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Photo: In the midst of white nights. Station Petrozavodsk. July 2, time 22:10

There is no exact astronomical definition of “white nights”. As we approach the summer solstice, the nights are getting brighter, reaching a peak of illumination on the solstice night – June 20-21. Moreover, the greater the breadth, the white nights are brighter and longer.

In the days close to the winter solstice, the opposite is true – dark days when the sun does not rise high enough over the horizon during the day to create normal lighting. This day, especially in cloudy weather, more like twilight, cars drive with headlights on. Visually, dawn can smoothly merge with the sunset. Like the white nights, the dark days have no clear astronomical definition. If we take the midday height of the sun above the horizon as low as 9 °, then at latitude, for example, of St. Petersburg, they will last from November 27 to January 15.

At the poles The white night is observed continuously 15-16 days before sunrise and the same after its sunset. At the North Pole, there are periods from March 3 to March 18 and from September 26 to October 11, at the South Pole, from March 23 to April 7 and from September 7 to 21. The differences in the length of the white nights, the polar day and the polar night at the poles are related to the fact that the area around the South Pole is at an altitude of 2800 meters above sea level, and at the North Pole the height is determined by the level of the Arctic Ocean.

In latitudes above the polar circle white nights and "dark days" can also be seen in the cities around the Arctic Circle: Salekhard, Murmansk, Norilsk, Vorkuta, Naryan-Mar – 3 weeks before the polar day (the polar night) and the same after it. In these cities, the white night gradually brightens until the sun ceases to go beyond the horizon, and the polar day does not come. In winter, the day gradually darkens until the sun ceases to rise from the horizon and the polar night falls.

In southern Karelia, including at latitude PetrozavodskWhite nights begin in the second half of May and end in mid-August. In Petrozavodsk, for the time of the white nights, the lights on the streets are not turned on. It is so light that you can do without artificial lighting.

AT St. Petersburg there is an “official” period of white nights: from June 11 to July 2. But many Petersburgers consider the beginning of the white nights on May 25-26, and the end – on July 16-17. During this period, the sun at the latitude of St. Petersburg at night descends beyond the horizon by no more than 9 °. June 20-21, at astronomical midnight (the moment of the lower climax), the sun sets below the horizon by about 7 °. Street lighting in Petersburg during the period of white nights turns on for a short time.

In the lower latitudes, nights are no longer called white, but light. For example, at latitude Moscow at midnight, the observer will notice only a slight lightening of the sky in the direction of the sun (in clear weather), artificial lighting in settlements will work all night.

Finally, starting at about 48 ° N. at midnight during the summer solstice you cannot see or record the slightest twilight. Such dark summer nights are sometimes called "southern".

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