View source for Day

{{Short description|Time of one rotation around a planet axis}}
{{For-multi|the part of the day when there is daylight|Daytime|other uses}}
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| image1            = Midtown Manhattan from Weehawken September 2021 panorama 1.jpg
| alt1              = Midtown Manhattan at afternoon
| image2            = Midtown Manhattan from Weehawken September 2021 panorama 2.jpg
| image3            = Midtown Manhattan from Weehawken September 2021 HDR panorama.jpg
| alt2              = Midtown Manhattan at sunset
| alt3              = Midtown Manhattan at evening
| footer            = A quarter-day cycle at [[Midtown Manhattan]], from afternoon to dusk
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A '''day''' is the [[time]] [[rotation period|period]] of a full [[Earth's rotation|rotation of the Earth]] with respect to the [[Sun]].  On average, this is 24 [[hour]]s (86,400 [[second]]s). As a day passes at a given location it experiences [[morning]], [[afternoon]], [[evening]], and [[night]]. This daily cycle drives [[circadian rhythm]]s in many organisms, which are vital to many [[life]] processes.

A collection of sequential days is organized into [[calendar]]s as [[Calendar date|dates]], almost always into [[week]]s, [[month]]s and [[year]]s. A [[solar calendar]] organizes dates based on the Sun's annual cycle, giving consistent start dates for the [[season|four seasons]] from year to year. A [[lunar calendar]] organizes dates based on the [[Moon]]'s [[lunar phase]].

In common usage, a day starts at [[midnight]], written as 00:00 or [[12 am|12:00 am]] in [[24-hour clock|24-]] or [[12-hour clock]]s, respectively. Because the time of midnight varies between locations, [[time zone]]s are set up to facilitate the use of a uniform [[standard time]]. Other conventions exist for marking the beginning and end of days, such as the [[Hebrew calendar|Jewish religious calendar]] which counts days beginning at sunset, or in [[astronomy]], in which a day begins at [[noon]] so that observations throughout a single night are recorded as happening on the same day.

In specific applications, the definition of a day is slightly modified, such as in the [[International System of Units|SI]] day (exactly 86,400 [[second]]s) used for [[computer]]s and [[Time metrology|standards keeping]], [[local mean time]] accounting of the [[Day length fluctuations|Earth's natural fluctuation of a solar day]], and [[stellar day]] and [[Sidereal time|sidereal day]] (using the [[celestial sphere]]) used for [[astronomy]].  In some countries outside of the [[tropics]], [[daylight saving time]] is practiced, and each year there will be one 23-hour civil day and one 25-hour civil day.  Due to slight variations in the rotation of the Earth, there are rare times when a [[leap second]] will get inserted at the end of a [[Coordinated Universal Time|UTC]] day, and so while almost all days have a duration of 86,400 seconds, there are these exceptional cases of a day with 86,401 seconds (in the half-century spanning 1972 through 2022, there have been a total of 27 leap seconds that have been inserted, so roughly once every other year). It is a [[Non-SI units mentioned in the SI|non-SI unit that is accepted for use with SI]].<ref>{{cite web|url=https://www.bipm.org/utils/common/pdf/si-brochure/SI-Brochure-9-EN.pdf|archive-url=https://web.archive.org/web/20200316121104/https://www.bipm.org/utils/common/pdf/si-brochure/SI-Brochure-9-EN.pdf|url-status=dead|archive-date=2020-03-16|work=Bureau International de Poids et Mesures|title=Non-SI units accepted for use with the SI, and units based on fundamental constants|pages=145-146}}</ref>

== Etymology ==
The term comes from the [[Old English]] term ''dæġ'' ({{IPA|/dæj/}}), with its cognates such as ''dagur'' in [[Icelandic language|Icelandic]], ''Tag'' in [[German language|German]], and ''dag'' in [[Norwegian language|Norwegian]], [[Danish language|Danish]], [[Swedish language|Swedish]] and [[Dutch language|Dutch]] – all stemming from a [[Proto-Germanic]] root ''*dagaz''.<ref name=":0">Mallory, James P.; Adams, Douglas Q. (2006). ''The Oxford Introduction to Proto-Indo-European and the Proto-Indo-European World''. Oxford, UK: Oxford University Press. p. 124. {{ISBN|978-0-19-929668-2}}.</ref>

== Definitions ==
=== Apparent and mean solar day ===
[[File:EpicEarth-Globespin-tilt-23.4.gif|thumb|Earth's rotation imaged by [[Deep Space Climate Observatory]], showing axis tilt]]
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'', the time it takes for the Sun to return to its culmination point (its highest point in the sky). Due to an orbit's eccentricity, the Sun resides in one of the orbit's [[Focus (geometry)|foci]] instead of the middle. Consequently, due to [[Kepler's laws of planetary motion#Second law|Kepler's second law]], the planet travels at different speeds at various positions in its orbit, and thus a solar day is not the same length of time throughout the orbital year. Because the Earth moves along an [[Semi-major and semi-minor axes#Semi-major and semi-minor axes of the planets|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<ref>{{cite web|title=Earth Orientation Parameters|url=http://hpiers.obspm.fr/eoppc/eop/eopc04/eopc04.62-now|publisher=International Earth Rotation and Reference Systems Service|archive-url=https://web.archive.org/web/20150426160146/http://hpiers.obspm.fr/eoppc/eop/eopc04/eopc04.62-now|archive-date=April 26, 2015|url-status=live}}</ref> (24.000 000 6 hours). There are currently about 365.2421875 solar days in one mean [[tropical year]].

Ancient custom has a new day starting at either the rising or setting of the Sun on the local horizon (Italian reckoning, for example, being 24 hours from sunset, old style).<ref>L. Holford-Stevens, ''The History of Time'' (Oxford 2005) p. 6</ref> The exact moment of, and the interval between, two sunrises or sunsets depends on the geographical position ([[longitude]] and latitude, as well as altitude), and the time of [[year]] (as indicated by ancient hemispherical [[sundial]]s).

A more constant day can be defined by the Sun passing through the local [[meridian (geography)|meridian]], which happens at local [[noon]] (upper [[culmination]]) or [[midnight]] (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 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).

In terms of Earth's rotation, the average day length is about 360.9856°. A day lasts for more than 360° of rotation because of the Earth's revolution around the Sun.  With a full year being slightly more than 360 days, the Earth's daily orbit around the Sun is slightly less than 1°, so the day is slightly less than 361° of rotation.

Elsewhere in the [[Solar System]] or other parts of the [[universe]], a day is a full rotation of other large [[astronomical object]]s with respect to its star.<ref>{{Cite web |title=day |url=https://www.britannica.com/science/day |access-date=2022-08-17 |website=[[Encyclopædia Britannica]] |language=en |archive-date=2022-07-10 |archive-url=https://web.archive.org/web/20220710214718/https://www.britannica.com/science/day |url-status=live }}</ref>

==== 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 zone|'' time zones'']] began to be adopted about the middle of the 19th century when [[Rail transport|railroads]] 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+00:00|UTC+00]], which uses [[Coordinated Universal Time|Coordinated Universal Time (UTC)]].

The most common convention starts the civil day at [[midnight]]: this is near the time of the [[Culmination|lower culmination]] of the Sun on the central meridian of the time zone. Such a day may be called a [[calendar day]].

A day is commonly divided into 24 hours, with each hour being made up of 60 minutes, and each minute composed of 60 seconds.

=== Sidereal day{{anchor|Extraterrestrial bodies}} ===
{{main|Earth's rotation#Stellar day|Sidereal time|Rotation period}}
[[File:PIA19547-Ceres-DwarfPlanet-Dawn-RC3-Animation-20150504.gif|thumb|Rotation of the dwarf planet [[Ceres (dwarf planet)|Ceres]]]]
A [[Sidereal time|sidereal day]] or [[stellar day]] is the span of time it takes for the Earth to make one entire [[rotation]]<ref>Certain authors caution against identifying "day" with rotation period. For example: {{cite web |first=Courtney |last=Seligman |title=Rotation Period and Day Length |url=http://cseligman.com/text/sky/rotationvsday.htm |url-status=live |archive-url=https://web.archive.org/web/20180929010908/http://cseligman.com/text/sky/rotationvsday.htm |archive-date=2018-09-29 |access-date=2011-06-03 |quote=A Cautionary Note: Because the rotation period of the Earth is almost the same as the length of its day, we sometimes get a bit sloppy in discussing the rotation of the sky, and say that the stars rotate around us once each day. In a similar way, it is not unusual for careless people to mix up the rotation period of a planet with the length of its day, or vice versa.}}</ref> with respect to the celestial background or a distant star (assumed to be fixed).<ref name=":1">{{Cite web |title=sidereal day |url=https://www.britannica.com/science/sidereal-day |access-date=2022-08-17 |website=[[Encyclopædia Britannica]] |language=en |archive-date=2022-09-26 |archive-url=https://web.archive.org/web/20220926003236/https://www.britannica.com/science/sidereal-day |url-status=live }}</ref> Measuring a day as such is used in [[astronomy]].<ref name=":1" /> A sidereal day is about 4 minutes less than a solar day of 24 hours (23 hours 56 minutes and 4.09 seconds), or 0.99726968 of a solar day of 24 hours.<ref name="Allen296">{{cite book |last1=Allen |first1=Clabon Walter |url=https://books.google.com/books?id=w8PK2XFLLH8C&pg=PA296 |title=Allen's Astrophysical Quantities |last2=Cox |first2=Arthur N. |date=2000 |publisher=[[Springer (publisher)|Springer]] |isbn=0-387-98746-0 |page=296 |author-link=Clabon Walter Allen |name-list-style=amp |access-date=2022-08-17 |archive-date=2011-12-09 |archive-url=https://web.archive.org/web/20111209062816/http://books.google.com/books?id=w8PK2XFLLH8C&pg=PA296 |url-status=live }}</ref> There are about 366.2422 stellar days in one mean tropical year (one stellar day more than the number of solar days).<ref>{{Cite web |last=Komhyr |first=Walter Dmyro |date=June 1980 |title=Operations Handbook – Ozone Observations with a Dobson Spectrophotometer |url=https://gml.noaa.gov/ozwv/dobson/papers/report6/appi.html |access-date=2022-08-17 |website=gml.noaa.gov |page=122 |archive-date=2021-06-12 |archive-url=https://web.archive.org/web/20210612111012/https://gml.noaa.gov/ozwv/dobson/papers/report6/appi.html |url-status=live }}</ref>

Besides a stellar day on [[Earth]], other bodies in the Solar System have day times, the durations of these being:<ref>{{cite web |title=Planetary Fact Sheet – Metric |url=https://nssdc.gsfc.nasa.gov/planetary/factsheet/ |archive-url=https://archive.today/20120719082605/http://nssdc.gsfc.nasa.gov/planetary/factsheet/ |archive-date=19 July 2012 |access-date=May 29, 2021 |work=nssdc.gsfc.nasa.gov [[NASA]]}}</ref><ref name="VRG-20190118">{{cite web |last=Griggs |first=Mary Beth |date=18 January 2019 |title=Shaky rings help scientists measure Saturn's days – Speedy planet |url=https://www.theverge.com/2019/1/18/18188429/rings-saturn-nasa-day-time-hours-duration |url-status=live |archive-url=https://web.archive.org/web/20190119035815/https://www.theverge.com/2019/1/18/18188429/rings-saturn-nasa-day-time-hours-duration |archive-date=19 January 2019 |access-date=18 January 2019 |work=[[The Verge]]}}</ref>
{| class="wikitable"
!Name
!Daylength (hours)
|-
|[[Mercury (planet)|Mercury]]
|{{val|4222.6}}
|-
|[[Venus]]
|{{val|2802}}
|-
|[[Moon|Earth's Moon]]
|708.7
|-
|[[Mars]]
|24.7
|-
|[[Ceres (dwarf planet)|Ceres]]
|9<ref>{{cite web |title=planets/dwarf-planets/ceres/in-depth |url=https://solarsystem.nasa.gov/planets/dwarf-planets/ceres/in-depth/ |url-status=live |archive-url=https://web.archive.org/web/20210602235747/https://solarsystem.nasa.gov/planets/dwarf-planets/ceres/in-depth/ |archive-date=June 2, 2021 |access-date=May 30, 2021 |work=nasa.gov}}</ref>–9.1<ref>{{cite web |author=Tate, Karl |date=21 November 2012 |title=Dwarf Planets of Our Solar System (Infographic) |url=https://www.space.com/18584-dwarf-planets-solar-system-infographic.html |url-status=live |archive-url=https://web.archive.org/web/20210518093249/https://www.space.com/18584-dwarf-planets-solar-system-infographic.html |archive-date=18 May 2021 |access-date=May 30, 2021 |work=www.[[space.com]]}}</ref>
|-
|[[Jupiter]]
|9.9
|-
|[[Saturn]]
|10.7
|-
|[[Uranus]]
|17.2
|-
|[[Neptune]]
|16.1
|-
|[[Pluto]]
|153.3
|}

=== In the International System of Units ===
{{Main|International System of Units}}

In the [[International System of Units]] (SI), a day [[Non-SI units mentioned in the SI|not an official unit, but is accepted for use with SI]].<ref name="Non-SI">{{cite web |author=BIPM |author-link=International Bureau of Weights and Measures |date=2014 |title=Non-SI units accepted for use with the SI, and units based on fundamental constants |url=http://www.bipm.org/en/publications/si-brochure/table6.html |url-status=dead |archive-url=https://web.archive.org/web/20141111155820/http://www.bipm.org/en/publications/si-brochure/table6.html |archive-date=2014-11-11 |access-date=2015-01-27 |website=SI Brochure |edition=8th |orig-year=2006}}</ref> A day, with symbol d, is defined using SI units as 86,400 seconds; the second is the base unit of time in [[SI units]]. In 1967–68, during the 13th CGPM (Resolution&nbsp;1),<ref>{{Cite web |title=SI Unit of Time (Second) |url=http://www.bipm.org/en/CGPM/db/13/1/ |url-status=dead |archive-url=https://web.archive.org/web/20110110122822/http://www.bipm.org/en/CGPM/db/13/1/ |archive-date=2011-01-10 |access-date=2015-10-17 |website=Resolution 1 of the 13th CGPM (1967/68) |publisher=Bureau International des Poids et Mesures (BIPM)}}</ref> the [[International Bureau of Weights and Measures]] (BIPM) redefined a second as "the duration of 9,192,631,770 periods of the [[Radioactive decay|radiation]] corresponding to the transition between two [[hyperfine structure|hyperfine]] [[energy level|levels]] of the [[Stationary state|ground state]] of the [[caesium]]-133 [[atom]]".<ref>{{Cite web |year=2014 |title=Unit of Time (Second) |url=http://www.bipm.org/en/publications/si-brochure/second.html |url-status=dead |archive-url=https://web.archive.org/web/20180613111627/https://www.bipm.org/en/publications/si-brochure/second.html |archive-date=2018-06-13 |access-date=2015-10-17 |website=SI Brochure: The International System of Units (SI) |publisher=Bureau International des Poids et Mesures (BIPM) |edition=8 |orig-year=2006}}</ref> This makes the SI-based day last exactly 794,243,384,928,000 of those periods.

=== In decimal and metric time ===
{{Main|decimal time|Metric time}}
[[File:Decimal Clock face by Pierre Daniel Destigny 1798-1805.jpg|thumb|Decimal clock face, made in around the start of the 19th century]]
Various [[Decimal time|decimal]] or [[metric time]] proposals have been made, but do not redefine the day, and use the day or [[sidereal day]] as a base unit. Metric time uses metric prefixes to keep time. It uses the day as the base unit, and smaller units being fractions of a day: a metric hour (''deci'') is {{frac|1|10}} of a day; a metric minute (''milli'') is {{frac|1|1000}} of a day; etc.<ref>{{Cite web |last=Veitch |first=Harriet |date=2008-04-02 |title=Why don't we have metric time? |url=https://www.smh.com.au/entertainment/books/why-dont-we-have-metric-time-20080402-gds868.html |access-date=2022-08-21 |website=The Sydney Morning Herald |language=en |archive-date=2022-08-21 |archive-url=https://web.archive.org/web/20220821154425/https://www.smh.com.au/entertainment/books/why-dont-we-have-metric-time-20080402-gds868.html |url-status=live }}</ref> Similarly, in decimal time, the length of a day is static to normal time. A day is also split into 10 hours, and 10 days comprise a ''décade –'' the equivalent of a week. 3 ''décades'' make a month.<ref name=":5">{{Cite journal |last=Vera |first=Hector |date=2009 |title=Decimal Time: Misadventures of a Revolutionary Idea, 1793–2008 |url=https://brill.com/view/journals/kron/9/1-2/article-p29_6.xml |journal=KronoScope |volume=9 |issue=1–2 |pages=29–48 |doi=10.1163/156771509X12638154745382 |issn=1567-715X |access-date=2022-08-21 |archive-date=2022-08-21 |archive-url=https://web.archive.org/web/20220821154425/https://brill.com/view/journals/kron/9/1-2/article-p29_6.xml |url-status=live |url-access=subscription }}</ref>{{Rp|page=35}} Various decimal time proposals which do not redefine the day: Henri de Sarrauton's proposal kept days, and subdivided hours into 100 minutes;<ref name=":5" />{{Rp|page=42}} in Mendizábal y Tamborel's proposal, the [[sidereal day]] was the basic unit, with subdivisions made upon it;<ref name=":5" />{{Rp|page=|pages=42–43}} and Rey-Pailhade's proposal divided the day 100 ''cés.<ref name=":5" />{{Rp|page=42}}''

=== Other definitions ===
The word refers to various similarly defined ideas, such as:
; Full day
* A full dark and light period, sometimes called a ''[[nychthemeron]]'' in English, from the Greek for ''[[night]]-day'',<ref>{{Cite web |title=Definition of NYCHTHEMERON |url=https://www.merriam-webster.com/dictionary/nychthemeron |url-status=live |archive-url=https://web.archive.org/web/20170202145445/https://www.merriam-webster.com/dictionary/nychthemeron |archive-date=2017-02-02 |access-date=2017-02-01 |website=www.merriam-webster.com |language=en}}</ref> or more colloquially the term {{nowrap|''24 hours''}}. Other languages may also have a separate word for a full day.
* A day counting approximation, for example "See you in three days." or "the following day"
* Part of a date: the day of the year (''doy'') in [[ordinal date]]s, day of the month (''dom'') in [[calendar date]]s or [[weekday|day of the week]] (''dow'') in [[week date]]s.
* Time regularly spend at paid work on a single [[work day]], cf. [[man-hour|man-day]] and [[workweek]].

; Daytime
* The period of light when the Sun is above the local [[horizon]] (that is, the time period from [[sunrise]] to [[sunset]])
* The time period from first-light "[[dawn]]" to last-light "[[dusk]]".

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

== Variations in length ==
{{See|Leap second|Tidal acceleration}}
Mainly due to [[Tidal acceleration|tidal deceleration]] – the Moon's [[Gravity|gravitational pull]] slowing down the [[Earth's rotation]] – the Earth's [[Rotation period|rotational period]] is slowing.<ref name=":4">{{Cite journal |last1=McCarthy |first1=Dennis D. |last2=Hackman |first2=Christine |last3=Nelson |first3=Robert A. |date=2008-11-01 |title=The Physical Basis of the Leap Second |url=https://iopscience.iop.org/article/10.1088/0004-6256/136/5/1906 |journal=The Astronomical Journal |volume=136 |issue=5 |pages=1906–1908 |doi=10.1088/0004-6256/136/5/1906 |bibcode=2008AJ....136.1906M |s2cid=124701789 |issn=0004-6256 |access-date=2022-08-20 |archive-date=2022-05-31 |archive-url=https://web.archive.org/web/20220531162009/https://iopscience.iop.org/article/10.1088/0004-6256/136/5/1906 |url-status=live |url-access=subscription }}</ref> Because of the way the second is defined, the mean length of a solar day is now about 86,400.002 seconds, and is increasing by about 2 [[millisecond]]s per century.<ref name=":2">{{Cite journal |last=Arbab |first=Arbab I. |date=January 2009 |title=The Length of the Day: A Cosmological Perspective |url=http://www.ptep-online.com/2009/PP-16-02.PDF |journal=Department of Physics, Faculty of Science, University of Khartoum |volume=1 |access-date=2022-08-20 |archive-date=2022-08-20 |archive-url=https://web.archive.org/web/20220820051602/http://www.ptep-online.com/2009/PP-16-02.PDF |url-status=usurped }}</ref>

Since the rotation rate of the Earth is slowing, the length of a {{Abbr|SI|International System of Units}} second fell out of sync with a second derived from the rotational period.<ref name=":4" /> This created the need for [[leap second]]s, which insert extra seconds into [[Coordinated Universal Time|Coordinated Universal Time (UTC)]].<ref name=":4" /> Although typically 86,400 {{Abbr|SI|International System of Units}} seconds in duration, a civil day can be either 86,401 or 86,399 SI seconds long on such a day. Other than the two-millisecond variation from tidal deceleration, [[Day length fluctuations|other factors minutely affect the day's length]], which creates an irregularity in the placement of leap seconds.<ref>{{cite web |date=2013 |title=IERS science background |url=https://www.iers.org/IERS/EN/Science/EarthRotation/EarthRotation.html |url-status=live |archive-url=https://web.archive.org/web/20160829050135/https://www.iers.org/IERS/EN/Science/EarthRotation/EarthRotation.html |archive-date=August 29, 2016 |access-date=August 6, 2016 |publisher=[[IERS]] |location=Frankfurt am Main}}</ref> Leap seconds are announced in advance by the [[International Earth Rotation and Reference Systems Service|International Earth Rotation and Reference Systems Service (IERS)]], which measures the Earth's rotation and determines whether a leap second is necessary.

=== Geological day lengths ===
Discovered by paleontologist [[John W. Wells]], the day lengths of geological periods have been estimated by measuring sedimentation rings in coral [[fossil]]s,<ref name=":3" /><ref name=":2" /> due to some biological systems being affected by the tide.<ref name=":2" /> The length of a day at the Earth's formation is estimated at 6 hours.<ref name=":2" /> Arbab I. Arbab plotted day lengths over time and found a curved line.<ref name=":2" /> Arbab attributed this to the change of water volume present affecting Earth's rotation.<ref name=":2" />
{| class="wikitable"
|-
| '''Date'''
| '''Geological period'''
| '''Number of days per year'''<ref name=":3">{{Cite journal|url=http://articles.adsabs.harvard.edu//full/1969LAstr..83..411K/0000411.000.html|title=Paléo-Astronomie|author=J.Kovalesky ''Bureau des Longitudes''|journal=L'Astronomie|year=1969|volume=83|page=411|bibcode=1969LAstr..83..411K|access-date=5 June 2021|archive-date=21 December 2019|archive-url=http://archive.wikiwix.com/cache/20191221210557/http://articles.adsabs.harvard.edu//full/1969LAstr..83..411K/0000411.000.html|url-status=live}}</ref>
| '''Duration of the day'''
|-
| Present
| Current
| 365
| 24 hours
|-
| −100 million years
| [[Cretaceous]]
| 380
| 23 hours and 20 minutes
|-
| −200 million years
| [[Triassic]]
| 390
| 22 hours and 40 minutes
|-
| −300 million years
| [[Carboniferous]]
| 400
| 22 hours
|-
| −400 million years
| [[Devonian]]
| 410
| 21 hours and 20 minutes
|-
| −500 million years
| [[Cambrian]]
| 425
| 20 hours and 40 minutes
|}

== Boundaries ==
[[File:Sun and Moon Nuremberg chronicle.jpg|thumb|208px|[[Sun]] and [[Moon]], [[Hartmann Schedel]]'s ''[[Nuremberg Chronicle]]'', 1493]]{{More citations needed section|date=May 2023}}
For most [[Diurnality|diurnal]] 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.

<!-- === Evening to evening === -->
In the [[Hebrew Bible]], [[Book of Genesis|Genesis]] 1:5 defines a day in terms of "evening" and "morning" before recounting the creation of the 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."
The [[Judaism|Jewish]] day begins at either sunset or nightfall (when three second-[[Magnitude (astronomy)|magnitude]] stars appear).
[[Middle Ages|Medieval]] Europe also followed this tradition, known as [[Florentine calendar|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]], [[Halloween]] (“All Hallows’ Eve”), and the Eve of [[Agnes of Rome|Saint Agnes]] are remnants of the older pattern when [[Holiday#Religious holidays|holidays]] began during the prior evening.

<!-- === Midnight to midnight === -->
The common convention among the [[Ancient Rome|ancient Romans]],<ref>See [https://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A2008.01.0212%3Asection%3D84 Plutarch, ''Quaestiones Romanae'', 84.] {{Webarchive|url=https://web.archive.org/web/20210209025420/http://www.perseus.tufts.edu/hopper/text?doc=Perseus:text:2008.01.0212:section%3D84 |date=2021-02-09 }}</ref> [[Ancient China|ancient Chinese]]<ref>[[s:zh:清史稿/卷48]]: 起''子正''，盡''夜子初''。</ref> 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. The [[International Meridian Conference]] of 1884 resolved
<blockquote>That the Conference expresses the hope that as soon as may be practicable the astronomical and nautical days will be arranged everywhere to begin at midnight.</blockquote>

<!-- === Morning to morning === -->
In [[ancient Egypt]] the day was reckoned from sunrise to sunrise.

Prior to 1926, Turkey had two time systems: ''Turkish'', counting the hours from sunset, and ''French'', counting the hours from midnight.

== Parts ==
{{See also|Category:Parts of a day}}
Humans have divided the day in rough periods, which can have cultural implications, and other effects on humans' biological processes. The parts of the day do not have set times; they can vary by lifestyle or hours of daylight in a given place.<ref>{{Cite web |title=Parts of the Day: Early morning, late morning, etc. |url=https://www.britannica.com/dictionary/eb/qa/parts-of-the-day-early-morning-late-morning-etc |access-date=2022-08-22 |website=Britannica Dictionary |language=en-US |archive-date=2022-08-22 |archive-url=https://web.archive.org/web/20220822024715/https://www.britannica.com/dictionary/eb/qa/parts-of-the-day-early-morning-late-morning-etc |url-status=live }}</ref>

=== Daytime ===
{{Main|Daytime}}
[[Daytime]] is the part of the day during which [[sunlight]] directly reaches the ground, assuming that there are no obstacles. The length of daytime averages slightly more than half of the 24-hour day. Two effects make daytime on average longer than night. The Sun is not a point but has an apparent size of about 32 [[minute of arc|minutes of arc]]. Additionally, the [[Earth's atmosphere|atmosphere]] [[refraction|refracts]] 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.<ref>{{frac|32′|2}} + 34′ = 50′</ref> Thus, daytime is on average around 7 minutes longer than 12 hours.<ref>{{sfrac|50°|60}} ÷ 360° × 2(for sunrise and set) × 24 hours ≈ 7 min</ref>

Daytime is further divided into [[morning]] and [[afternoon]]. Morning occurs between [[sunrise]] and [[noon]].<ref>{{Cite web |title=Definition of MORNING |url=https://www.merriam-webster.com/dictionary/morning |access-date=2022-08-22 |website=www.merriam-webster.com |language=en |archive-date=2022-08-22 |archive-url=https://web.archive.org/web/20220822030634/https://www.merriam-webster.com/dictionary/morning |url-status=live }}</ref> Afternoon occurs between noon and [[sunset]],<ref>{{Cite web |title=Definition of AFTERNOON |url=https://www.merriam-webster.com/dictionary/afternoon |access-date=2022-08-22 |website=www.merriam-webster.com |language=en |archive-date=2022-08-22 |archive-url=https://web.archive.org/web/20220822030634/https://www.merriam-webster.com/dictionary/afternoon |url-status=live }}</ref> or between noon and the start of evening. This period of time sees humans' highest [[Thermoregulation|body temperature]],<ref>{{cite book|last=Refinetti|first=Roberto|title=Circadian Physiology|date=2006|edition=2nd|publisher=Taylor & Francis Group|isbn=978-0-8493-2233-4|page=556}}</ref> an increase of [[traffic collision]]s,<ref>{{cite book|last=McCabe|first=Paul T.|title=Contemporary Ergonomics|date=2004|publisher=CRC Press|isbn=0-8493-2342-8|page=588}}</ref> and a decrease of [[productivity]].<ref>{{cite book|last=Ray|first=James T.|title=Human Performance as a Function of the Work–Rest Cycle|publisher=[[National Academy of Sciences]]|date=1960|page=11}}</ref> 

=== Twilight ===
{{Main|Twilight}}
[[File:Twilight-dawn_subcategories.svg|thumb|right]]
[[Twilight]] is the period before [[sunrise]] and after [[sunset]] in which there is natural light but no direct sunlight.<ref name="USNO">{{cite web |title=Definitions from the US Astronomical Applications Dept |url=http://aa.usno.navy.mil/faq/docs/RST_defs.php |archive-url=https://web.archive.org/web/20190927072432/http://aa.usno.navy.mil/faq/docs/RST_defs.php |archive-date=2019-09-27 |access-date=2011-07-22 |publisher=[[United States Naval Observatory|USNO]]}}</ref> The morning twilight begins at [[dawn]] and ends at sunrise, while the evening twilight begins at sunset and ends at [[dusk]]. Both periods of twilight can be divided into [[civil twilight]], [[nautical twilight]], and [[astronomical twilight]]. Civil twilight is when the sun is up to 6 degrees below the horizon; nautical when it is up to 12 degrees below, and astronomical when it is up to 18 degrees below.<ref>{{Cite web |title=Glossary of Marine Navigation |url=http://msi.nga.mil/MSISiteContent/StaticFiles/NAV_PUBS/APN/Gloss-1.pdf |url-status=dead |archive-url=https://web.archive.org/web/20170829062838/http://msi.nga.mil/MSISiteContent/StaticFiles/NAV_PUBS/APN/Gloss-1.pdf |archive-date=2017-08-29 |access-date=2022-08-22 }}</ref>
[[File:Adolf Stademann Flusslandschaft im Mondschein.jpg|thumb|[[Night in paintings (Western art)|Night in art]]]]

[[Evening]] can begin before sunset and overlaps both twilight and night. Evening begins around 5 or 6 pm, or when the sun sets, and ends when one goes to bed.<ref name="oed">{{cite web |title=evening, n. |url=https://www.oed.com/dictionary/evening_n1 |website=www.oed.com |publisher=Oxford English Dictionary |access-date=18 September 2023|quote=The close of day, esp. the time from about 6 p.m., or sunset if earlier, to bedtime; the period between afternoon and night.}}</ref><ref name="Britannica">{{cite web|title=Definition of evening in English|url=https://www.britannica.com/dictionary/eb/qa/parts-of-the-day-early-morning-late-morning-etc|website=Britannica|publisher=[[Britannica]]|access-date=17 Sep 2023}}</ref><ref>{{Cite web |title=Definition of 'evening' |url=https://www.collinsdictionary.com/dictionary/english/evening |website=[[Collins Dictionary]] |access-date=2022-08-22 |archive-date=2021-05-01 |archive-url=https://web.archive.org/web/20210501040345/https://www.collinsdictionary.com/dictionary/english/evening |url-status=live }}</ref>

=== Night ===
{{Main|Night}}
[[Night]] or ''nighttime''<ref>{{cite dictionary |title=Nighttime |url=https://dictionary.cambridge.org/dictionary/english/nighttime |dictionary=Cambridge Dictionary |publisher=Cambridge University Press & Assessment |access-date=2025-08-04}}</ref><ref>{{cite dictionary |title=Nighttime |url=https://www.collinsdictionary.com/dictionary/english/nighttime |dictionary=Collins Dictionary|publisher=Harper Collins |access-date=2025-08-04}}</ref><ref>{{cite dictionary |title=Nighttime |url=https://www.merriam-webster.com/dictionary/nighttime |dictionary=Merriam-Webster.com Dictionary |publisher=Merriam-Webster |access-date=2025-08-04}}</ref> is the period in which the sky is [[Darkness|dark]];<ref>{{Cite web |title=Night |url=https://dictionary.cambridge.org/dictionary/english/night |access-date=2022-08-22 |website=[[Cambridge Advanced Learner's Dictionary|Cambridge Dictionary]] |archive-date=2022-04-01 |archive-url=https://web.archive.org/web/20220401194705/https://dictionary.cambridge.org/dictionary/english/night |url-status=live }}</ref> the period between [[dusk]] and [[dawn]] when no light from the sun is visible.<ref>{{Cite web |title=Definition of NIGHT |url=https://www.merriam-webster.com/dictionary/night |access-date=2022-08-22 |website=www.merriam-webster.com |language=en |archive-date=2022-08-22 |archive-url=https://web.archive.org/web/20220822024714/https://www.merriam-webster.com/dictionary/night |url-status=live }}</ref> [[Light pollution]] during night can impact human and animal life, for example by disrupting sleep.<ref>{{Cite journal |last1=Blume |first1=Christine |last2=Garbazza |first2=Corrado |last3=Spitschan |first3=Manuel |date=2019 |title=Effects of light on human circadian rhythms, sleep and mood |journal=Somnologie |volume=23 |issue=3 |pages=147–156 |doi=10.1007/s11818-019-00215-x |doi-access=free |issn=1432-9123 |pmc=6751071 |pmid=31534436}}</ref><ref>{{Cite web |title=Light pollution harms wildlife and ecosystems|url=https://darksky.org/resources/what-is-light-pollution/effects/wildlife-ecosystems |access-date=2023-10-30 |website=DarkSky International |language=en}}</ref>

===In other cultures===
Divisions of the day into parts vary between cultures. For example, in Brazil, the day is divided into four periods of six hours each:<ref>{{cite web |url=https://www.portugues.com.br/gramatica/seis-meia-tarde-ou-noite.html |title=Seis e meia da tarde ou da noite? |website=Português |language=pt |access-date=2025-06-28}}</ref><ref>{{cite web |url=https://brasilescola.uol.com.br/gramatica/cinco-meia-manha-ou-madrugada.htm |title=Cinco e meia da manhã ou da madrugada? |website=UOL |language=pt |access-date=2025-06-28}}</ref><ref>{{cite web |url=https://www12.senado.leg.br/manualdecomunicacao/estilos/hora |title=Manual de Comunicação da Secom |publisher=Federal Senate, Brazil |language=pt |access-date=2025-06-28}}</ref> 

{|class="wikitable"
!Period!!Name
|-
|00:00–05:59||{{lang|pt|madrugada}}
|-
|06:00–11:59||{{lang|pt|manhã}}
|-
|12:00–17:59||{{lang|pt|tarde}}
|-
|18:00–23:59||{{lang|pt|noite}}
|}

== See also ==
{{div col|colwidth=30em}}
* [[Determination of the day of the week]]
* [[Holiday]]
* [[ISO 8601]]
* [[Season]], for a discussion of daylight and darkness at various latitudes
* [[Synodic day]]
* [[World Meteorological day]]
{{div col end}}

== References ==
{{Reflist}}

==External links==
* {{Commons category-inline}}
* {{Wiktionary-inline}}
* {{Wikiquote-inline}}

{{Time topics}}
{{Time measurement and standards}}
{{Parts of a day}}
{{SI units}}

{{Authority control}}

[[Category:Day| ]]
[[Category:Orders of magnitude (time)]]
[[Category:Units of time]]