Dissecting Light Pollution
Every astronomer knows the artificial skyglow that hangs over populated areas, washing out almost everyone's view of the universe to a greater or lesser degree. In the last two generations, light pollution has spread from a problem in cities to a major astronomical disruption almost everywhere.
But some aspects of light pollution are not widely appreciated by amateur astronomers. Knowledge is power; here are facts that may help you avoid some of the problem and combat the rest more effectively.
Glare versus skyglow. The most annoying and destructive problem is light that beams directly into your eye from a bright bulb. This is called glare; it comes from fixtures that are poorly designed or improperly aimed, in other words most of those currently in use. When glare crosses property lines and creates a nuisance, it's called "light trespass." Glare is often the easiest problem to avoid — by setting up your telescope in a shadowy corner, erecting a tarpaulin to shade the telescope, or negotiating with your neighbors or local government to have the offending light turned off or replaced with a modern one of better design.
Skyglow is what the term "light pollution" properly denotes. One way to measure it is to compare it to the night sky's natural background light. The sky does have a certain minimum surface brightness even in the most pristine, unspoiled environment. This natural skyglow has four sources: faint airglow in the upper atmosphere (a permanent, low-grade aurora), sunlight reflected off interplanetary dust (zodiacal light), starlight scattered in the atmosphere, and background light from faint, unresolved stars and galaxies. Airglow peaks around the maximum of the 11-year sunspot cycle; the other sources vary with the hour of night and the seasons. But their combined average is well known.
A typical suburban sky today is about 5 to 10 times brighter at the zenith than the natural sky. In city centers the zenith may be 25 or 50 times brighter than the natural background.