I'm still waiting for a flat earth explanation if this on another thread:
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, and 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.
The opposite phenomenon,
polar night, occurs in winter, when the sun stays below the
horizon throughout the day.
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.
[1] 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.
Because of
atmospheric refraction, and also because the sun is a disc rather than a point, the midnight sun may be experienced at latitudes slightly below the polar circle, though not exceeding one degree (depending on local conditions). For example, Iceland is known for its midnight sun, even though most of it (
Grímsey is the exception) is slightly south of the Arctic Circle. For the same reasons, the period of sunlight at the poles is slightly longer than six months. Even the northern extremities of
Scotland (and places at similar latitudes, such as
St. Petersburg) experience
twilight throughout the night in the northern sky at around the summer solstice.
Observers at heights appreciably above sea level can experience extended periods of midnight sun as a result of the "dip" of the horizon viewed from altitude.
