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polar night
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. "Night" is understood as the center of the Sun being below a free horizon. Since the atmosphere bends the rays of the Sun, "light wins over darkness" by a few degrees. Hence the area that is affected by polar night is somewhat smaller than the area of midnight sun. The polar circle is located at a latitude between these two areas, at the latitude of approx. 66.5 degrees. For instance, in the most northern city of Sweden, Kiruna (located at the northern latitude of 67°51'), the polar night lasts for around 28 twenty-four hour periods, while the midnight sun lasts around 50 twenty-four hour periods.
==Description==
A common misconception is that the polar shortest day is totally dark everywhere inside the polar circle. In places very close to the poles it is completely dark, but regions located at the inner border of the polar circles experience polar twilight instead of polar night. In fact, polar regions typically get more twilight throughout the year than equatorial regions.
For regions inside the polar circles, the maximum lengths of the time that the Sun is completely below the horizon varies from about 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).
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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