explain how pollution in air obstruct star viewing
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Explanation:
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.
Full-cutoff shielding in light fixtures is the essential remedy for both glare and skyglow. A lamp should send all its light more or less down where the light is intended to be used, not upward or sideways. "Full cutoff" is usually taken to mean that no light rays from the fixture go above the horizon, and that at least 90 percent of the light is blocked in the near-sideways range from 0° to 20° below the horizontal plane.
Light that shines in this near-sideways range contributes nothing to most lighting needs. It is merely a dazzling annoyance in the eyes of people nearby and dissipates uselessly into the distance. Light spilling upward, of course, is wasted totally. Tremendous above-the-horizon waste is tolerated because it goes unseen; people who install lights don't normally check them at night from high in the air! The electricity cost of this wasted light has been put at $1 billion to $2 billion annually in the United States.
Near-horizontal light is especially destructive. A light ray aimed straight up is usually not the worst kind. It escapes into space quickly, passing through what astronomers call one "air mass." A ray aimed 10° above the horizon, on the other hand, passes through 5.6 times as much atmosphere — 5.6 air masses — polluting all the way. A ray that skims the horizon pollutes up to 10 air masses, though not much of the light is left by the time it goes through the last few of them.
The situation is comparable to atmospheric extinction of starlight arriving in the opposite direction. When a light ray travels straight up through clear air from sea level, only 20 to 30 percent of it is absorbed or scattered by the atmosphere. The rest escapes harmlessly into space. When the same ray is aimed 5° above the horizon, about 90 percent of it is absorbed or scattered. Thus it causes three or four times as much pollution, when all the damage is summed up over an area many miles across. (That, anyway, is the situation in clear air. Aerosols can complicate the picture.)
Add the fact that most lights provide some blockage at high angles, and it becomes clear that most of the light-pollution war will be won or lost in the narrow battleground just a little way above the horizon. At least this is true at sites fairly far from the offending lights — the semirural areas that seem to have suffered the worst degradation in the last 20 years. Right inside a city, rays at higher angles (and reflected from the ground) are the primary problem.
Harsh lights in residential neighborhood
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