The Arctic Circle is popularly thought to demarcate the latitude at which darkness is continuous during the winter solstice. The Circle is defined where the minimum zenith angle to the centre of the Sun is 90 degrees on 21/22 December. Sunrise and sunset occur when the centre of the the Sun is at a zenith angle of 90 degrees 50', and therefore the Sun is observed at the Arctic Circle throughout the year. Night begins at the end oi twilight, sometime alter sunset. The limiting zenith angle for civil twilight is 96 degrees, for nautical twilight 102 degrees, and for astronomical twilight 108 degrees. In the northern win ter 24-hour night is not experienced at latitudes south of 72 degrees 33', and astronomical polar night is not observed at any point on the Earth's land surface. Only live settlements, two in Canada, two in Russia, and one in Svalbard are within the limit of nautical polar night.
