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'{{No footnotes|date=January 2012}} [[File:Polar-Night Longyearbyen.jpg|thumb|250px|right|Characteristic polar night blue twilight, [[Longyearbyen]], [[Svalbard]], located at 78° north.]] The '''polar night''' occurs when the night lasts for more than 24 hours. This occurs only inside the [[polar circle]]s. The opposite phenomenon, the [[midnight sun|polar day, or midnight sun]], occurs when the sun stays above the horizon for more than 24 hours. ==Description== A common misconception is that the [[shortest day]] is totally dark everywhere inside the polar circle. In places very close to the poles it is completely dark, but regions located at the inner border of the polar circles experience polar [[twilight]] instead of polar night. In fact, [[polar region]]s typically get more twilight throughout the year than [[tropics|equatorial regions]]. For regions inside the polar circles, the maximum lengths of the time that the sun is completely below the horizon varies from about 20 hours at the [[Arctic Circle]] and [[Antarctic Circle]] {{Citation needed|date=October 2010}} to 179 days at the [[Geographical pole|Poles]]{{Citation needed|date=February 2011}}. However not all this time is classified as polar night, since sunlight may be visible because of [[refraction]] {{Citation needed|date=February 2011}}. Also, the time when the sun is '''above''' the horizon at the poles is 186 days. The numerical [[wiktionary:asymmetry|asymmetry]] occurs because the time when the sun is ''partially'' above the horizon is counted towards the "daytime". Also, the above numbers are average numbers: owing to the ellipticity of the Earth's orbit, where the South pole receives a week more of sun-below-horizon than the North pole (see [[equinox]]). == Types of polar night == [[File:Morketidettermiddag.JPG|thumb|250px|right|Early afternoon during the polar night in [[Tromsø]], [[Norway]].]] [[File:Nordkinnhalvoya-polar-night.jpg|thumb|right|250px|Polar night on [[Nordkinn Peninsula]], mainland Europe's northernmost peninsula.]] As there are various kinds of [[twilight]], there also exist various kinds of polar night. Each kind of polar night is defined as when it's darker than the corresponding kind of twilight. The descriptions below are based on relatively clear skies, so the sky will be darker in the presence of dense cloud. ===Polar twilight=== Polar twilight occurs in areas that are located at the inner border of the polar circles, where the sun will be on or below the horizon all day on the [[winter solstice]], although there may still be enough light for normal outdoor activities during civil twilight because of light scattering by the upper atmosphere and refraction. It happens when there is no true ''[[daylight]]'' and only ''[[civil twilight]]'' occurs at the [[solar culmination]], meaning that the sun is less than 6° below the horizon. Street-lamps may remain on and a person looking at a window from within a brightly lit room may see their reflection even at noon, as the level of outdoor [[illuminance]] will be below that of many illuminated indoor spaces. However, it is possible to conduct outdoor activities without any artificial light during the time of civil twilight. Sufferers of [[Seasonal Affective Disorder]] tend to seek out therapy with artificial light, as the psychological benefits of daylight require relatively high levels of ambient light (up to 10,000 [[lux]]) which are not present in any stage of twilight, thus, the midday twilights experienced anywhere inside the polar circles are still "polar night" in that sense. === Civil polar night === The ''civil polar night'' period produces only a faint glow of light visible at midday. It happens when there is no ''[[civil twilight]]'' and only ''[[nautical twilight]]'' occurs at the [[solar culmination]]. Civil twilight happens when the sun is between 0 and 6° below the horizon, and civil night when it is lower than that. Therefore the civil polar night is limited to latitudes above 72° 34', which is exactly 6° inside the polar circle. Nowhere on mainland Europe is this definition met. On the Norwegian territory of [[Svalbard]], however, civil polar night lasts from about 11 November until 30 January. [[Dikson (urban-type settlement)|Dikson]], in Russia, experiences civil polar night for approximately a month. During dense cloud cover places like the coast of [[Finnmark]] (about 70°) in [[Norway]] will get a darker "day". === Nautical polar night === During the ''nautical polar night'' period there is no trace of natural light, except for around midday. It happens when there is no ''[[nautical twilight]]'' and only ''[[astronomical twilight]]'' occurs at the [[solar culmination]]. Nautical twilight happens when the sun is between six and twelve degrees below the horizon. There is a location at the [[horizon]] around midday with more light than others because of refraction. During nautical night the sun is lower than 12° below the horizon, so nautical polar night is limited to latitudes above 78° 34', which is exactly 12° within the polar circle, or 11.5° from the pole. [[Alert, Nunavut|Alert]], [[Nunavut]], the northernmost settlement in [[Canada]] and the world, experiences this from late November to mid January. === Astronomical polar night === The ''astronomical polar night'' is a period of continuous ''[[night]]'' where no ''[[astronomical twilight]]'' occurs. Astronomical twilight happens when the sun is between twelve and eighteen degrees below the horizon and astronomical night when it is even lower. Thus, the astronomical polar night is limited to latitudes above 84° 34', which is exactly 18° within the polar circle, or five and a half degrees from the pole. During the astronomical polar night stars of the sixth magnitude, which are the dimmest stars visible to the naked eye, will be visible throughout the entire [[day]]. There are no permanent settlements anywhere in this range of latitude. This portion of the [[Arctic Ocean]] is generally permanently ice-capped{{fact|date=January 2013}}. Some scientific stations in [[Antarctica]], including the [[Amundsen-Scott South Pole Station]], experience this. ==Effects on humans== The period of polar night can trigger [[clinical depression|depression]] in some people. People who suffer from [[seasonal affective disorder]] are particularly susceptible to these conditions.{{Citation needed|date=March 2010}} The polar night may also be implicated in some instances of [[solipsism syndrome]].{{Citation needed|date=March 2010}} ==Popular culture== The concept of a night of almost one month long has been the subject of the vampire movies ''[[Frostbiten]]'' and ''[[30 Days of Night (film)|30 Days of Night]]''. In these films, the vampires are drawn to the long duration of darkness, allowing them to terrorize the area as they please. ==Further reading== * {{cite journal | last1 = Hansen | first1 = V. | last2 = Lund | first2 = E. | last3 = Smith-Sivertsen | first3 = T. | year = 1998 | month = March | title = Self-reported mental distress under the shifting daylight in the high north | journal = [[Psychological Medicine]] | volume = 28 | issue = 2 | pages = 447–452 | doi = 10.1017/S0033291797006326 | pmid=9572101 }} ==References== {{Empty section|date=March 2012}} ==External links== * [http://collections.ic.gc.ca/simply_science/toc.htm The polar night and polar darkness] * [http://weather.cs.uit.no/video/index-2006.html Many years' of webcam pictures from Tromsø, Norway.] These clearly show the progression into and away from winter at a latitude within the Arctic Circle. {{DEFAULTSORT:Polar Night}} [[Category:Earth phenomena]] [[Category:Arctic geography terminology]] [[be:Палярная ноч]] [[be-x-old:Палярная ноч]] [[ca:Nit polar]] [[cs:Polární noc]] [[da:Polarnat]] [[de:Polarnacht]] [[et:Polaaröö]] [[es:Noche polar]] [[eu:Gau polar]] [[fa:شب قطبی]] [[fr:Nuit polaire]] [[xal:Кииснә сө]] [[ko:극야]] [[hr:Polarna noć]] [[it:Notte polare]] [[kk:Полюстік тун]] [[lv:Polārā nakts]] [[lb:Polarnuecht]] [[lt:Poliarinė naktis]] [[mk:Поларна ноќ]] [[nl:Poolnacht]] [[ja:極夜]] [[no:Polarnatt]] [[pl:Noc polarna]] [[pt:Noite polar]] [[ru:Полярная ночь]] [[se:Skábma]] [[sq:Nata polare]] [[simple:Polar night]] [[sk:Polárna noc]] [[sh:Polarna noć]] [[fi:Kaamos]] [[sv:Polarnatt]] [[uk:Полярна ніч]] [[vi:Ban đêm vùng cực]] [[zh:极夜]]'