Polar night

Early afternoon in Tromsø, Norway, during the Polar Night, viewed from the upper reaches of the city centre towards the mainland side.

The polar night occurs when the night lasts for more than 24 hours. This only occurs inside the polar circles. The opposite phenomenon, when the sun stays above the horizon for a long time is called the polar day, or midnight sun.

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 midnight sun is experienced, this is not true. Due to 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.

In regions inside the polar circles, the length 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 there may be plenty of sunlight because of refraction. Also, one might notice that the time when the sun is above the horizon at the poles is said to be 186 days. The asymmetry in numbers is because the time when the sun is partially above the horizon is counted towards the "daytime".

Types of polar night

Since there are various kinds of twilight, there are also various kinds of polar night. This is because the polar night is defined as the period during where no twilight occurs. These latitude definitions are based on relatively clear skies, so the sky will be darker if there are thick clouds about.

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 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 will probably resort to 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 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. In mainland Europe, there are no places fulfilling this definition. On the Norwegian Territory of Svalbard, however, civil polar night lasts from about 12 November until the end of 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".

Nautical polar night

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.

Astronomical polar night

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 ever 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.

Effects on humans

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.

External links

• The polar night and polar darkness
• 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.