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By Root 1021 0

lighting itself, for not only do countless individual lights contribute to it, but different kinds of artificial light—incandescent, low- or high-pressure sodium vapor, low- or high-pressure mercury vapor, tungsten-halogen, fluorescent, and LED—affect the surroundings in different ways. And no matter the kind of light, the effect of lighting in any one place is always variable because its intensity and apparent brilliance are affected by weather, by the dust and gas in the atmosphere, and by the cloudiness or clarity of the sky. The direction and path of the illumination makes a difference as well. "Light traversing a path at a shallow angle above the horizontal ... will cause more sky glow, since it will encounter more particles and droplets from which to be scattered on its way through the atmosphere," wrote astronomer Bob Mizon. The type of surface the light eventually falls on matters as well: whether it's wet or dry, smooth or rough, dark or light determines the reflectivity of any light.

The Palomar scientists and town and county officials attempted to mitigate the light problem at the observatory by creating zoning ordinances. For decorative lights, such as those used to illuminate advertising, and lighting at outdoor sales areas, they established strict shielding requirements, which would direct the artificial light downward. They also established an 11:00 P.M. curfew for nonessential lighting, and in the fifteen-mile radius around the observatory, ordinances forbade decorative lighting altogether. Riverside County replaced its mercury vapor streetlights with more efficient sodium vapor lamps, which don't interfere with the spectra of astronomical objects.

Even with concerted efforts to contain light pollution in southern California, the seeing at Palomar is becoming more and more deeply compromised. The mercury vapor lights from surrounding cities have grown so bright that astronomers can no longer observe some parts of the spectrum of celestial objects. In 1999 one astronomer noted that "the city lights can be directly seen through gaps in the mountains, meaning that city light is making its way directly into the telescopes, without first even being reflected by the sky. Many observers have given up looking at objects in the southwestern sky, because the light pollution is so bad in that direction."

As much as light pollution obscures an understanding of deep space, it obscures an understanding of time, for seeing outward "is equivalent to looking backward in time," Richard Preston notes.

The universe—as we see it—could be imagined as a series of concentric shells centered on the earth—shells of lookback time. The shells closest to the earth contain images of galaxies near us in time and space. Farther out are shells containing images of remote galaxies—galaxies as they existed before our time. Still farther out is the shell of the early universe. Some of the photons reaching a telescope's mirror are nearly as old as the universe itself. The quasars are brilliant pinpoints of light that seem to surround the earth on all sides, shining out of deep time. Beyond the quasars, the observable universe has a horizon, which could be imagined as the inner wall of a shell. This horizon is the limit of lookback time, which is also an image of the beginning.

The Hubble Space Telescope—the first space-based observatory—orbits beyond the distortions of the atmosphere and the effects of light pollution, and it sends back to Earth clearer and deeper views of the universe than previously possible. But space-based telescopes—extraordinarily expensive and precarious—can't fully replace what happens when human thought meets the dark sky, whether at rarefied Palomar, where scientists gather and play cowboy pool as they wait for their precious observation time; in a remote pasture where an amateur astronomer collects starlight with a homemade telescope built of wood and mirrors; or at the back door of a farmhouse, where a child looks up and stares.

Perhaps what we've lost with the disappearance of the night sky is more profound