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 Earth's daily rotation. Experts believe that daylight is the time from sunrise to sunset.
The Earth's axis of rotation is tilted, so the length of daylight changes throughout the year. In winter, the day is the shortest, and its duration varies with changes in latitude. In the north, winter daylight hours are 4–5 hours, and the rest of the time is darkness. And even further north there is no sun at all - the polar night, but in the summer there is no time to sleep - there is no night at all. As soon as the sun went below the horizon, and twilight began, almost immediately they end - the sun rises again.
But no matter how long the daylight hours last, 6 hours or 18, the night will last just enough to take 24 hours together 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, these are 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 so - the days of the spring and autumn equinoxes. Although, if we take into account the phenomenon of refraction of the solar disk and its size (0.5 arc minutes), tonature, using these physical effects, adds a few more minutes to the length of the day. After all, daylight is the time from the appearance of the upper edge of the solar disk above the horizon to the departure of its lower (in relation to the morning) edge beyond the horizon, and this is another two minutes of the movement of the solar disk. And it's 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 its highest height and the day is longest. In the polar regions, the nights during this period are very short and "white", that is, twilight without darkness. But December 21-22 is the shortest day and the longest night. And in the polar 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. Their solstice is in December and the longest nights are in June.
Biorhythms and daylight hours
Nature has adapted living organisms to the change of light and dark times of the day. If animals (and humans) are kept in the "12 hours a day, 12 hours a night" mode for several weeks, and then abruptly switched to the "18 hours light, 6 hours dark" mode, then active wakefulness and sleep disorders begin.
In human society, disruption of biorhythms in the daily cycle leads tostress, 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 length of daylight hours.
Different latitudes have different daylight hours. Moscow, located at 55 degrees north latitude, has daylight hours from 7 hours in December-January to 17 hours in June-July.
Daylight hours in St. Petersburg 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 here is about 18.5 hours. This creates the effect of white nights when the sun goes down only briefly. Officially, white nights last from May 25 to July 17. But in December-January it gets dark at five o'clock in the evening.
