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A day is approximately the period of time during which the Earth completes one rotation on its axis.^[1] A solar day is the length of time which elapses between the Sun reaching its highest point in the sky two consecutive times.^[2]

In 1960, the second was redefined in terms of the orbital motion of the Earth in the year 1900, and was designated the SI base unit of time. The unit of measurement "day", was redefined as 86,400 SI seconds and symbolized d. In 1967, the second and so the day were redefined by atomic electron transition.^[3]

References

^ Weisstein, Eric W. (2007). "Day". Retrieved 2011-05-31.
^ Weisstein, Eric W. (2007). "Solar Day". Retrieved 2011-05-31.
^ BIPM (2014) [2006]. "Unit of time (second)". SI Brochure (8th ed.).

External links

Media related to Day at Wikimedia Commons
The dictionary definition of day at Wiktionary
Quotations related to Day at Wikiquote

Time

Key concepts

Measurement
and standards

Chronometry	UTC UT TAI Unit of time Orders of magnitude (time)
Measurement systems	Italian six-hour clock Thai six-hour clock 12-hour clock 24-hour clock Relative hour Daylight saving time Chinese Decimal Hexadecimal Hindu Metric Roman Sidereal Solar Time zone
Calendars	Main types Solar Lunar Lunisolar Gregorian Julian Hebrew Islamic Solar Hijri Chinese Hindu Panchang Maya List
Clocks	Main types astronomical astrarium atomic quantum hourglass marine sundial watch mechanical stopwatch water-based Cuckoo clock Digital clock Grandfather clock History Timeline

Philosophy of time

Human experience
and use of time

Time in science

Geology	Geological time age chron eon epoch era period Geochronology Geological history of Earth
Physics	Absolute space and time Arrow of time Chronon Coordinate time Instant Proper time Spacetime Theory of relativity Time domain Time translation symmetry Time reversal symmetry
Other fields	Chronological dating Chronobiology Circadian rhythms Clock reaction Glottochronology Time geography

v t e Time measurement and standards
Chronometry Orders of magnitude Metrology
International standards	Coordinated Universal Time offset UT ΔT DUT1 International Earth Rotation and Reference Systems Service ISO 31-1 ISO 8601 International Atomic Time 12-hour clock 24-hour clock Barycentric Coordinate Time Barycentric Dynamical Time Civil time Daylight saving time Geocentric Coordinate Time International Date Line IERS Reference Meridian Leap second Solar time Terrestrial Time Time zone 180th meridian
Obsolete standards	Ephemeris time Greenwich Mean Time Prime meridian
Time in physics	Absolute space and time Spacetime Chronon Continuous signal Coordinate time Cosmological decade Discrete time and continuous time Proper time Theory of relativity Time dilation Gravitational time dilation Time domain Time-translation symmetry T-symmetry
Horology	Clock Astrarium Atomic clock Complication History of timekeeping devices Hourglass Marine chronometer Marine sandglass Radio clock Watch stopwatch Water clock Sundial Dialing scales Equation of time History of sundials Sundial markup schema
Calendar	Gregorian Hebrew Hindu Holocene Islamic (lunar Hijri) Julian Solar Hijri Astronomical Dominical letter Epact Equinox Intercalation Julian day Leap year Lunar Lunisolar Solar Solstice Tropical year Weekday determination Weekday names
Archaeology and geology	Chronological dating Geologic time scale International Commission on Stratigraphy
Astronomical chronology	Galactic year Nuclear timescale Precession Sidereal time
Other units of time	Instant Flick Shake Jiffy Second Minute Moment Hour Day Week Fortnight Month Year Olympiad Lustrum Decade Century Saeculum Millennium
Related topics	Chronology Duration music Mental chronometry Decimal time Metric time System time Time metrology Time value of money Timekeeper

v t e Parts of a day
Daytime	Morning Noon Afternoon Evening
Twilight	Dawn Sunrise Sunset Dusk Blue hour
Night	Midnight Witching hour Brahmamuhurta
Related	Belt of Venus Golden hour Terminator Daylight Moonlight Midnight sun Polar night Zodiacal light

v t e SI units
Base units	ampere candela kelvin kilogram metre mole second
Derived units with special names	becquerel coulomb degree Celsius farad gray henry hertz joule katal lumen lux newton ohm pascal radian siemens sievert steradian tesla volt watt weber
Other accepted units	astronomical unit dalton day decibel degree of arc electronvolt hectare hour litre minute minute and second of arc neper tonne
See also	Conversion of units Metric prefixes Historical definitions of the SI base units 2019 revision of the SI System of units of measurement
Category

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 In 1960, the [[second]] was redefined in terms of the orbital motion of the Earth in the year 1900, and was designated the [[SI base unit]] of [[time]]. The [[Units of measurement|unit of measurement]] "day", was redefined as 86,400 SI seconds and symbolized ''d''. In 1967, the second and so the day were redefined by [[atomic electron transition]].<ref name=Second>{{cite web |url=https://www.bipm.org/en/publications/si-brochure/second.html |title=Unit of time (second) |edition=8th |date=2014 |orig-year=2006 |website=SI Brochure |author=BIPM |authorlink=International Bureau of Weights and Measures}}</ref>
-== Boundaries ==
-[[File:Sun and Moon Nuremberg chronicle.jpg|thumb|left|208px|[[Sun]] and [[Moon]], Hartmann Schedel's ''[[Nuremberg Chronicle]]'', 1493]]
-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. Common convention among the [[Ancient Rome|ancient Romans]],<ref>See [http://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A2008.01.0212%3Asection%3D84 Plutarch, ''Quaestiones Romanae'', 84.]</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 last a full 24 hours until 24:00 (i.e. 00:00 of the next day).
-In [[ancient Egypt]], the day was reckoned from sunrise to sunrise. 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.{{citation needed|date=May 2019}} Days such as [[Christmas Eve]], [[Halloween]], and the Eve of [[Agnes of Rome|Saint Agnes]] are remnants of the older pattern when [[Holiday#Religious holiday|holidays]] began during the prior evening. Prior to 1926, [[Turkey]] had two time systems: Turkish (counting the hours from sunset) and French (counting the hours from midnight).
-Validity of [[Ticket (admission)|tickets]], 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]]) 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. 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]] (Dutch Railways) is valid for 28 hours, from 0:00 to 28:00 (that is, 4:00 the next day); the validity of a pass on [[Transport for London]] (TfL) services is until the end of the "transport day"&nbsp;– that is to say, until 4:30 am on the day after the "expires" date stamped on the pass.
 == See also ==

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