TheInfoList

The word day has a number of meanings, depending on the context it is used such as of astronomy, physics, and various calendar systems. As a term in physics and astronomy it is approximately the period during which the Earth completes one rotation around its axis, which takes about 24 hours. A
solar day A synodic day (or synodic rotation period or solar day) is the rotation period, period for a celestial object to rotate once in relation to the star it is orbiting, and is the basis of solar time. The synodic day is distinguished from the sidereal ...
is the length of time which elapses between the Sun reaching its highest point in the sky two consecutive times. Days on other planets are defined similarly and vary in length due to differing rotation periods, that of Mars being slightly longer and sometimes called a sol. The
unit of measurement A unit of measurement is a definite magnitude Magnitude may refer to: Mathematics *Euclidean vector, a quantity defined by both its magnitude and its direction *Magnitude (mathematics), the relative size of an object *Norm (mathematic ...
"day" (symbol d) is defined as 86,400 SI seconds. The
second The second (symbol: s, also abbreviated: sec) is the of in the (SI) (french: Système International d’unités), commonly understood and historically defined as of a – this factor derived from the division of the day first into 24 s, th ...
is designated the
SI base unit The SI base units are the standard defined by the (SI) for the seven of what is now known as the : they are notably a basic set from which all other can be . The units and their physical quantities are the for , the for , the for , the f ...

of
time Time is the continued of and that occurs in an apparently succession from the , through the , into the . It is a component quantity of various s used to events, to compare the duration of events or the intervals between them, and to of ...

. Previously, it was defined in terms of the orbital motion of the Earth in the year 1900, but since 1967 the second and so the day are defined by
atomic electron transition Atomic electron transition is a change of an electron The electron is a subatomic particle In physical sciences, subatomic particles are smaller than atom An atom is the smallest unit of ordinary matter In classical physics and ...
. A civil day is usually 24 hours, plus or minus a possible
leap second A leap second is a one-second adjustment that is occasionally applied to Coordinated Universal Time (UTC), to accommodate the difference between precise time (International Atomic Time (TAI), as measured by atomic clocks) and imprecise solar tim ...

in
Coordinated Universal Time Coordinated Universal Time or UTC is the primary time standard A time standard is a specification for measuring time: either the rate at which time passes; or points in time; or both. In modern times, several time specifications have been o ...
(UTC), and occasionally plus or minus an hour in those locations that change from or to
daylight saving time#REDIRECT Daylight saving time Daylight saving time (DST), also daylight savings time or daylight time (the United States and Canada) and summer time (United Kingdom, European Union, and others), is the practice of advancing clocks (typically by ...
. Day can be defined as each of the twenty-four-hour periods, reckoned from one
midnight Midnight is the transition time from one day A day is approximately the period during which the Earth completes one rotation around its axis, which takes around 24 hours. A solar day is the length of time which elapses between the Sun reachin ...

to the next, into which a week, month, or year is divided, and corresponding to a rotation of the earth on its axis. However, its use depends on its context; for example, when people say 'day and
night Night (also described as night time, night-time, or nighttime, unconventionally spelled as ''nite'') is the period of ambient Ambient or Ambiance or Ambience may refer to: Music and sound * Ambience (sound recording), also known as atmospher ...

', 'day' will have a different meaning: the interval of light between two successive nights, the time between
sunrise 250px, Skyline of Kaohsiung harbour, Taiwan at sunrise. Sunrise (or sunup) is the moment when the upper rim of the Sun appears on the horizon in the morning. The term can also refer to the entire process of the solar disk crossing the horizon a ...

and
sunset Sunset, also known as sundown, is the daily disappearance of the Sun below the horizon The horizon is the apparent line that separates earth Earth is the third planet from the Sun and the only astronomical object known to harbor ...

; the time of light between one night and the next. For clarity when meaning 'day' in that sense, the word "
daytime On Earth, daytime is the period of the day during which a given location experiences Daylight, natural illumination from direct sunlight. Daytime occurs when the Sun appears above the local horizon, that is, anywhere on the globe's Hemisphere ...

" may be used instead, though context and phrasing often makes the meaning clear. The word ''day'' may also refer to a day of the
week A week is a time unit A unit of time or midst unit is any particular time interval, used as a standard way of measuring or expressing duration. The SI base unit, base unit of time in the International System of Units (SI) and by extension mo ...

or to a
calendar date A calendar date is a reference to a particular day represented within a calendar A calendar is a system of organizing days. This is done by giving names to periods of time, typically days, weeks, months and years. A calendar date, da ...
, as in answer to the question, "On which day?" The biologically determined living patterns (
circadian rhythm A circadian rhythm (), or circadian cycle, is a natural, internal process that regulates the sleep–wake cycle and repeats roughly every 24 hours. It can refer to any process that originates within an organism (i.e., Endogeny (biology), endogenou ...

s) of humans and many other species relate to Earth's solar day and the day-night cycle.

# Introduction

## Apparent and mean solar day

Several definitions of this universal human concept are used according to context, need and convenience. 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 its culmination point (its highest point in the sky). 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. Because the Earth moves along an eccentric orbit around the Sun while the Earth spins on an inclined axis, this period can be up to 7.9 seconds more than (or less than) 24 hours. In recent decades, the average length of a solar day on Earth has been about 86,400.002 seconds (24.000 000 6 hours) and there are currently about 365.2421875 solar days in one mean
tropical year A tropical year (also known as a solar year) is the time Time is the indefinite continued sequence, progress of existence and event (philosophy), events that occur in an apparently irreversible process, irreversible succession from the past, ...
. Ancient custom has a new day start at either the rising or setting of the Sun on the local horizon (Italian reckoning, for example, being 24 hours from sunset, oldstyle). The exact moment of, and the interval between, two sunrises or sunsets depends on the geographical position (
longitude Longitude (, ) is a geographic coordinate A geographic coordinate system (GCS) is a coordinate system associated with position (geometry), positions on Earth (geographic position). A GCS can give positions: *as Geodetic coordinates, ...

as well as latitude), and the time of
year A year is the orbital period The orbital period is the time a given astronomical object takes to complete one orbit around another object, and applies in astronomy Astronomy (from el, ἀστρονομία, literally meaning the ...
(as indicated by ancient hemispherical
sundial A sundial is a horological device that tells the time of day (in modern usage referred to as civil time In modern usage, civil time refers to statutory time scales designated by civilian authorities, or to local time indicated by clocks. Mo ...

s). A more constant day can be defined by the Sun passing through the local
meridian Meridian, or a meridian line may refer to Science * Meridian (astronomy), imaginary circle in a plane perpendicular to the planes of the celestial equator and horizon **Central meridian (planets) * Meridian (geography), an imaginary arc on the E ...
, which happens at local
noon Noon (or midday) is 12 o'clock in the daytime On Earth, daytime is the period of the day during which a given location experiences Daylight, natural illumination from direct sunlight. Daytime occurs when the Sun appears above the local h ...

(upper
culmination In observational astronomy, culmination is the instant of time of the transit of a astronomical object, celestial object (the Sun, the Moon, a planet, a star, constellation or a deep-sky object) across the observer's meridian (astronomy), local me ...
) or
midnight Midnight is the transition time from one day A day is approximately the period during which the Earth completes one rotation around its axis, which takes around 24 hours. A solar day is the length of time which elapses between the Sun reachin ...

(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 The celestial equator is the great circle A great circle, also known as an orthodrome, of a sphere of a sphere A sphere (from Greek language, Greek —, "globe, ball") is a Geometry, geometrical object in solid geometry, three-dimensional ...
; 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).

## Stellar day

A ''day'', understood as the span of time it takes for the Earth to make one entire
rotation A rotation is a circular movement of an object around a center (or point) of rotation. The plane (geometry), geometric plane along which the rotation occurs is called the ''rotation plane'', and the imaginary line extending from the center an ...

with respect to the celestial background or a distant star (assumed to be fixed), is called a ''
stellar day Stellar means anything related to one or more star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star to Eart ...
''. This period of rotation is about 4 minutes less than 24 hours (23 hours 56 minutes and 4.09 seconds) and there are about 366.2422 stellar days in one mean tropical year (one stellar day more than the number of solar days). Other planets and moons have stellar and solar days of different lengths from Earth's. Besides a stellar day on
Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% is Water distribution on Earth, covered wit ...

, other bodies in the Solar System have day times, the durations of these being: *name: daylength in hours *
Pluto Pluto (minor-planet designation A formal minor planet designation is, in its final form, a number–name combination given to a minor planet (asteroid, centaur (minor planet), centaur, trans-Neptunian object and dwarf planet but not comet ...

: 153.3 *
Neptune Neptune is the eighth and farthest-known Solar planet from the Sun. In the Solar System, it is the fourth-largest planet by diameter, the third-most-massive planet, and the densest giant planet. It is 17 times the mass of Earth, slightly mor ...

:16.1 *
Uranus Uranus is the seventh planet from the Sun. Its name is a reference to the Greek god of the sky, Uranus, who, according to Greek mythology Greek mythology is the body of myths originally told by the Ancient Greece, ancient Greeks, and ...

: 17.2 *
Saturn Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with an average radius of about nine and a half times that of Earth. It only has one-eighth the average density of Earth; how ...

: 10.7 *
Jupiter Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a gas giant A gas giant is a giant planet composed mainly of hydrogen Hydrogen is the chemical element with the Symbol (chemistry), symbol H and at ...

: 9.9 *
Ceres Ceres most commonly refers to: * Ceres (dwarf planet) Ceres (; minor-planet designation: 1 Ceres) is the smallest recognized dwarf planet, the closest dwarf planet to the Sun, and the List of notable asteroids, largest object in the main astero ...
: 9 - 9.1 hours *
Mars Mars is the fourth planet A planet is an astronomical body orbiting a star or Stellar evolution#Stellar remnants, stellar remnant that is massive enough to be Hydrostatic equilibrium, rounded by its own gravity, is not massive enough to ...

: 24.7 *
Earth's Moon The Moon is Earth's only Claimed moons of Earth, proper natural satellite. At one-quarter the diameter of Earth (comparable to the width of Australia), it is the largest natural satellite in the Solar System relative to the size of its plane ...

:708.7 *
Venus Venus is the second planet from the Sun. It is named after the Venus (mythology), Roman goddess of love and beauty. As List of brightest natural objects in the sky, the brightest natural object in Earth's night sky after the Moon, Venus can ...

: 2802.0 *
Mercury Mercury usually refers to: * Mercury (planet) Mercury is the smallest planet in the Solar System and the closest to the Sun. Its orbit around the Sun takes 87.97 Earth days, the shortest of all the Sun's planets. It is named after the Roman g ...

: 4222.6

## Daytime vs. nightime

A day, in the sense of
daytime On Earth, daytime is the period of the day during which a given location experiences Daylight, natural illumination from direct sunlight. Daytime occurs when the Sun appears above the local horizon, that is, anywhere on the globe's Hemisphere ...

that is distinguished from night time, is commonly defined as the period during which
sunlight Sunlight is a portion of the given off by the , in particular , , and light. On , sunlight is and through , and is obvious as when the Sun is above the . When direct is not blocked by s, it is experienced as sunshine, a combination of b ...

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 Image:1931 Hawkes Bay Earthquake - Minutes of First Meeting of the Hawkes Bay Earthquake Relief Fund Committee (24631618026).jpg, Minutes of First Meeting of the Hawkes Bay Earthquake Relief Fund Committee Minutes, also known as minutes of meeting ...
atmosphere An atmosphere (from the greek words ἀτμός ''(atmos)'', meaning 'vapour', and σφαῖρα ''(sphaira)'', meaning 'ball' or 'sphere') is a layer or a set of layers of gases surrounding a planet or other material body, that is held in ...

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. Thus, daytime is on average around 7 minutes longer than 12 hours.

# Etymology

The term comes from the
Old English Old English (, ), or Anglo-Saxon, is the earliest recorded form of the English language English is a West Germanic languages, West Germanic language first spoken in History of Anglo-Saxon England, early medieval England, which has eventu ...
''dæg'', with its cognates such as ''dagur'' in
Icelandic Icelandic refers to anything of, from, or related to Iceland and may refer to: *Icelandic people *Icelandic language *Icelandic alphabet *Icelandic cuisine See also

* Icelander (disambiguation) * Icelandic Airlines, a predecessor of Icelandai ...
, ''Tag'' in
German German(s) may refer to: Common uses * of or related to Germany * Germans, Germanic ethnic group, citizens of Germany or people of German ancestry * For citizens of Germany, see also German nationality law * German language The German la ...

, and ''dag'' in
Norwegian Norwegian, Norwayan, or Norsk may refer to: *Something of, from, or related to Norway, a country in northwestern Europe *Norwegians, both a nation and an ethnic group native to Norway *Demographics of Norway *The Norwegian language, including the t ...
,
Danish Danish may refer to: * Something of, from, or related to the country of Denmark * A national or citizen of Denmark, also called a "Dane", see Demographics of Denmark * Danish people or Danes, people with a Danish ancestral or ethnic identity * Danis ...
,
Swedish Swedish or ' may refer to: * Anything from or related to Sweden, a country in Northern Europe * Swedish language, a North Germanic language spoken primarily in Sweden and Finland * Swedish alphabet, the official alphabet used by the Swedish langua ...
and
Dutch Dutch commonly refers to: * Something of, from, or related to the Netherlands * Dutch people () * Dutch language () *Dutch language , spoken in Belgium (also referred as ''flemish'') Dutch may also refer to:" Castle * Dutch Castle Places * ...
- all stemming from a
Proto-Germanic Proto-Germanic (abbreviated PGmc; also called Common Germanic) is the reconstructed Reconstruction may refer to: Politics, history, and sociology *Reconstruction (law), the transfer of a company's (or several companies') business to a new ...
root ''*dagaz''. , ''day'' is the 205th most common word in
US English American English (AmE, AE, AmEng, USEng, en-US), sometimes called United States English or U.S. English, is the set of varieties Variety may refer to: Science and technology Mathematics * Algebraic variety, the set of solutions of a syst ...
, and the 210th most common in UK English.

# International System of Units (SI)

A day, symbol d, defined as 86,400 seconds, is not an SI unit, but is accepted for use with SI. The second is the base unit of time in
SI units The International System of Units, known by the international abbreviation SI in all languages and sometimes pleonastically as the SI system, is the modern form of the metric system The metric system is a system of measurement A syste ...
. In 1967–68, during the 13th CGPM (Resolution 1), the
International Bureau of Weights and Measures The International Bureau of Weights and Measures (french: Bureau international des poids et mesures, BIPM) is an intergovernmental organisation An intergovernmental organization (IGO) or international organization is an organization composed ...
(BIPM) redefined a second as
... the duration of 9 192 631 770 periods of the
radiation upThe international symbol for types and levels of ionizing radiation (radioactivity) that are unsafe for unshielded humans. Radiation, in general, exists throughout nature, such as in light and sound. In physics Physics (from grc ...

corresponding to the transition between two hyperfine levels of the
ground state The ground state of a quantum-mechanical system is its lowest-energy In , energy is the that must be to a or to perform on the body, or to it. Energy is a ; the law of states that energy can be in form, but not created or destro ...
of the
caesium Caesium (IUPAC spelling) (American and British English spelling differences, also spelled cesium in American English) is a chemical element with the Symbol (chemistry), symbol Cs and atomic number 55. It is a soft, silvery-golden alkali ...

133
atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of atom ...

.
This makes the SI-based day last exactly 794,243,384,928,000 of those periods. There are 365.25 days in a Julian year.

## Leap seconds

Mainly due to tidal effects, the Earth's rotational period is not constant, resulting in minor variations for both solar days and stellar "days". The Earth's day has increased in length over time due to
tide Tides are the rise and fall of s caused by the combined effects of the forces exerted by the and the , and the of the . s can be used for any given locale to find the predicted times and (or ""). The predictions are influenced by many ...

s raised by the
Moon The Moon is Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% is Water distri ...

which slow
Earth's rotation Earth's rotation or Earth's spin is the rotation A rotation is a circular movement of an object around a center (or point) of rotation. The plane (geometry), geometric plane along which the rotation occurs is called the ''rotation plan ...
. 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
millisecond A millisecond (from ''milli- ''Milli-'' (symbol m) is a unit prefixA unit prefix is a specifier or mnemonic that is prepended to units of measurement to indicate multiples or fractions of the units. Units of various order of magnitude, sizes are ...
s per century (an average over the last 2,700 years). The length of a day circa 620 million years ago has been estimated from rhythmites (alternating layers in
sandstone Sandstone is a clastic sedimentary rock of a clast (sand grain), derived from a basalt Basalt (, ) is a fine-grained extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron ('' mafic ' ...

) as having been about 21.9 hours. In order to keep the civil day aligned with the apparent movement of the Sun, a day according to Coordinated Universal Time (UTC) can include a negative or positive
leap second A leap second is a one-second adjustment that is occasionally applied to Coordinated Universal Time (UTC), to accommodate the difference between precise time (International Atomic Time (TAI), as measured by atomic clocks) and imprecise solar tim ...

. Therefore, although typically 86,400 SI seconds in duration, a civil day can be either 86,401 or 86,399 SI seconds long on such a day. Leap seconds are announced in advance by the International Earth Rotation and Reference Systems Service (IERS), which measures the Earth's rotation and determines whether a leap second is necessary.

## Civil day

For civil purposes, a common clock time is typically defined for an entire region based on the local mean solar time at a central meridian. Such ''time zones'' began to be adopted about the middle of the 19th century when
railroads Rail transport (also known as train transport) is a means of transport, transferring passengers and goods on wheeled vehicles running on rails, which are located on Track (rail transport), tracks. In contrast to road transport, where the vehic ...

with regularly occurring schedules came into use, with most major countries having adopted them by 1929. As of 2015, throughout the world, 40 such zones are now in use: the central zone, from which all others are defined as offsets, is known as UTC±00, which uses Coordinated Universal Time (UTC). The most common convention starts the civil day at
midnight Midnight is the transition time from one day A day is approximately the period during which the Earth completes one rotation around its axis, which takes around 24 hours. A solar day is the length of time which elapses between the Sun reachin ...

: this is near the time of the lower culmination of the Sun on the central meridian of the time zone. Such a day may be referred to as a calendar day. A day is commonly divided into 24 hours of 60 minutes, with each minute composed of 60 seconds.

## Decimal and metric time

In the 19th century, an idea circulated to make a decimal fraction ( or ) of an astronomical day the base unit of time. This was an afterglow of the short-lived movement toward a decimalisation of
timekeeping Time is the indefinite continued progress of existence Existence is the ability of an entity to interact with physical or mental reality Reality is the sum or aggregate of all that is real or existent within a system, as opposed to that wh ...
and the
calendar A calendar is a system of organizing days. This is done by giving names to periods of time, typically days, weeks, months and years. A calendar date, date is the designation of a single, specific day within such a system. A calendar is al ...
, which had been given up already due to its difficulty in transitioning from traditional, more familiar units. The most successful alternative is the '' centiday'', equal to 14.4 minutes (864 seconds), being not only a shorter multiple of an hour (0.24 vs 2.4) but also closer to the SI multiple ''
kilosecond An order of magnitude An order of magnitude is an approximation of the logarithm of a value relative to some contextually understood reference value, usually ten, interpreted as the base of the logarithm and the representative of values of ma ...
'' (1,000 seconds) and equal to the traditional Chinese unit, ''kè''.

# Colloquial

The word refers to various similarly defined ideas, such as: ; Full day * 24
hour An hour (metric symbol, symbol: h; also abbreviated hr) is a unit of measurement, unit of time conventionally reckoned as of a day and scientifically reckoned as 3,599–3,601 seconds, depending on conditions. There are 60 minutes in an hour, a ...
s (exactly) (a nychthemeron) * A day counting approximation, for example "See you in three days." or "the following day" * The full day covering both the dark and light periods, beginning from the start 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'' in English, from the Greek for ''
night Night (also described as night time, night-time, or nighttime, unconventionally spelled as ''nite'') is the period of ambient Ambient or Ambiance or Ambience may refer to: Music and sound * Ambience (sound recording), also known as atmospher ...

-day''; or more colloquially the term . In other languages, is also often used. Other languages also have a separate word for a full day. * Part of a date: the day of the year (''doy'') in
ordinal date An ordinal date is a calendar date typically consisting of a ''year'' and a ''day of the year'' ranging between 1 and 366 (starting on January 1), though year may sometimes be omitted. The two numbers can be formatted as YYYY-DDD to comply with t ...
s, day of the month (''dom'') in
calendar date A calendar date is a reference to a particular day represented within a calendar A calendar is a system of organizing days. This is done by giving names to periods of time, typically days, weeks, months and years. A calendar date, da ...
s or
day of the week File:Heptagram of the celestial bodies of the weekdays.png, Heptagram of the seven celestial bodies of the week The names of the days of the week in many languages are derived from the names of the classical planets in Hellenistic astrology, w ...

(''dow'') in week dates. * Time regularly spend at paid work on a single work day, cf. man-day and
workweek The weekdays and weekend are the complementary parts of the week devoted to Labour (economics), labour and Leisure, rest, respectively. The legal weekdays (British English), or workweek (American English), is the part of the seven-day week devo ...
. ; Daytime * The period of light when the Sun is above the local
horizon The horizon is the apparent line that separates the surface of a celestial body In astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that st ...

(that is, the time period from
sunrise 250px, Skyline of Kaohsiung harbour, Taiwan at sunrise. Sunrise (or sunup) is the moment when the upper rim of the Sun appears on the horizon in the morning. The term can also refer to the entire process of the solar disk crossing the horizon a ...

to
sunset Sunset, also known as sundown, is the daily disappearance of the Sun below the horizon The horizon is the apparent line that separates earth Earth is the third planet from the Sun and the only astronomical object known to harbor ...

) * The time period from 06:00–18:00 (6:00 am – 6:00 pm) or 21:00 (9:00 pm) or another fixed clock period overlapping or offset from other time periods such as "
morning Morning is the period from sunrise 250px, Skyline of Kaohsiung harbour, Taiwan at sunrise. Sunrise (or sunup) is the moment when the upper rim of the Sun appears on the horizon in the morning. The term can also refer to the entire process of ...

", "
evening Evening is the period of a day A day is approximately the period during which the Earth completes one rotation around its axis, which takes around 24 hours. A solar day is the length of time which elapses between the Sun reaching its highest ...

", or "
night Night (also described as night time, night-time, or nighttime, unconventionally spelled as ''nite'') is the period of ambient Ambient or Ambiance or Ambience may refer to: Music and sound * Ambience (sound recording), also known as atmospher ...

". * The time period from first-light "
dawn Dawn is the time that marks the beginning of twilight Twilight is the of the lower when the Sun is not directly visible because it is below the . Twilight is produced by in the upper atmosphere, illuminating the lower atmosphere so ...

" to last-light "
dusk Dusk occurs at the darkest stage of twilight File:Twilight-dawn subcategories.svg, 300px, Morning twilight: astronomical, nautical and civil stages at dawn. The apparent disk of the Sun is shown to scale. Twilight is the diffuse sky radia ...

". ;Other * A specific period of the day, which may vary by context, such as "the school day" or "the work day".

# Boundaries

For most
diurnal Diurnal ("daily Daily or The Daily may refer to: Journalism * Daily newspaper A newspaper is a Periodical literature, periodical publication containing written News, information about current events and is often typed in black ink with a ...
animals, the day naturally begins at dawn and ends at sunset. Humans, with their cultural norms and scientific knowledge, have employed several different conceptions of the day's boundaries. In the
Hebrew Bible The Hebrew Bible or Tanakh (; Hebrew Hebrew (, , or ) is a Northwest Semitic languages, Northwest Semitic language of the Afroasiatic languages, Afroasiatic language family. Historically, it is regarded as the language of the Israelites ...

, Genesis 1:5 defines a day in terms of "evening" and "morning" before recounting the creation of a sun to illuminate it: "And God called the light Day, and the darkness he called Night. And the evening and the morning were the first day." Common convention among the ancient Romans, ancient Chinese s:zh:清史稿/卷48: 起''子正''，盡''夜子初''。 and in modern times is for the civil day to begin at midnight, i.e. 00:00, and to last a full 24 hours until 24:00 (i.e. 00:00 of the next day). In
ancient Egypt Ancient Egypt was a civilization  A civilization (or civilisation) is a that is characterized by , , a form of government, and systems of communication (such as ). Civilizations are intimately associated with additional char ...

the day was reckoned from sunrise to sunrise. The
Jewish Jews ( he, יְהוּדִים ISO 259-2 , Israeli pronunciation ) or Jewish people are an ethnoreligious group and nation originating from the Israelites Israelite origins and kingdom: "The first act in the long drama of Jewish history is ...
day begins at either sunset or nightfall (when three second-
magnitude Magnitude may refer to: Mathematics *Euclidean vector, a quantity defined by both its magnitude and its direction *Magnitude (mathematics), the relative size of an object *Norm (mathematics), a term for the size or length of a vector *Order of ...
stars appear). Medieval Europe also followed this tradition, known as Florentine 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 is the evening or entire day before Christmas Day Christmas is an annual festival commemorating Nativity of Jesus, the birth of Jesus Christ, observed primarily on December 25 as a religious and cultural celebration amo ...

,
Halloween Halloween or Hallowe'en (a contraction of "All Hallows' evening"), less commonly known as Allhalloween, All Hallows' Eve, or All Saints' Eve, is a celebration observed in many countries on 31 October, the eve of the Western Christian feast o ...

, and the Eve of Saint Agnes are remnants of the older pattern when
holidays A holiday is a day set aside by custom or by law on which normal activities, especially business or work including school, are suspended or reduced. Generally, holidays are intended to allow individuals to celebrate or commemorate an event or tr ...
began during the prior evening. Prior to 1926,
Turkey Turkey ( tr, Türkiye ), officially the Republic of Turkey, is a country located mainly on Anatolia Anatolia,, tr, Anadolu Yarımadası), and the Anatolian plateau. also known as Asia Minor, is a large peninsula in Western Asia an ...

had two time systems: Turkish (counting the hours from sunset) and French (counting the hours from midnight). Validity of tickets, passes, etc., for a day or a number of days may end at midnight, or closing time, whichever occurs earlier. However, if a service (e.g.,
public transport Public transport (also known as public transportation, public transit, mass transit, or simply transit) is a system of transport Transport (in British English), or transportation (in American English), is the Motion, movement of humans ...

) operates from for example, 06:00 to 01:00 the next day (which may be noted as 25:00), the last hour may well count as being part of the previous day. For services depending on the day ("closed on Sundays", "does not run on Fridays", and so on) there is a risk of ambiguity. For example, a day ticket on the
Nederlandse Spoorwegen Nederlandse Spoorwegen (NS; ; en, "Dutch Railways") is a Dutch state-owned company, company, the principal Rail transport in the Netherlands, passenger railway operator in the Netherlands. Founded in 1938, NS provides rail services on the Dutc ...

(Dutch Railways) is valid for 28 hours, from 00:00 to 28:00 (that is, 4:00 the next day); the validity of a pass on
Transport for London Transport for London (TfL) is a local government body responsible for most of the in , . TfL has responsibility for various rail networks including the and , as well as London's , , principal road routes, cycling provision, , and . It doe ...
(TfL) services runs until the end of the "transport day" – that is to say, until 4:30 am on the day after the "expires" date stamped on the pass.

# Midnight sun

In places which experience the , daytime may extend beyond one 24-hour period and could even extend to months.

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24-hour clock The 24-hour clock, popularly referred to in the United States and some other countries as military time, is the convention of timekeeping in which the day runs from midnight to midnight and is divided into 24 hours. This is indicated by the hour ...
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Daylight Daylight is the combination of all direct and indirect sunlight Sunlight is a portion of the given off by the , in particular , , and light. On , sunlight is and through , and is obvious as when the Sun is above the . When direct is ...

* Day length fluctuations *
Determination of the day of the week The determination of the day of the week for any date may be performed with a variety of algorithm of an algorithm (Euclid's algorithm) for calculating the greatest common divisor (g.c.d.) of two numbers ''a'' and ''b'' in locations named A and B ...
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Holiday A holiday is a day set aside by Norm (social), custom or by law on which normal activities, especially business or work including school, are suspended or reduced. Generally, holidays are intended to allow individuals to celebrate or commemorate ...

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ISO 8601 ISO 8601 is an international standard An international standard is a technical standard A technical standard is an established norm (social), norm or requirement for a repeatable technical task which is applied to a common and repeated use of rule ...
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Season A season is a division of the year based on changes in weather Weather is the state of the atmosphere An atmosphere (from the greek words ἀτμός ''(atmos)'', meaning 'vapour', and σφαῖρα ''(sphaira)'', meaning 'ball' or ...

, for a discussion of daylight and darkness at various latitudes * Synodic day *
Sidereal time Sidereal time ( ) is a timekeeping Time is the indefinite continued progress of existence Existence is the ability of an entity to interact with physical or mental reality Reality is the sum or aggregate of all that is real or existen ...

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World Meteorological day World Meteorological Day was established in 1961 to commemorate the World Meteorological Organization creation on 23 March, 1950. This organization announces a slogan for World Meteorology Day every year, and this day is list of minor secular obse ...
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Zmanim ''Zmanim'' ( he, זְמַנִּים, literally "times", singular ''zman'') are specific times of the day in Jewish law ''Halakha'' (; he, הֲלָכָה, ), also transliterated Transliteration is a type of conversion of a text from one ...