Light Pollution

A composite photograph of Earth's surface at night taken from space shows the alarming spread of surface lighting at night. The east coast of the United States is clearly recognizable in any wide-angle picture of the planet at night. The problem is that most of this light goes to waste. Instead of shining down on the ground to illuminate our streets at night, they instead scatter huge amounts of light into the night sky. This is what astronomers call "light pollution." Light pollution is no good for anybody. It wastes uncountable billions of dollars per year in electricity, scatters unwanted light in the eyes of drivers and ruins the natural beauty of the night sky. In urban inner cities, so much light from unshielded high-pressure sodium streetlights is scattered into the sky that only the brightest stars and planets can be seen. I remember one particularly horrid night in New York where despite a clear sky and unrestricted visibility, I found the limiting visual magnitude to be just slightly better than +2.0. I could barely make out Polaris to the north, but Zubenelgenubi (the brightest star in Libra, magnitude +2.8) could not be seen at all. The sky that night in Flushing Meadow resembled the view one might have from inside a milk bottle. It did not help on that late August night that the U.S. Open tennis tournament was being played to my south while the bright lights of Shea Stadium flared not far to the north. The city skyline loomed brilliantly to the west and the lights of suburban Long Island soared the view to the east. That is the horror of light pollution at its very worst. Unless you're content with views of the planets and their brightest moons, a view from the city will surely spoil anyone's enthusiasm for astronomy. In urban areas, this background sky glow from city lights is brighter than the stars trying to shine through it. When this is the case, there's no way for that object to shine through.

In some suburban communities, light pollution is being pushed back. Some communities are replacing high-pressure sodium lights with low-pressure sodium. These lights shine at wavelengths that are not quite as damaging as are high-pressure lights. Many of these lights are also shielded so that the light they emit is directed at the ground where it is needed, not into eyes of drivers or into the evening sky. This can yield as much as two full magnitudes of improvement in the transparency of the sky. Many communities have passed laws mandating the use of lights that restrict sky glow, and recently the Massachusetts state legislature passed a law mandating such measures for the entire state.2 The best solution however for dealing with light pollution is to get away from it altogether. In many areas, driving about an hour away from bright city lights will do wonders to clear the view. Stars down to near the naked-eye threshold creep into view and the sky turns a deep clear black except for one milky colored band that refuses to vanish. It may take a city-based observer exposed to his first dark sky a moment or two to realize what that glow is. When he does however, it becomes obvious why our galaxy is named the "Milky Way." If you are serious about astronomy, do whatever it takes to find a good dark-sky site. No matter how good your eyes, your technique and your equipment it will not do you a lick of good if the glory of the sky is hidden behind streetlight glare. If you do not belong to one, now is the time to find a good astronomy club. Nearly all clubs have a line on a good dark-sky site where their members go to observe on a regular basis, scheduled or otherwise. For your telescope, you may wish to consider a light-pollution rejection filter; but for your eyes, there is no better way to deal with light pollution than to be rid of it.

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