In the parts of the world that are close to the poles, the way the Earth tilts means that the summer months in those regions are marked by constant daylight. Parts of Scandinavia, for instance, are known as the “land of the midnight Sun”. In mid-winter, these areas experience the opposite — total darkness for 24 hours a day.
The midnight sun is a natural phenomenon that occurs in the summer months in places north of the Arctic Circle or south of the Antarctic Circle, when the Sun remains visible at the local midnight.
Around the summer solstice (approximately 21 June in the Northern Hemisphere and 23 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, 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.
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. 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 south of the Arctic Circle or north of the Antarctic 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.
Picture Credit : Google