Polar night

Polar night is a phenomenon that occurs in the northernmost and southernmost regions of Earth when the Sun remains below the horizon for more than 24 hours. This only occurs 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.

There are multiple ways to define twilight, the gradual transition to and from darkness when the Sun is below the horizon. "Civil" twilight occurs when the Sun is between 0 and 6 degrees below the horizon. Nearby planets like Venus and bright stars like Sirius are visible during this period. "Nautical" twilight continues until the Sun is 12 degrees below the horizon. During nautical twilight, the horizon is visible enough for navigation. "Astronomical" twilight continues until the Sun has sunk 18 degrees below the horizon. Beyond 18 degrees, refracted sunlight is no longer visible. True night is defined as the period when the sun is 18 or more degrees below either horizon.

Since the atmosphere refracts sunlight, polar day is longer than polar night, and the area that experiences polar night is slightly smaller than the area that experiences polar day. The polar circles are located at latitudes 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).

The length of polar night varies by latitude from 24 hours just inside the polar circles to 179 days at the poles. As there are various kinds of twilight, there also exist various kinds of polar twilight that progress towards true 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.

As mentioned, a location experiencing polar night does not mean that the location will be in full darkness; in most cases, due to sunlight being refracted over the horizon, a location experiencing polar night will actually be in one of the various phases of polar twilight. As in locations experiencing daylight, the middle of the day will typically be the brightest time in locations experiencing polar twilight.

For example, a typical day during civil polar twilight in Vadsø, Norway will begin with night, astronomical twilight, nautical twilight, and civil twilight in that order (with each successive phase including more light than the last). Following civil twilight, the day will progress through the other phases in the opposite order (nautical twilight, then astronomical twilight, then night to end the day).

Civil polar twilight occurs at latitudes above 72°34' North or South, where the Sun will be below the horizon all day on the winter solstice, but by less than 6° at solar noon. There is then no true daylight at the solar culmination, only civil twilight. During civil polar twilight, there is still enough light for most normal outdoor activities at midday because of light scattering by the upper atmosphere and refraction. However, during dense cloud cover, places like the coast of Finnmark (about 70°) in Norway will experience a "day" that is darker than usual. Street lamps may therefore remain on even at midday, and a person looking at a window from within a brightly lit room might still be able to see their reflection, as the level of outdoor illuminance will be below that of many illuminated indoor spaces.