Question: I live 30 miles north of Phoenix, and still have a hard time finding some of the stars you show in your sky maps. Why is that? — KS, New River, AZ
Answer: You, and many other people living within about 50 miles of city center, are the unwilling victims of light pollution. Light pollution is the scattered light that saturates the sky above large metropolitan areas, the vast majority of which is due to billboards, streetlights and dusk-to-dawn yard lights.
Billboards are the main culprits, since they’re illuminated from below to allow easy (read “cheap”) replacement of lamps. Of course, this means most of their light goes straight up into the sky. A billboard west of me on I-17 casts a distinct beam of light upward fully 30° above the horizon — and it’s half a mile away!
Nationally, in one year, we send $1 billion worth of light straight up into the sky. If it all continued unimpeded into the depths of space, it wouldn’t interfere with our astronomy. Unfortunately, the atmosphere is not 100% transparent. Even in clear air much of this light bounces back towards Earth and around the sky. On hazy days the “cloud bulb” above Phoenix can be seen for hundreds of miles. Result: of the 6000+ stars bright enough to be seen by the human eye, the average city dweller sees only the brightest 50-60.