About the white nights the Russians hear almost annually – mainly, thanks to rich cultural life of St. Petersburg where at this time there takes place the theater festival with such name. Though as the natural phenomenon can be observed the white nights not only in Russia, but also in other countries which territories occupy subpolar areas – in Norway, Denmark, Sweden, Iceland, in the northern areas of Canada and Alaska.
White nights as atmospheric phenomenon
The southern border of the white nights passes on width 49 °. There the white night can be observed only once a year - June 22. Further to the North duration of this period increases, and nights become more and more bright.
The phenomenon is experts call still civil twilight. Generally, evening twilight is time when the sun already was behind the horizon, but signs of a sunset are still visible. The earth is lit with diffused light, i.e. beams of already disappeared star, are accepted by an upper atmosphere and partially dissipate, and partially are reflected and light the earth. Objects are well visible without artificial lighting, the line of the horizon is accurately distinguishable, but it is not a daylight any more – in a clear weather in a sky the first brightest stars are visible.
Depending on illumination or, strictly speaking, from position of the sun concerning the line of the horizon, experts allocate civil, navigation and astronomical twilight.
Civil twilight lasts from the moment of visible sunset till time when the corner between the line of the horizon and the center of a solar disk makes 6 °, from 6 ° up to 12 ° - navigation, from 12 ° up to 18 ° - astronomical twilight.
Thus, the white night is the phenomenon when evening twilight smoothly passes into morning, passing night, i.e. the period of the minimum illumination of the earth's surface.
It is a little astronomy
If to consider the phenomenon from the astronomical point of view, it is necessary to remember that the terrestrial axis is located at an angle to the ecliptic plane, i.e. to the planet orbit plane around the Sun, and this inclination does not change.
Generally, the tilt angle of a terrestrial axis changes. It spins in space and at different times "looks" to different places of the star sky. However the period of this movement, in human understanding, is very big – nearly 26 thousand years.
Thus, in the course of the movement of Earth on an orbit the Sun lights that northern, that southern hemisphere. And an inclination of a terrestrial axis such is that in some points of an orbit the sunshine fall on one of poles almost perpendicularly. On the lit hemisphere - summer. In polar regions the polar day when the sun is not behind the horizon many days in a row is observed at this time. Other hemisphere worries winter as it is lit poorly. Sunshine kind of slide on the Earth's surface and badly heat it. The pole is in a shadow, polar night there. In subpolar areas of the lit hemisphere the Sun though sits down, but for a while and is close to the line of the horizon. It is so close that can light the surface of the planet with the beams disseminated in the atmosphere. There come the white nights.