Around the summer solstice (approximately 21 June in the Northern Hemisphere and 22 December in the Southern Hemisphere), the sun is visible for the full 24 hours, given fair weather. The number of days per year with potential midnight sun increases the closer towards either pole one goes. Although approximately defined by the polar circles, in practice the midnight sun can be seen as much as 55 miles (90 km) outside the polar circle, as described below, and the exact latitudes of the farthest reaches of midnight sun depend on topography and vary slightly year-to-year.
Because there are no permanent human settlements south of the Antarctic Circle, apart from research stations, the countries and territories whose populations experience the midnight sun are limited to those crossed by the Arctic Circle: the Canadian Yukon, Nunavut, and Northwest Territories; the nations of Iceland, Finland, Norway, Sweden, Denmark (Greenland), Russia; and the State of Alaska in the United States. A quarter of Finland's territory lies north of the Arctic Circle, and at the country's northernmost point the sun does not set at all for 60 days during summer. In Svalbard, Norway, the northernmost inhabited region of Europe, there is no sunset from approximately 19 April to 23 August. The extreme sites are the poles, where the sun can be continuously visible for half the year. The North Pole has midnight sun for 6 months from late March to late September.
Since the axial tilt of the Earth is considerable (approximately 23 degrees 27 minutes), the sun does not set at high latitudes in local summer. The sun remains continuously visible for one day during the summer solstice at the polar circle, for several weeks only 100 km (62 mi) closer to the pole, and for six months at the pole. At extreme latitudes, the midnight sun is usually referred to as polar day.
At the poles themselves, the sun rises and sets only once each year on the equinox. During the six months that the sun is above the horizon, it spends the days continuously moving in circles around the observer, gradually spiralling higher and reaching its highest circuit of the sky at the summer solstice.