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Polar day and polar night

09-21-2011 13:47 BJT

The midnight sun is a natural phenomenon occurring in summer months at latitudes north and nearby to the south of the Arctic Circle, and south and nearby to the north of the Antarctic Circle where the sun remains visible at the local midnight. Given fair weather, the sun is visible for a continuous 24 hours, mostly north of the Arctic Circle and south of the Antarctic Circle. The number of days per year with potential midnight sun increases the farther poleward one goes from the equator.

The midnight sun is a natural phenomenon occurring in summer months at latitudes north and nearby to the south of the Arctic Circle, and south and nearby to the north of the Antarctic Circle where the sun remains visible at the local midnight.

There are no permanent human settlements south of the Antarctic Circle, so the countries and territories whose populations experience it are limited to the ones crossed by the Arctic Circle, e.g. Canada (Yukon, Northwest Territories, and Nunavut), Denmark (Greenland), Finland, Lapland, Norway, Russia, Sweden, the United States (Alaska), and extremities of Iceland. A quarter of Finland's territory lies north of the Arctic Circle and at the country's northernmost point the sun does not set for 73 days during summer. In Svalbard, Norway, the northernmost inhabited region of Europe, there is no sunset from approximately 19 April to 23 August. The extreme sites are the poles where the sun can be continuously visible for a half year.

The polar night occurs when the night lasts for more than 24 hours. This occurs only inside the polar circles. 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. 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.

The polar night occurs when the night lasts for more than 24 hours. This occurs only inside the polar circles.

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.

Since there are various kinds of twilight, there are also various kinds of polar night, such as polar twilight, civil polar night, nautical polar night, and astronomical polar night.

Editor:Zheng Limin |Source: CNTV.CN

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