Due to the high light pollution the average city dweller almost never see a truly dark nebaKazhetsya the darkness - it is quite simple: The sun sets and the sky darkens. Sometimes it appears the stars, sometimes not. But the darkness everything is much more complicated. In fact, there is a special range designed specifically to measure different degrees of darkness. Bortle scale to assess the degree of darkness of the sky was established in the year 2001, and measures the darkness of the sky in the range of 1 to 9. The mark 1 on the scale - it's really dark sky. Then begins the mass of technical details (such as stars and constellations that can be seen with the naked eye), but one - this is when you can not see your car in the parking lot, even if it is in front of you. 2 - this is when you begin to see a faint light on the horizon, and even if it is seen the Milky Way, it is not easy to discern. 3 - is the sky that can be seen in every village, and 4 - the sky that can be observed on the border of the countryside and suburbs. 5 - this is heaven in the suburbs. 6 - it is also a "suburban heaven", but more vivid. 7 - so the sky could be seen on the edge of the city and suburban areas. 8 - a city night sky. 9 - sky brightly lit city. When you get to the level of 9, you can hardly see any at night the stars or constellations, even with a telescope. You can see only the big celestial bodies such as the Moon.
Dark skies, really dark, like those that inspired the great Virgil and Hesiod, like the ones that hung over the biblical city - disappear. The more houses built, cities and suburbs, the higher the level of light pollution, and the sky becomes lighter.
The International Dark Sky Association is currently trying to keep the darkness of the first type, helping to spread the fixtures that emit diffuse light, and which reduce light pollution to a minimum. They also map the places where everyone, including astronomers can see a truly dark sky.