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Encyclopedia > Polar day
The midnight sun at Nordkapp, Norway

The midnight sun is a phenomenon, occurring in latitudes north of the arctic circle and south of the antarctic circle, where the sun is visible during at least 24 hours. Since there are no permanent human settlements south of the antarctic circle, the countries and territories whose population experiences it are limited to Alaska, northern Canada, Greenland, Iceland (due to refraction), northern Norway, Sweden and Finland, and extremities of Russia such as Novaya Zemlya or Murmansk.

The opposite phenomenon, polar night, is observed in winter when the sun is sufficiently below the horizon, so that there is insufficient sunlight or no light at all. (A lingering dusk may occur, if the sun is only 12 degrees or less below the horizon.)

Since the earth's axis is tilted with respect to the ecliptic by approximately 23 degrees 26 minutes (commonly rounded to 23 degrees and a half), the sun does not set at high latitudes in (local) summer. The duration of the midnight sun increases from one day during the summer solstice at the polar circle to approximately six months at the poles. At extreme latitudes, it is usually referred to as polar day. The length of the time when the sun is above the horizon varies from 20 hours at the Arctic Circle and Antarctic Circle to 186 days at the Poles.

Due to refraction, the midnight sun may be experienced at latitudes slightly below the polar circle, though not exceeding a degree (depending on local conditions). For example, it is possible to experience the midnight sun in Iceland, even though it is slightly south of the arctic circle.

Locations at latitudes up to 60 degrees experience midnight twilight instead. The sun is just below the horizon, so that daytime activities, such as reading, are still possible without resorting to artificial light. These are the white nights experienced, for example, in St. Petersburg, Russia from about 11 June to 11 July.

Influence on humans

The period of midnight sun is reportedly very taxing on the human body. Peaks in the suicide figures and increased severity of mental disorders have been demonstrated to occur in summer months. Polar nights affect people more severely. People who suffer from seasonal affective disorder are particularly susceptible.

External link

  • Explanation of various related phenomona (http://collections.ic.gc.ca/simply_science/toc.htm)

  Results from FactBites:
Polar night - Wikipedia, the free encyclopedia (547 words)
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.
This is because polar night is the period during which no twilight occurs; but there are various kinds of twilight.
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.
season - definition of season - Labor Law Talk Dictionary (1574 words)
One of the divisions of the year, marked by alterations in the length of day and night, or by distinct conditions of temperature, moisture, etc., caused mainly by the relative position of the earth with respect to the sun.
In the temperate and polar regions, seasons are marked by changes in the amount of sunlight, which in turn often cause cycles of dormancy in plants and hibernation in animals.
A common misconception is that, within the Arctic and Antarctic Circles, the sun rises once in the spring and sets once in the fall; thus, the day and night are erroneously thought to last uninterrupted for 183 calendar days each.
  More results at FactBites »



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