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Midnight sun, also known as polar day, 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. When midnight sun is seen in the Arctic, the Sun appears to move from left to right. In Antarctica, the equivalent apparent motion is from right to left. This occurs at latitudes ranging from approximately 65°44' to exactly 90° north or south, and does not stop exactly at the Arctic Circle or the Antarctic Circle, due to refraction.
The opposite phenomenon, polar night, occurs in winter, when the Sun stays below the horizon throughout the day.
The largest city in the world north of the Arctic Circle, Murmansk, Russia, experiences midnight sun from 22 May to 22 July (62 days).
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 72 days during summer. Nuorgam, Lapland, Finland — Sunrise, Sunset, and Daylength, May 2022
In Svalbard, 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 about 6 months, from approximately 18 March to 24 September. South Pole, Antarctica has midnight sun and experiences this from approximately 20 September to 23 March (about 6 months).
Places sufficiently close to the poles, such as Alert, Nunavut, experience times where it does not get entirely dark at night yet the Sun does not rise either, combining effects of midnight sun and polar night, reaching civil twilight during the "day" and astronomical twilight at "night".
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White Nights have become a common symbol of Saint Petersburg, where they occur from about 11 June to 1 July, and the last 10 days of June are celebrated with cultural events known as the White Nights Festival. The phenomenon carries similar significance for Fairbanks, Alaska, where an annual Midnight Sun Game baseball competition has been contested since 1906 in the twilight surrounding midnight on June 21. (1991). 9780553289190, Bantam Books. ISBN 9780553289190
The northernmost tip of Antarctica also experiences white nights near the Southern Hemisphere summer solstice.
At the poles themselves, the Sun rises and sets only once each year on the . During the six months that the Sun is above the horizon, it spends the days appearing to continuously move in circles around the observer, gradually spiraling higher and reaching its highest circuit of the sky at the summer solstice, before beginning to sink lower, setting just after the autumnal equinox.
If a precise moment for the genuine "midnight sun" is required, the observer's longitude, the local civil time, and the equation of time must be taken into account. The moment of the Sun's closest approach to the horizon coincides with its passing due north at the observer's position, which occurs only approximately at midnight in general. Each degree of longitude east of the Greenwich meridian makes the vital moment exactly 4 minutes earlier than midnight as shown on the clock, while each hour that the local civil time is ahead of coordinated universal time (UTC, also known as GMT) makes the moment an hour later. These two effects must be added. Furthermore, the equation of time (which depends on the date) must be added: a positive value on a given date means that the Sun is running slightly ahead of its average position, so the value must be subtracted.H. Spencer Jones, General Astronomy (Edward Arnold, London, 1922), Chapters I-III
As an example, at the North Cape of Norway at midnight on June 21/22, the longitude of 25.9 degrees east makes the moment 103.2 minutes earlier by clock time; but the local time, 2 hours ahead of GMT in the summer, makes it 120 minutes later by clock time. The equation of time at that date is -2.0 minutes. Therefore, the Sun's lowest elevation occurs 120 - 103.2 + 2.0 minutes after midnight: at 00.19 Central European Summer time. On other nearby dates the only thing different is the equation of time, so this remains a reasonable estimate for a considerable period. The Sun's altitude remains within half a degree of the minimum of about 5 degrees for about 45 minutes either side of this time.
When it rotates on its own axis, it sometimes moves closer to the Sun. During this period of Earth's rotation from May to July, Earth tilts at an angle of 23.5 degrees above its own axis in its orbit. This causes the part of Norway located in the Arctic region at the North Pole of Earth to move very close to the Sun and during this time the length of the day increases. It can be said that it almost never subsides. Night falls in Norway's Hammerfest at this particular time of year.
Even though at the Arctic Circle the center of the Sun is, per definition and without refraction by the atmosphere, only visible during one summer night, some part of midnight sun is visible at the Arctic Circle from approximately 12 June until 1 July. This period extends as one travels north: At Cape Nordkinn, the northernmost point of Continental Europe, midnight sun lasts approximately from 14 May to 29 July. On the Svalbard archipelago further north, it lasts from 20 April to 22 August.Trygve B. Haugan, ed. Det Nordlige Norge Fra Trondheim Til Midnattssolens Land (Trondheim: Reisetrafikkforeningen for Trondheim og Trøndelag. 1940)
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.
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