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, the polar day is longer than the polar night, and 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 approximately 66.5 degrees. In the most northern city of Sweden, Kiruna, at 67°51'N, the polar night lasts for around 28 twenty-four-hour periods, while the midnight sun lasts around 50 twenty-four-hour periods. While it is day in the Arctic Circle it is night in the Antarctic circle.

The polar shortest day is not totally dark everywhere inside the polar circle, but only in places within about 5° of the poles, and only when the moon is well below the horizon. 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 zero a quarter degree beyond 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: the ellipticity of the Earth's orbit makes the South Pole receive a week more of Sun-below-horizon than the North Pole (see equinox).

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