Polar night
The polar night in the polar regions is a period around the winter solstice during which the sun cannot be seen directly for several days or months. The polar night is the opposite of the polar 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 (although the sun still rises partially above the horizon, only the lower edge does not). The closer you get to the North or South Pole, the longer the polar night lasts. The polar night is caused by the inclination of the earth's axis by about 23.4 °.
definition
Between the poles and the polar circles, the sun does not rise completely above the horizon for at least one day . However, even on the darkest day of the year (winter solstice) it does not get completely dark everywhere beyond the polar circles. That is u. a. because the sun is closer to the horizon due to refraction (refraction of light in the earth's atmosphere ) higher than its diameter, so it can seem to rise completely despite the “theoretical” polar night. The opposite applies to places that are just below the Arctic Circle: Because of the refraction, the phenomenon that the sun does not rise on at least one day a year only occurs a little closer to the poles (from about ± 67.16 ° latitude).
From this latitude onwards, only half the solar disk makes it over the horizon, from around ± 67.41 ° not even the upper edge of the sun can be seen . At this point, however, it is lightly twilight, like just before sunrise or sunset, because the sun is only just below the horizon. The closer you get to the pole, the deeper below the horizon the sun is. From about 73.2 ° latitude, it is always too dark to read the newspaper because civil twilight is no longer even reached. From about 79.2 ° latitude the brightest stars can be seen continuously, from 85.2 ° the sun no longer reaches the earth.
root cause
Since the earth orbits the sun and the earth's axis maintains its inclination of 23.4 ° independently of this orbital movement, the poles face the sun in the respective summer and turn away from it in winter. Therefore, the sun at the geographic poles is below the horizon for about half a year. During this period, polar stratospheric clouds can form at altitudes of 22 to 29 km; these only form at temperatures below −78 ° C.
Web links
- Webcam and weather data from Tromsø University
- Sunrises and sunsets for famous places