Polar night is a phenomenon where night lasts for more than 24 hours that occurs in the northernmost and southernmost regions of Earth. This occurs only inside the polar circles. The opposite phenomenon, polar day, or midnight sun, occurs when the Sun remains 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 refracts sunlight, polar day is longer than 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 approximately 66.5°. While it is day in the Arctic Circle, it is night in the Antarctic Circle, and vice versa. Any planet or moon with a sufficient axial tilt that rotates with respect to its star significantly more frequently than it orbits the star (and with no tidal locking between the two) will experience the same phenomenon (a nighttime lasting more than one rotation period). Description The polar shortest day is not totally dark everywhere inside the polar circle, but only in places within about 5.5° 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 to a few days 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. The time when any part of the Sun is above the horizon at the poles is 186 days. The preceding numbers are average numbers: the ellipticity of Earth's orbit makes the South Pole receive a week more of Sun-below-horizon than the North Pole (see equinox). Types of polar night As there are various kinds of twilight, there also exist various kinds of polar night. Each kind of polar night is defined as when it is darker than the corresponding kind of twilight. The descriptions below are based on relatively clear skies, so the sky will be darker in the presence of dense clouds. Polar