The word "day" has two meanings. The first is the time of day when it is light outside, and the second is the light part of the time of the Earth’s daily rotation. Experts believe that daylight hours are the time from sunrise to sunset.
The axis of rotation of the Earth is tilted, so the length of daylight changes during the year. In winter, the day is the shortest, and its duration changes with a change in geographical latitude. In the north, winter daylight hours are 4–5 hours, and the rest of the time is darkness. And even to the north there is no sun at all - a polar night, but there is no time to sleep in the summer - there is absolutely no night. Only the sun went beyond the horizon, and dusk began, almost immediately they end - the sun rises again.
But no matter how long the daylight hours, 6 hours or 18 hours, the night will last exactly as long as it takes 24 hours with the day — a calendar day. And if the night in June is only 5 hours, then the day will be 19. But there are interesting periods in the calendar year. In 2010 - 2020, this is March 20, June 20-21, September 22-23 and December 21-22. On these days in March and September on Earth, night and day are equal. They are called so - the days of the spring and autumn equinox. Although, taking into account the phenomenon of refraction of the solar disk and its size (0.5 arc minutes), nature uses these physical effects to add a few more minutes to the length of the day. After all, daylight hours are the time from the appearance of the upper edge of the solar disk above the horizon to the departure of its lower (relative to the morning) edge beyond the horizon, and this is another two minutes of movement of the solar disk. And it is at the equator. And in our latitudes it is another 3-4 minutes or more. In addition, due to 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 they are very short and “white, ” that is, twilight without darkness. But December 21-22 is the shortest day, and the night is the longest. And in the circumpolar regions and to the north, the day may not begin at all. But on the other side of the globe, in Australia, South Africa and South America, everything is exactly the opposite. They have a solstice in December and the longest nights in June.
Biorhythms and daylight
Nature has adapted living organisms to the change of daylight and dark. If animals (and humans) are kept for several weeks in the 12 o’clock, 12 o’clock mode and then abruptly switch to 18 o’clock, 6 o’clock, then disturbances of active wakefulness and sleep begin.
In human society, the violation of biorhythms in the daily cycle leads to stress, up to the development of diseases - depression, insomnia, pathologies of the heart and blood vessels, and even cancer. There was even the concept of "seasonal depression" associated with the winter daylight hours.
At different latitudes - different daylight hours. Moscow, being at the 55th degree of northern latitude, has daylight hours from 7 hours in December – January to 17 hours in June – July.
St. Petersburg daylight hours also depend on the time of year. And since St. Petersburg is located at 60 degrees north latitude, the length of the day in June is about 18.5 hours. This creates the effect of white nights when the sun only leaves briefly. Officially, white nights last from May 25 to July 17. But in December – January it gets dark at five in the evening.
