Night
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'''Night''' or '''nighttime''' is the period of [[time]] when the [[sun]] is below the [[horizon]]. The opposite of night is [[day]] (or "[[daylight|daytime]]" to distinguish it from "day" as used for a 24-hour period). Time of day varies based on factors such as [[season]], [[latitude]], [[longitude]] and [[timezone]].
==Duration and geography==
[[Image:Nightfall europe-and-afrika 20050507-184500.jpg|thumb|right|245px|A composite image showing the [[Terminator (solar)|terminator]] dividing night from day, running across Europe and Africa. Observers on the surface of the earth along this terminator will see a [[sunset]].]]
Nights are shorter than days on average due to two factors. One, the sun is not a point, but has an apparent size of about 32 minutes of arc. Two, the atmosphere refracts sunlight so that some of it reaches the ground when the sun is below the horizon by about 34 minutes of arc. The combinaton of these two factors means that light reaches the ground when the centre of the sun is below the horizon by about 50 minutes of arc.
Without these effects, day and night would be the same length at the [[Autumnal equinox|autumnal]] (autumn/fall) and [[Vernal equinox|vernal]] (spring) [[equinox]]es, the moments when the sun passes over the [[equator]]. In reality, around the equinoxes the day is almost 14 minutes longer than the night at the equator, and even more closer to the [[geographical pole|pole]]s. The summer and winter [[solstice]]s mark the shortest night and the [[winter solstice|longest night]], respectively.
The closer a location is to the [[North Pole|North]] or [[South Pole]], the larger the range of variation in the night's length. Although equinoxes occur with a day and night close to equal length, before and after an equinox the ratio of night to day changes more rapidly in locations near the poles than in locations between the [[Tropic of Cancer]] and the [[Tropic of Capricorn]]. In the [[Northern Hemisphere]], [[Denmark]] has shorter nights in June than [[India]] has. In the [[Southern Hemisphere]], [[Antarctica]] has longer nights in June than [[Chile]] has. The Northern and Southern Hemispheres of the world experience the same patterns of night length at the same [[latitude]]s, but the cycles are 6 months apart so that one hemisphere experiences long nights (winter) while the other is experiencing short nights (summer).
Between the pole and the polar circle, the variation in daylight hours is so extreme that for a portion of the summer, there is no longer an intervening night between consecutive days and in the winter there is a period that there is no intervening day between consecutive nights.
==On other celestial bodies==
The phenomenon of day and night is due to the rotation of a [[celestial body]] about its axis, creating the illusion of the sun rising and setting. Different bodies spin at very different rates, however. Some may spin much faster than [[Earth]], while others spin extremely slowly, leading to very long days and nights. The planet [[Venus]] rotates once every 243 days – by far the slowest rotation period of any of the major planets. In contrast, the gas giant [[Jupiter]]'s [[sidereal day]] is only 9 hours and 56 minutes.<ref>{{cite web | author = Seidelmann, P. K.; Abalakin, V. K.; Bursa, M.; Davies, M. E.; de Burgh, C.; Lieske, J. H.; Oberst, J.; Simon, J. L.; Standish, E. M.; Stooke, P.; Thomas, P. C. | year = 2001 | url = http://www.hnsky.org/iau-iag.htm | title = Report of the IAU/IAG Working Group on Cartographic Coordinates and Rotational Elements of the Planets and Satellites: 2000 | publisher = HNSKY Planetarium Program | accessdate = 2007-02-02 }}</ref> A planet may experience large temperature variations between day and night, such as [[Mercury (planet)|Mercury]], the closest planet to the sun. This is one consideration in terms of [[planetary habitability]] or the possibility of [[extraterrestrial life]].
==Impact on life==
[[Image:Big-eared-townsend-fledermaus.jpg|right|thumb|[[Bat]]s are just one of the thousands of species of animals that are active during the night]]
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The disappearance of sunlight, the primary [[energy]] source for [[life]] on Earth, has dramatic impacts on the [[morphology (biology)|morphology]], [[physiology]] and [[behavior]] of almost every organism. Some animals [[sleep]] during the night, while other [[nocturnal animal]]s including [[moths]] and [[cricket (insect)|cricket]]s are active during this time. The effects of day and night are not seen in the [[animal kingdom]] alone, [[plant]]s have also evolved adaptations to cope best with the lack of sunlight during this time. For example, [[crassulacean acid metabolism]] in a unique type of [[carbon fixation]] which allows [[photosynthesis|photosynthetic]] plants to store [[carbon dioxide]] in their tissues as [[organic acid]]s during the night, which can then be used during the day to synthesize [[carbohydrate]]s. This allows them to keep their [[stomata]] closed during the daytime, preventing [[transpiration]] of precious [[water]].
==Humans and the night==
===Social and economic factors===
[[Image:MATTTREVITHICKBW.JPG|thumb|200px|left|A busy street at nighttime]]
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Throughout the rest of history, night has primarily been a time of resting and [[sleep]] for humans, since little work or labor can be done in the dark. On the other hand, clandestine activities such as [[romance (love)|romance]], [[sexual intercourse|sex]], [[prostitution]], and [[crime|criminal]] and [[police]] activity flourish.
As artificial [[lighting]] has improved, especially after the [[Industrial Revolution]], night-time activity has increased and become a significant part of the [[economy]] in most places. Many establishments, such as [[nightclub]]s, [[bar (establishment)|bar]]s, [[convenience store]]s, fast-food restaurants, gas stations, distribution facilities, and police stations now operate 24 hours a day or stay open as late as 1 or 2 a.m. Even without artificial light, [[moon]]light sometimes makes it possible to travel or work outdoors at night.
===Cultural aspects===
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[[Image:Nott painting.jpg|thumb|left|[[Nótt]], the Norse goddess of the night, rides her horse in this 19th century painting by [[Peter Nicolai Arbo]].]]
Night is often associated with danger and evil, because bandits and dangerous animals can be concealed by darkness. The belief in [[Magic (paranormal)|magic]] often includes the idea that magic and magicians are more powerful at night. Similarly, mythical and folkloric creatures as [[vampire]]s, and [[werewolf|werewolves]] are thought to be more active at night. [[Ghost]]s are believed to wander around almost exclusively during night-time. In almost all cultures, there exist stories and legends warning of the dangers of night-time. In fact, the Saxons called the darkness of night the 'death mist'.{{Fact|date=July 2007}}
==See also==
{{commonscat|Night}}
{{Spoken Wikipedia|En-Night.ogg|2006-12-10}}
<!-- Ensure these are kept alphabetical (and try to merge them into the text, if you can) -->
*[[Earth clock]]
*[[Midnight]]
*[[Night sky]]
*[[Nightlife]]
*[[Nocturne]]
*[[Olbers' paradox]]
==References==
{{reflist}}
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[[Category:Night| ]]
[[Category:Time in astronomy]]
[[Category:Parts of a day]]
[[ar:ليل]]
[[ay:Aruma]]
[[az:Gecə]]
[[bg:Нощ]]
[[ca:Nit]]
[[cs:Noc]]
[[ch:Puengi]]
[[cy:Nos]]
[[da:Nat (tidsrum)]]
[[pdc:Nacht]]
[[de:Nacht]]
[[es:Noche]]
[[eo:Nokto]]
[[eu:Gau]]
[[fr:Nuit]]
[[gl:Noite]]
[[ko:밤 (시간)]]
[[io:Nokto]]
[[id:Malam]]
[[it:Notte (parte del giorno)]]
[[he:לילה]]
[[la:Nox]]
[[lv:Nakts]]
[[lt:Naktis]]
[[mk:Ноќ]]
[[ml:രാത്രി]]
[[nah:Yohualli]]
[[myv:Ве]]
[[nl:Nacht]]
[[new:इरवु (तमिल संकिपा)]]
[[ja:夜]]
[[no:Natt]]
[[nn:Natt]]
[[nrm:Nyit]]
[[pt:Noite]]
[[ro:Noapte]]
[[qu:Tuta]]
[[ru:Ночь]]
[[scn:Notti]]
[[sk:Noc]]
[[fi:Yö]]
[[sv:Natt]]
[[ta:இரவு]]
[[te:రాత్రి]]
[[uk:Ніч]]
[[yi:נאכט]]
[[zh-yue:夜晚]]
[[bat-smg:Naktės]]
[[zh:晚上]]