word "day" has two meanings.First - this is the time of day when it's light, and the second - this is a light of the time the daily rotation of the Earth.Experts believe that the daylight hours - the time from sunrise to sunset.
the Earth's rotation axis is tilted, so in the course of the year to change the length of daylight.In winter, the shortest day and the duration of it varies with latitude.In the north, the winter light day is 4-5 hours, and the rest of the time - darkness.But further north the sun does not happen at all - the polar night, but once summer sleep - not at night.Only the sun disappeared behind the horizon, and twilight began, almost immediately they come to an end - the sun rises again.
But no matter how much daylight lasted 6 hours or 18, last night just enough to take a day with 24 hours - day calendar.And if the night in June, only 5 hours, the day is 19. But in a calendar year there are interesting times.In 2010 - 2020 years is 20 March, June 20-21, September 22-23, and December 21-22.These days in March and September in the world night and day are equal.They are called - the days of spring and autumn equinox.Though, given the phenomenon of refraction of the solar disk and its size (0.5 minutes of arc), to the duration of the day nature, using these physical effects adds a few minutes.After a light day - this time from the appearance of the border below the top of the solar disk above the horizon before the departure of the lower (relative to the morning) the border below the horizon, and it's two minutes of motion of the solar disk.And it is on the equator.In our latitudes it is still 3-4 minutes or more.Furthermore, due to refraction phenomena - refraction of light rays in the atmosphere - the sun has already seen, although, according to geometric calculations, it is still below the horizon.The same is observed at sunset.
A June 20-21 - the summer solstice, when the sun rises to the greatest height, and the longest day.In the polar regions of the night in this period very short, and "white", ie. E., Without the twilight darkness.But on December 21-22 - the shortest day and the night is the longest.And in the polar regions to the north and the day may not start.But on the other side of the globe, in Australia, South Africa and South America are the exact opposite.They solstice in December, and the longest night - in June.
Biorhythms and daylight
Nature organisms adapted to the changing light and dark time of day.If the animal (and human) for several weeks to keep in the "12 hours a day, 12 - the night", and then abruptly transfer mode "18 hours of light and 6 hours of darkness," then begin violations of active wakefulness and sleep.
In human society, jet lag in the daily cycle leads to stress, until the development of diseases - depression, insomnia, abnormalities of the heart and blood vessels, and even cancer.There was even a concept of "seasonal depression" related to the long daylight hours of winter.
at different latitudes - different light day.Moscow, being at 55 degrees north latitude, has a light day of 7 hours in December and January to 17 in June and July.
light day St. Petersburg, too, depends on the season.And as St. Petersburg is located at 60 degrees north latitude, the length of the day here in June is about 18.5 hours.This creates the effect of the White Nights, when the sun is only a short time away.Officially white nights last from May 25 to July 17.But in December and January dark at five o'clock in the evening.
