The polar night occurs when the night lasts for more than 24 hours. This occurs only inside the polar circles. The opposite phenomenon, the polar day, or midnight sun, occurs when the sun stays above the horizon for more than 24 hours.
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A common misconception is that the shortest day is totally dark at each point where the midnight sun occurs inside the polar circle. In places very close to the poles this is true, but in regions that are located at the inner border of the polar circles where the midnight sun is experienced, this is not the case. Because of twilight, these regions experience polar twilight instead of the polar night. In fact, polar regions typically get more twilight throughout the year than equatorial regions.
For regions inside the polar circles, the lengths of the time when the sun is below the horizon varies from 20 hours at the Arctic Circle and Antarctic Circle to 179 days at the Poles. 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: 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).
As there are various kinds of twilight, there also exist various kinds of polar night. This is because the polar night is defined as the period during which no twilight occurs. These latitude definitions are based on relatively clear skies, so the sky will be darker in the presence of dense cloud.
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. 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.
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. The civil polar night is limited to latitudes above 72° 33', 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, 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".
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, although there is a location at the horizon with more light than others because of refraction. The nautical polar night is limited to latitudes above 78° 33', which is exactly 12° within the polar circle, or 11.5° from the pole. Alert, Nunavut, the northernmost settlement in Canada and the world, experiences this from late November to mid January.
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. Thus, the astronomical polar night is limited to latitudes above 84° 33', 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. Some scientific stations in Antarctica, including the Amundsen-Scott South Pole Station, experience this.
The period of polar night can trigger depression in some people. People who suffer from seasonal affective disorder are particularly susceptible to these conditions. The polar night may also be implicated in some instances of solipsism syndrome.
The concept of a night of almost one month long has been the subject of the vampire movies Frostbiten and 30 Days of Night. In these films, the vampires are drawn to the long duration of darkness, allowing them to do as they please.