Comets and Light Pollution
Yet most light pollution is unnecessary. It is not an inevitable result of having well-lit streets and cities. As much as three-fourths of the murky glow you see blotting out the stars at night is waste light that beams directly skyward from light fixtures that are poorly designed or improperly aimed. (See "Bright Lights, Big Problems".)
A standard security light, for instance, may send a third to half of its rays above horizontal directly into the sky rather than down toward the ground where the light does any good. The upward part is pure waste. This waste goes completely unnoticed (except by astronomers), because people who install outdoor lights don't normally inspect their handiwork from high up in the sky at night!
If the fixture is replaced with a well-designed, "full-cutoff shielded" fixture of various styles now available one that directs all the light down where it's supposed to go the bulb wattage can be cut by a third to a half for a big electricity saving. The quality of illumination is actually improved, because of the reduction in annoying glare the near-horizontal beams that dazzle your eyes directly from a bulb. And we regain some of the lost starry heavens.
International Dark-Sky Association at 3225 N. First Ave., Tucson, AZ 85719. Sky & Telescope supports the IDA's efforts.