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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 scattering of light by the upper atmosphere and refraction. It happens when there is no true daylight and only civil twilight occurs at the solar culmination. Street-lamps may remain on and one looking out a window from a brightly lit room may see their reflection even at noon, as the level of outdoor illuminance will remain 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.
The civil polar night is the period during which there is 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 zero and six degrees below the horizon. The civil polar night is limited to latitudes above 72° 33', which is exactly 6 degrees inside the polar circle. Nowhere in 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, in Russia, experiences civil polar night for around a month. If there are thick clouds, it gets darker, and places like the coast of Finnmark (about 70°) in Norway will get a rather dark "day".
The nautical polar night is the period that no trace of light can be seen anywhere but the sky is not completely dark at 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, although there is still a place at the horizon with clearly more light than other places because of refraction. The nautical polar night is limited to latitudes above 78° 33', which is exactly 12 degrees within the polar circle, or eleven and a half degrees from the pole. Alert, Nunavut, the northernmost settlement in Canada and the world, experiences this from late November to mid January.
Finally, the astronomical polar night is the period of continuous night where no astronomical twilight occurs. Astronomical twilight happens when the sun is between twelve and eighteen degrees below the horizon. Thus, the astronomical polar night is limited to latitudes above 84° 33', which is exactly 18 degrees 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. Some scientific stations in Antarctica, including the Amundsen-Scott South Pole Station, experience this.