White Nights or civil twilight?

What if someone thinks that the White Nights - the exclusive privilege of Russia's northern capital, it is confusing exclusively on the conscience of the media.White nights are amazing, but this atmospheric phenomenon, repeated annually, and watch it possible in many Russian cities, as well as throughout the t
erritory of Iceland, Greenland, Finland, in some polar regions of Sweden, Denmark, Norway, Estonia, Canada, the United Kingdom and Alaska.The zone of the White Nights starts at 49 ° N.Here there is only one white night of the year.The further north, the brighter the night and the long period of observation.

White Nights - an amazing phenomenon that experts rather dryly called civil twilight.And, actually, that this twilight?This certain part of the day - depending on whether any of the morning or evening twilight in question - when the sun has already or not yet clear, because it is below the horizon line.At this time, the Earth's surface illuminated by sunlight, which partly scattered upper atmospheric layers and partly reflected by them.

Assuming that night - a period of a minimum illumination of the earth's surface, the twilight - the time of its partial illumination.Thus, the white nights - a smooth flow of evening twilight in the morning twilight, bypassing the period of minimum illumination, ienight, just as Pushkin wrote about it.

But why Twilight was "civil"?The fact is that experts have identified a number of gradations of twilight, depending on the position of the sun relative to the horizon.All the difference is in the value of the angle between the horizon and the center of the solar disk.Civil twilight - the brightest "twilight" period - the time between the visible and the moment of sunset, when the angle between the horizon and the center of the sun is 6 °.There are also navigation - angle of 6 ° to 12 ° and astronomical twilight - angle of 12 ° to 18 °.When the value of the angle exceeds 18 ° «Twilight" period is over and the night falls.

As atmospheric processes are more or less clear, the issue may be raised more globally.Why at certain times the sun sinks behind the horizon just a few degrees?What caused the emergence of the White Nights with the astronomical point of view?

Short Course astronomy

high school astronomy course provides familiarity with the material in a quite sufficient level.That is, people who finished school quite able to understand how it happens with the universal point of view.

First, the Earth's axis, as, indeed, and the axes of all the other planets, are at an angle to the plane of motion of the planets around the sun, ie,to the plane of the ecliptic.Changing the value of this angle is for such a long period of time - 26 years 000 - which in this case can be neglected.

Secondly, when moving in an orbit some definite period of time, the Earth relative to the Sun is positioned so that one of its poles light rays fall almost vertically.In this particular place the sun is at its zenith for days - there is the polar day.A little farther to the south angle of incidence of solar radiation on the earth's surface changes.The sun sinks behind the horizon, but so slightly that flow smoothly into the evening twilight in the morning, passing the period of minimum illumination of the earth's surface.This is the white nights.

to appeal to the Sun reigns hemisphere summer.The further south, the darker and longer nights.Other hemisphere during this period experiencing the delights of winter as "sliding" on the surface of the planet rays poorly, it is heated.

the end of this short course should be noted that the White Nights is not the exclusive privilege of the Northern Hemisphere.The same phenomena are observed in the Southern Hemisphere.Simply zone white nights of the southern hemisphere has to the world's oceans and watch the beauty of the phenomenon may be except that the sailors.