Examine individual changes

'[[File:Polar-Night Longyearbyen.jpg|thumb|right|Characteristic polar night blue twilight, [[Longyearbyen]], [[Svalbard]], [[Norway]] located at 78° north.]] [[File:Amundsen-Scott marsstation ray h.jpg|thumb|right|Polar night at the [[South Pole]], [[Antarctica]].]] [[File:Polar night in Naryan-Mar.jpg|thumbnail|right|Polar night in [[Naryan-Mar]], [[Russia]]. December 23, 2014, 11:27 (noon)]] The '''polar night''' occurs when the night lasts for more than 24&nbsp;hours. This occurs only inside the [[polar circle]]s.<ref>{{cite book|last=Burn|first=Chris|title=The Polar Night|url=http://nwtresearch.com/sites/default/files/the-polar-night.pdf|publisher=The Aurora Research Institute|accessdate=28 September 2015}}</ref> The opposite phenomenon, the polar day, or [[midnight sun]], occurs when the [[Sun]] stays above the horizon for more than 24&nbsp;hours. "Night" is understood as the center of the Sun being below a free horizon. Since the atmosphere [[refraction|bends the rays]] of the Sun, the polar day is longer than the polar night, and the area that is affected by polar night is somewhat smaller than the area of midnight sun. The [[polar circle]] is located at a latitude between these two areas, at the [[latitude]] of approximately 66.5 degrees. In the most northern city of Sweden, [[Kiruna]], at 67°51'N, the polar night lasts for around 28 twenty-four-hour periods, while the midnight sun lasts around 50 twenty-four-hour periods. While it is day in the Arctic Circle it is night in the Antarctic Circle. ==Description== The polar [[shortest day]] is not totally dark everywhere inside the polar circle, but only in places within about 5° of the poles, and only when the moon is well below the horizon. 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 zero a quarter degree beyond the [[Arctic Circle]] and [[Antarctic Circle]] 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]]. Also, the time when the Sun is above the horizon at the poles is 186 days. The numerical 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: the ellipticity of the Earth's orbit makes the South Pole receive 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]] in Norway, 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 clouds. ===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]]. There is then no true [[daylight]] at the [[solar culmination]], only [[civil twilight]]. This means that the Sun is below the horizon, but by less than 6°. During civil twilight, there may still be enough light for normal outdoor activities because of light scattering by the upper atmosphere and refraction. [[Street lamp]]s 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. 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&nbsp;[[lux]]) which are not present in any stage of twilight, thus, the midday twilights experienced anywhere inside the polar circles are still "polar night" for this purpose. Thus, polar twilight is limited to latitudes more than 66.5 degrees from the equator, in other words, north of the Arctic Circle or south of the Antarctic Circle. In fact, on either one of these circles, at least half of the sun's disk will still be above the horizon at midday all year round, and because of refraction even more will be visible. For the sun to be completely invisible at midday one would have to be about 50 minutes of an arc inside the circle (see [[Sunrise]]). This also depends on one's elevation and the elevation of the horizon. === 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°&nbsp;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 13 November until 29 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&nbsp;70°) in [[Norway]] will get a darker "day". On the Canadian territory of [[Resolute, Nunavut]] however civil polar night lasts from about 28 November until 13 January. === Nautical polar night === During the nautical polar night period, there is no trace of daylight, except 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°&nbsp;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. [[Ny-Alesund]] experiences this from 11 December to 30 December. === 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 lower than that. Thus, the astronomical polar night is limited to latitudes above 84°&nbsp;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]]. This happens when the sun is between 18 and 23.5 degrees below the horizon. These conditions last about 11 weeks at the poles. The [[Amundsen–Scott South Pole Station]] experiences this. The North Pole experiences this from November 14 to January 29.<ref name="arctic polar night livescience">{{cite web|last1=Rao|first1=Joe|title=The Myth of Arctic Daylight and Darkness Exposed|url=http://www.livescience.com/32814-arctic-daylight-darkness-myth-equinox.html|website=[[Live Science]]|accessdate=14 April 2017|date=21 September 2010}}</ref> ==In popular culture== The concept of a night of almost one month in length has been the subject of the vampire movies ''[[Frostbite (2006 film)|Frostbite]]'' 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 openly kill and feed at will. ==References== {{reflist}} ==Further reading== * {{cite journal | last1 = Hansen | first1 = V. | last2 = Lund | first2 = E. | last3 = Smith-Sivertsen | first3 = T. |date=March 1998 | 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 }} ==External links== * [https://web.archive.org/web/20120904173515/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. {{Authority control}} {{Parts of a day}} {{DEFAULTSORT:Polar Night}} [[Category:Earth phenomena]] [[Category:Arctic geography terminology]] [[Category:Night]]'