In the 1950s, the city of Tucson, Arizona, enacted one of the first dark-sky ordinances in the US to safeguard viewing conditions at Kitt Peak National Observatory, about 60 miles away. A small town at the time, Tucson is now a metropolitan area of more than a million people, but the culture of the dark sky still holds. Light fixtures direct their light down instead of up, have lower illumination levels in general and are designed to go off during the late night hours. I live within the city limits, but on the eastern edge of urban development. In my development of more than 100 homes, there are no street lights, just small fixtures by the mailbox in front of the house. As a result, I can see lower-magnitude stars near the horizon on a clear night. Try this in a typical city; it's often hard to see any but the brightest stars most nights. I grew up in Los Angeles, and to see the stars, we had to go camping or drive far away from the city.
Lower nocturnal light levels offer benefits other than astronomical observing. In the desert, because of the intense daytime heat, many animals are nocturnal, and dark skies provide a protective cover for their feeding and hunting activities. According to another source:
The ecological effects of artificial night lighting are profound and increasing. Each year, over four million migrating birds are killed in collisions with lighted communications towers in the United States. Dispersing mountain lions miss crucial landscape linkages because they avoid lit areas. Increased night lighting disrupts important behaviors and physiological processes with significant ecological consequences.95
Reducing light trespass from a project site also makes one a good neighbor. Many studies have found that lower illumination levels can also be safer, because there is less contrast between light and dark areas, allowing one's eyes to see better into the dark. For older people, whose eyes don't adjust as fast, it can take several minutes to adjust from brightly lit parking areas to darker zones.
I always wonder why large office buildings in cities have so many floors lit up during the night when no one is working. Some simple engineering, providing multiple lighting zones for each floor, would allow the janitors and other nighttime workers to have light for their work without wasting so much energy. Even corporations who pay millions of dollars to put their names on the top of all buildings may not realize how much adverse public relations they are creating by keeping the lights on at night.
LEED provides one credit point in the category of "sustainable sites" for meeting certain established criteria for night lighting levels and light trespass from a building site. These standards have been established and maintained by the Illuminating Engineering Society of North America. As one might expect, the criteria for exterior lighting vary, ranging from more stringent (for projects located in darker rural areas and park settings) to relatively less stringent (for projects located in the brightly lit centers of major cities). Interior lighting that can be seen from outside should be reduced by 50% or more between the hours of 11 p.m. and 5 a.m., or else shielded from outside view.
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