Daylight in Moscow and St. Petersburg

The word "day" has two meanings. The first is the time of day when the street is light, and the second is the light part of the Earth's daily rotation time. Experts believe that a light day is the time from sunrise to sunset.

The axis of the Earth's rotation is tilted, so the length of the light day varies throughout the year. In winter, the day is the shortest, and its duration varies with a change in geographical latitude. In the north, the winter light day is 4-5 hours, and the rest of the time is darkness. And still more to the north in general the sun does not happen - a polar night, but in the summer to sleep there is no time - there is no night at all. Only the sun went beyond the horizon, and twilight began, almost immediately they end - the sun rises again.

But no matter how long the light day lasts, 6 hours or 18, the night will last exactly enough to take 24 hours a day with the day - a calendar day. And if the night in June is only 5 hours, then the day will be 19. But in the calendar year there are interesting periods. In 2010-2020, this is March 20 , June 20-21 , September 22-23 and December 21-22. These days in March and September on Earth, night and day are equal. They are called - the days of the spring and autumn equinox. Although, if we take into account the phenomenon of refraction of the solar disk and its dimensions (0.5 arc minutes), by the time of day, nature, using these physical effects, adds a few more minutes. After all, a light day is the time from the appearance of the upper edge of the solar disk above the horizon before the departure of its lower (in relation to the morning) edge over the horizon, and this is another two minutes of the movement of the solar disk. And this is on the equator. And in our latitudes it is still 3-4 minutes or more. In addition, because of the phenomenon of refraction - the refraction of light rays in the atmosphere - the sun is already visible, although, according to geometric calculations, it is still beyond the horizon. The same is observed at sunset.

And June 20-21 is the summer solstice, when the sun rises to the highest altitude, and the day is the longest. In the circumpolar regions of the night, during this period, very short and "white", i.e. twilight without darkness. But December 21-22 is the shortest day, and the night is the longest. And in the circumpolar regions and north of the day, it may not start at all. But on the other side of the globe, in Australia, South Africa and South America everything is exactly the opposite. They have the solstice in December, and the longest nights in June.

Biorhythms and light day

Nature has adapted living organisms to change the light and dark time of the day. If animals (and humans) are kept for several weeks in the "12 hours a day, 12 - night" mode, and then abruptly switched to "18 hours of light, 6 hours of darkness", then active sleep and sleep disturbances begin.

In human society, the disturbance of biorhythms in the daily cycle leads to stress, up to the development of diseases - depression, insomnia, cardiac and vascular pathologies and even cancer. There was even the concept of "seasonal depression" associated with the winter length of daylight hours.

At different latitudes - a different light day. Moscow, being at the 55th degree of the northern latitude, has a light day from 7 hours in December-January to 17 hours in June-July.

St. Petersburg's light day also depends on the time of the year. And since St. Petersburg is located at the 60th degree of northern latitude, the length of the day in June here is about 18.5 hours. This creates the effect of white nights, when the sun only briefly leaves. Officially, the white nights last from May 25 to July 17. But in December and January it gets dark at the fifth hour of the evening.