit away from the primary, and a corresponding...

for details. The length of one day was about 21.9 hours 620 million years ago as recorded by rhythmites (alternating layers in sandstone
Sandstone
Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...

); when the Earth was new about 4.5 billion years ago, it was probably around six hours as determined by computer simulations.{{citation needed|date=May 2011}} The length of day for the Earth or Proto-Earth before the event which created our moon by an impact is yet unknown.

## Etymology

The term comes from the Old English dæg, with its cognates such as Tag in German
German language
German is a West Germanic language, related to and classified alongside English and Dutch. With an estimated 90 – 98 million native speakers, German is one of the world's major languages and is the most widely-spoken first language in the European Union....

, and dag in Norwegian
Norwegian language
Norwegian is a North Germanic language spoken primarily in Norway, where it is the official language. Together with Swedish and Danish, Norwegian forms a continuum of more or less mutually intelligible local and regional variants .These Scandinavian languages together with the Faroese language...

, Danish
Danish language
Danish is a North Germanic language spoken by around six million people, principally in the country of Denmark. It is also spoken by 50,000 Germans of Danish ethnicity in the northern parts of Schleswig-Holstein, Germany, where it holds the status of minority language...

, Swedish
Swedish language
Swedish is a North Germanic language, spoken by approximately 10 million people, predominantly in Sweden and parts of Finland, especially along its coast and on the Åland islands. It is largely mutually intelligible with Norwegian and Danish...

and Dutch
Dutch language
Dutch is a West Germanic language and the native language of the majority of the population of the Netherlands, Belgium, and Suriname, the three member states of the Dutch Language Union. Most speakers live in the European Union, where it is a first language for about 23 million and a second...

.

## International System of Units (SI)

A day is defined as 86,400 seconds.

A day on the UTC time scale
Time scale
A time scale specifies divisions of time.*Time standard, a specification of either the rate at which time passes, or points in time, or both*Duration , a quantity of time...

can include a negative or positive leap second
Leap second
A leap second is a positive or negative one-second adjustment to the Coordinated Universal Time time scale that keeps it close to mean solar time. UTC, which is used as the basis for official time-of-day radio broadcasts for civil time, is maintained using extremely precise atomic clocks...

, and can therefore have a length of 86,399 or 86,401 seconds.

The International Bureau of Weights and Measures
International Bureau of Weights and Measures
The International Bureau of Weights and Measures , is an international standards organisation, one of three such organisations established to maintain the International System of Units under the terms of the Metre Convention...

(BIPM) currently defines a second as
… the duration of 9 192 631 770 periods of the radiation
Radioactive decay is the process by which an atomic nucleus of an unstable atom loses energy by emitting ionizing particles . The emission is spontaneous, in that the atom decays without any physical interaction with another particle from outside the atom...

corresponding to the transition between two hyperfine
Hyperfine structure
The term hyperfine structure refers to a collection of different effects leading to small shifts and splittings in the energy levels of atoms, molecules and ions. The name is a reference to the fine structure which results from the interaction between the magnetic moments associated with electron...

levels
Energy level
A quantum mechanical system or particle that is bound -- that is, confined spatially—can only take on certain discrete values of energy. This contrasts with classical particles, which can have any energy. These discrete values are called energy levels...

of the ground state
Stationary state
In quantum mechanics, a stationary state is an eigenvector of the Hamiltonian, implying the probability density associated with the wavefunction is independent of time . This corresponds to a quantum state with a single definite energy...

of the caesium
Caesium
Caesium or cesium is the chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-gold alkali metal with a melting point of 28 °C , which makes it one of only five elemental metals that are liquid at room temperature...

133 atom
Atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...

.

This makes the SI-based day last exactly 794,243,384,928,000 of those periods.

### Decimal and metric time

{{Main|Metric time}}
In the 19th century it had also been suggested to make a decimal fraction ({{frac|1|10,000}} or {{frac|1|100,000}}) of an astronomic day the base unit of time. This was an afterglow of decimal time
Decimal time
Decimal time is the representation of the time of day using units which are decimally related. This term is often used to refer specifically to French Revolutionary Time, which divides the day into 10 decimal hours, each decimal hour into 100 decimal minutes and each decimal minute into 100 decimal...

and calendar
French Republican Calendar
The French Republican Calendar or French Revolutionary Calendar was a calendar created and implemented during the French Revolution, and used by the French government for about 12 years from late 1793 to 1805, and for 18 days by the Paris Commune in 1871...

, which had been given up already for its difficulty to comply with familiar units. The still most successful candidate is the centiday = 14.4 minutes, as a shorter quarter
Quarter
Quarter is one fourth, ¼, or 25%, and may refer to:*Quarter , a section or area, usually of a town*Quarter , valued at one-fourth of a U.S...

of an hour and also close to the SI target kilosecond
Kilosecond
A kilosecond is 1000 seconds , so there are 86.4 kiloseconds in a 24 hour day, and 604.8 kiloseconds in a week. The second is the International System of Units base unit of time, combine with the prefix kilo- which means 1000, results in the definition of a kilosecond...

and old Chinese ke
Ke (unit)
The ke is a traditional Chinese unit of decimal time lasting approximately a quarter of a western hour. Traditionally the ke divides a day into 100 equal intervals of 14.4 minutes . The ke is equivalent to the centiday , a non-SI prefixed unit...

.

## Astronomy

A day of exactly 86,400 SI seconds is the astronomical unit of time (the second is not preferred in astronomy).

For a given planet, there are three types of day defined in astronomy
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...

:
• stellar day - an entire rotation of a planet with respect to the distant star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...

s
• sidereal day
Sidereal time
Sidereal time is a time-keeping system astronomers use to keep track of the direction to point their telescopes to view a given star in the night sky...

- a single rotation of a planet with respect to the vernal equinox
• mean solar day
Solar time
Solar time is a reckoning of the passage of time based on the Sun's position in the sky. The fundamental unit of solar time is the day. Two types of solar time are apparent solar time and mean solar time .-Introduction:...

- average time of a single rotation of a planet with respect to the sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

as the central star

For Earth, the stellar day and the sidereal day are nearly of the same length and about 3 minutes 56 seconds shorter than the solar day. In fact, the Earth spins 366 times about its axis during a 365-day year, because the Earth's revolution about the Sun removes one apparent turn of the Sun about the Earth.

## Colloquial

The word refers to various relatedly defined ideas, including the following:
• 24 hours (exactly)
• the period of light when the Sun is above the local horizon
Horizon
The horizon is the apparent line that separates earth from sky, the line that divides all visible directions into two categories: those that intersect the Earth's surface, and those that do not. At many locations, the true horizon is obscured by trees, buildings, mountains, etc., and the resulting...

(that is, the time period from sunrise
Sunrise
Sunrise is the instant at which the upper edge of the Sun appears above the horizon in the east. Sunrise should not be confused with dawn, which is the point at which the sky begins to lighten, some time before the sun itself appears, ending twilight...

to sunset
Sunset
Sunset or sundown is the daily disappearance of the Sun below the horizon in the west as a result of Earth's rotation.The time of sunset is defined in astronomy as the moment the trailing edge of the Sun's disk disappears below the horizon in the west...

);
• the full day covering a dark and a light period, beginning from the beginning of the dark period or from a point near the middle of the dark period;
• a full dark and light period, sometimes called a nychthemeron
Nychthemeron
Nychthemeron , occasionally nycthemeron or nuchthemeron is a period of 24 consecutive hours...

in English
English language
English is a West Germanic language that arose in the Anglo-Saxon kingdoms of England and spread into what was to become south-east Scotland under the influence of the Anglian medieval kingdom of Northumbria...

, from the Greek
Greek language
Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...

for night-day;
• the time period from 6:00 AM
12-hour clock
The 12-hour clock is a time conversion convention in which the 24 hours of the day are divided into two periods called ante meridiem and post meridiem...

to 6:00 PM or 9:00 PM or some other fixed clock period overlapping or set off from other time periods such as "morning
Morning
The word morning originally referred to the sunrise. Morning precedes midday, afternoon, and night in the sequence of a day.Morning is the part of the day usually reckoned from dawn to noon...

", "evening
Evening
Evening is the period between the late afternoon and night when daylight is decreasing, around dinner time at 6pm. Though the term is subjective, evening is typically understood to begin before sunset, during the close of the standard business day and extend until nightfall, the beginning of night...

", or "night
Night
Night or nighttime is the period of time when the sun is below the horizon. This occurs after dusk. The opposite of night is day...

".

## Civil day

For civil purposes a common clock time has been defined for an entire region based on the mean local solar time at some central meridian. Such time zone
Time zone
A time zone is a region on Earth that has a uniform standard time for legal, commercial, and social purposes. In order for the same clock time to always correspond to the same portion of the day as the Earth rotates , different places on the Earth need to have different clock times...

s began to be adopted about the middle of the 19th century when railroad
Rail transport
Rail transport is a means of conveyance of passengers and goods by way of wheeled vehicles running on rail tracks. In contrast to road transport, where vehicles merely run on a prepared surface, rail vehicles are also directionally guided by the tracks they run on...

s with regular schedules came into use, with most major countries having adopted them by 1929. For the whole world, 40 such time zones are now in use. The main one is "world time" or Coordinated Universal Time
Coordinated Universal Time
Coordinated Universal Time is the primary time standard by which the world regulates clocks and time. It is one of several closely related successors to Greenwich Mean Time. Computer servers, online services and other entities that rely on having a universally accepted time use UTC for that purpose...

(UTC).

The present common convention has the civil day starting at midnight, which is near the time of the lower culmination of the mean Sun on the central meridian of the time zone. A day is commonly divided into 24 hours of 60 minutes of 60 seconds each.

## Leap seconds

To keep the civil day aligned with the apparent movement of the Sun, positive or negative leap second
Leap second
A leap second is a positive or negative one-second adjustment to the Coordinated Universal Time time scale that keeps it close to mean solar time. UTC, which is used as the basis for official time-of-day radio broadcasts for civil time, is maintained using extremely precise atomic clocks...

s may be inserted.

A civil clock day is typically 86,400 SI seconds long, but will be 86,401 s or 86,399 s long in the event of a leap second.

Leap seconds are announced in advance by the International Earth Rotation and Reference Systems Service
International Earth Rotation and Reference Systems Service
The International Earth Rotation and Reference Systems Service , formerly the International Earth Rotation Service, is the body responsible for maintaining global time and reference frame standards, notably through its Earth Orientation Parameter and International Celestial Reference System ...

which measures the Earth's rotation and determines whether a leap second is necessary. Leap seconds occur only at the end of a UTC month, and have only ever been inserted at the end of June 30 or December 31.

## Boundaries of the day

For most diurnal animals, the day naturally begins at dawn and ends at sunset.Humans, with our cultural norms and scientific knowledge, have supplanted Nature with several different conceptions of the day's boundaries. The Jewish
Judaism
Judaism ) is the "religion, philosophy, and way of life" of the Jewish people...

day begins at either sunset or at nightfall (when three second-magnitude
Magnitude (astronomy)
Magnitude is the logarithmic measure of the brightness of an object, in astronomy, measured in a specific wavelength or passband, usually in optical or near-infrared wavelengths.-Background:...

stars appear). Medieval
Middle Ages
The Middle Ages is a periodization of European history from the 5th century to the 15th century. The Middle Ages follows the fall of the Western Roman Empire in 476 and precedes the Early Modern Era. It is the middle period of a three-period division of Western history: Classic, Medieval and Modern...

Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...

followed this tradition, known as Florentine
Florentine calendar
The Florentine calendar was used in Italy in the Middle Ages. In this system, the new day begins at sunset. When the reference of a birth was, for example, "two hours into the day", this meant two hours after sunset...

reckoning: in this system, a reference like "two hours into the day" meant two hours after sunset and thus times during the evening need to be shifted back one calendar day in modern reckoning. Days such as Christmas Eve
Christmas Eve
Christmas Eve refers to the evening or entire day preceding Christmas Day, a widely celebrated festival commemorating the birth of Jesus of Nazareth that takes place on December 25...

, Halloween
Halloween
Hallowe'en , also known as Halloween or All Hallows' Eve, is a yearly holiday observed around the world on October 31, the night before All Saints' Day...

, and the Eve of Saint Agnes are the remnants of the older pattern when holidays began the evening before. Present common convention is for the civil day to begin at midnight, that is 00:00 (inclusive), and last a full 24 hours until 24:00 (exclusive).

In ancient Egypt
Ancient Egypt
Ancient Egypt was an ancient civilization of Northeastern Africa, concentrated along the lower reaches of the Nile River in what is now the modern country of Egypt. Egyptian civilization coalesced around 3150 BC with the political unification of Upper and Lower Egypt under the first pharaoh...

, the day was reckoned from sunrise to sunrise. Muslim
Muslim
A Muslim, also spelled Moslem, is an adherent of Islam, a monotheistic, Abrahamic religion based on the Quran, which Muslims consider the verbatim word of God as revealed to prophet Muhammad. "Muslim" is the Arabic term for "submitter" .Muslims believe that God is one and incomparable...

s fast from daybreak to sunset each day of the month of Ramadan
Ramadan is the ninth month of the Islamic calendar, and the month in which the Quran was revealed.Ramadan is the holiest of months in the Islamic calendar, and fasting in this month is one of the Five Pillars of Islam. The month is spent by Muslims fasting during the daylight hours from dawn to...

. The "Damascus Document
Damascus Document
The Damascus Document or Damascus Rule is one of the most interesting texts of the Dead Sea Scrolls because it is the only Qumran sectarian work that was known before the discovery of the Dead Sea Scrolls....

", copies of which were also found among the Dead Sea scrolls
The Dead Sea Scrolls are a collection of 972 texts from the Hebrew Bible and extra-biblical documents found between 1947 and 1956 on the northwest shore of the Dead Sea, from which they derive their name...

, states regarding Sabbath
Shabbat
Shabbat is the seventh day of the Jewish week and a day of rest in Judaism. Shabbat is observed from a few minutes before sunset on Friday evening until a few minutes after when one would expect to be able to see three stars in the sky on Saturday night. The exact times, therefore, differ from...

observance that "No one is to do any work on Friday from the moment that the sun's disk stands distant from the horizon by the length of its own diameter," presumably indicating that the monastic community responsible for producing this work counted the day as ending shortly before the sun had begun to set.

In the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

, nights are named after the previous day, e.g. "Friday night" usually means the entire night between Friday and Saturday. This is the opposite of the Jewish pattern. This difference from the civil day often leads to confusion. Events starting at midnight are often announced as occurring the day before. TV-guides tend to list nightly programs at the previous day, although programming a VCR
Videocassette recorder
The videocassette recorder , is a type of electro-mechanical device that uses removable videocassettes that contain magnetic tape for recording analog audio and analog video from broadcast television so that the images and sound can be played back at a more convenient time...

requires the strict logic of starting the new day at 00:00 (to further confuse the issue, VCRs set to the 12-hour clock notation will label this "12:00 AM"). Expressions like "today", "yesterday" and "tomorrow" become ambiguous during the night.

Validity of ticket
A ticket is a voucher that indicates that one has paid for admission to an event or establishment such as a theatre, movie theater, amusement park, zoo, museum, concert, or other attraction, or permission to travel on a vehicle such as an airliner, train, bus, or boat, typically because one has...

s, passes, etc., for a day or a number of days may end at midnight, or closing time, when that is earlier. However, if a service (e.g. public transport
Public transport
Public transport is a shared passenger transportation service which is available for use by the general public, as distinct from modes such as taxicab, car pooling or hired buses which are not shared by strangers without private arrangement.Public transport modes include buses, trolleybuses, trams...

) operates from for example, 6:00 to 1:00 the next day (which may be noted as 25:00), the last hour may well count as being part of the previous day (also for the arrangement of the timetable
Public transport timetable
A public transport timetable is a representation of public transport information to assist a passenger with planning a trip using public transport. A timetable details when vehicle will arrive and depart specified locations and may be organised for by route or for a particular stop...

). For services depending on the day ("closed on Sundays", "does not run on Fridays", and so on) there is a risk of ambiguity. As an example, for the Nederlandse Spoorwegen
Nederlandse Spoorwegen
Nederlandse Spoorwegen , or NS, is the principal passenger railway operator in the Netherlands.Its trains operate over the tracks of the Dutch national railinfrastructure, operated by ProRail, which was split off from NS in 2003...

(Dutch Railways), a day ticket is valid 28 hours, from 0:00 to 28:00 (that is, 4:00 the next day). To give another example, the validity of a pass on London Regional Transport
London Regional Transport
London Regional Transport was the organisation responsible for the public transport network in Greater London, UK from 1984-2000. In common with all London transport authorities from 1933 to 2000, the public name and operational brand of the organisation was London Transport.The organisation was...

services is until the end of the "transport day" -- that is to say, until 4:30 am on the day after the "expiry" date stamped on the pass.

## 24 hours vs daytime

To distinguish between a full day and daytime, the word nychthemeron
Nychthemeron
Nychthemeron , occasionally nycthemeron or nuchthemeron is a period of 24 consecutive hours...

(from Greek for a night and a day) may be used in English for the former, or more colloquially the term {{nowrap|24 hours}}. In other languages, the latter is also often used. Other languages also have a separate word for a full day, such as יממה in Hebrew, dygn in Swedish, etmaal in Dutch, doba in Polish, сутки (sutki) in Russian, суткі (sutki) in Belarusian, døgn in Danish and доба́ (doba) in Ukrainian. In Italian, giorno is used to indicate a full day, while means daytime. In Spanish, singladura is used, but only as a marine unit of length, being the distance covered in 24 hours.{{inline relevance|date=June 2011}}

• 10{{smallsup, times from 10 kiloseconds to 100 kiloseconds
• Calculating the day of the week
Calculating the day of the week
This article details various mathematical algorithms to calculate the day of the week for any particular date in the past or future.A typical application is to calculate the day of the week on which someone was born or some other special event occurred....

• Daylight
Daylight
Daylight or the light of day is the combination of all direct and indirect sunlight outdoors during the daytime. This includes direct sunlight, diffuse sky radiation, and both of these reflected from the Earth and terrestrial objects. Sunlight scattered or reflected from objects in outer space is...

• Daylight saving time
Daylight saving time
Daylight saving time —also summer time in several countries including in British English and European official terminology —is the practice of temporarily advancing clocks during the summertime so that afternoons have more daylight and mornings have less...

• Meteorological day
Meteorological day
A meteorological day refers to the 24-hour period in which all observations for the corresponding day occur. It often refers to the 08.00-08.00 time span...

• Season
Season
A season is a division of the year, marked by changes in weather, ecology, and hours of daylight.Seasons result from the yearly revolution of the Earth around the Sun and the tilt of the Earth's axis relative to the plane of revolution...

, for a discussion of daylight and darkness at various latitudes
• Synodic day
Synodic day
A synodic day is the period of time it takes for a planet to rotate once in relation to the body it is orbiting ....