Editing Day (section)

== 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 }}</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 06:00–18:00 (6:00 [[12-hour clock|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]]", "[[afternoon]]", or "[[evening]]".
* 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".