Polar night

The polar night in the polar regions, a period around the winter solstice, where the sky for several days to months remains dark. The polar night is the opposite of the Arctic day with its midnight sun.

At the geographic North Pole and South Pole, the polar night lasts almost half a year, at the polar circles exactly one day (the sun but still partially rises over the horizon, only the lower edge not ). The closer you get to the North or South Pole, the longer the polar night. The polar night is caused by the tilt of Earth's axis by about 23.4 °.

Definition

Between the poles and the polar circles the sun does not rise for at least one day completely over the horizon. The Finnish knows this special term " Kaamos ". However, it is completely dark even on the darkest day of the year (winter solstice) not anywhere beyond the Arctic Circle. This is, inter alia, that, because of refraction ( refraction in the atmosphere ), the sun appears to be significantly higher near the horizon than it is in reality; so it can thus seemingly completely absorbed in spite of " theoretical " polar night. The opposite is true also for places that do not lie in the area between the Arctic Circle and Pole: Because of the refraction of light in the atmosphere ( see astronomical refraction), it is not exactly on the Arctic Circle, but only slightly polnäher (from about ± 67.16 ° width) the appearance that the sun on a day of the year does not work out.

From this latitude it provides only half the solar disk above the horizon, from about ± 67.41 ° not even the upper edge of the sun. At this time, however, it is light as dim shortly before sunrise or sunset, because the sun is just below the horizon. The closer you get to the pole, the deeper below the horizon where the sun is. From about 73.2 ° latitude it is always too dark to read the newspaper, because not even the civil twilight is achieved. From about 79.2 ° latitude, the brightest stars are constantly seen, from 85.2 ° constantly causes no twilight of the sun upon the earth.

Cause

As the earth revolves around the sun and the Earth's axis of this orbital motion independently maintains its inclination of 23.4 °, the poles are facing in each summer the sun and away in the winter of her. Therefore, the sun is at the geographic poles for about half a year below the horizon. During this period, it may result in the formation of polar stratospheric clouds in the 22 to 29 km altitude, these are formed only at temperatures below -88.3 ° C.