An Astronomical Explanation of the Midnight Sun

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The land of the midnight sun lies north of the arctic circle (i. e., north of +66°33′) - and it exists only around the date of the north summer solstice, usually June 21 (for the northern hemisphere).

Definition: astronomically speaking, the phenomenom of the midnight sun occurs if the center of the sun's disk is visible at or above the horizon at the time of the true local midnight, which can differ substantially from the ordinary midnight (always at 12:00 pm) because of factors like daylight saving time, geographical longitude with regard to the time zone's longitude and a component depending on the date of the year (called the equation of time). These deviations can together make up more than 1 hour.

Occurence: the earth's equator does not lie in the plane of its movement around the sun, but is tilted 23°27′ against it. The orientation of this inclination remains unaltered while the earth revolves around the sun during the course of a year (the earth's north pole is tilted approximately into the direction of the constellation of Gemini). As a result, during certain periods of the year, the north pole is inclined towards the sun to a varying degree; at other times, it is the south pole (these periods are called north spring and summer; or north fall and winter respectively). For an observer on earth, it appears as if the sun moves along a line against the background of stars. That line (called the ecliptic) appears tilted with respect to the celestial equator and therefore, the sun's so-called declination (the angular distance from the celestial equator) varies over the course of a year.
   At the time of the north summer solstice, the sun's declination is at its maximum (+23°27′), and so the sun shines not only on the earth's day side (up to the north pole), but over the north pole towards the earth's night side, down to a geographical latitude that is the same angular distance away from the north pole as the sun is above the celestial equator. Its geographical latidude is therefore +66°33′ (90° - 23°27′). This line is called the arctic cicle and it is so important for the friends of the midnight sun because the midnight sun can only be observed from the area between the arctic circle and the respective pole. At lower latitudes (towards the equator), it never appears.

North and South: as the year goes by and the north fall equinox approaches (usually around September 23), the sun's declination eventually shrinks towards zero and the region with midnight sun becomes smaller and smaller (i. e., you would have to travel farther to the north to see it) until it finally collapses at the north pole. At the same date, the midnight sun begins to shine at the south pole, and it extends its land more and more to the north until it reaches the southern arctic cicle (geographical latitude -66°33′) at the date of the north winter solstice (usually around December 21) which is of course the same as the south summer solstice. Then it begins to shrink again until south and north pole exchange their roles again at the date of the north vernal equinox (usually around March 20).

Approximations and Reality: that you can see the midnight sun from a location on the arctic circle exactly once in a year, is an approximation. It holds only if the earth was a perfect sphere and there were no mountains, trees or houses that could obstruct the line of sight. It also assumes that there is no refraction in the earth's atmosphere. The first point can make the midnight sun unobservable at a given location whereas the second (because there is an atmosphere and therefore the sunlight is refracted) makes the setting sun appear higher above the horizon (approx. 1/2 degree or 0°30′) than it is (geometrically). In practice, this leads to the result that the midnight sun can even be observed from locations that are approx. 1/2 degree below the arctic circle (approx. 55 km towards the equator). If you are content with just observing the sun's upper rim at true midnight, this makes up another 1/2 degree in geographical latitude (some tourism managers make up this calculation).

Getting there: for western Europe, this land lies in Scandinavia, in the area where the borders of Norway, Sweden and Finland converge. You can reach it rather comfortably by train. This is not the case for the North American continent (Alaska and nothern Canada require long car rides on gravel roads). The southern hemisphere's midnight sun is virtually unreachable because the southern arctic circle corresponds roughly with the borders of Antarctica.

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