The night sky is no longer as dark as it was a few generations ago. Countless lights that illuminate our streets, parking lots, and stadiums lose some of their light to the night sky. There, it’s scattered by molecules of air and particles of smog and dust known as aerosols. The result is sky glow, a phenomenon astronomers call light pollution.
The Defense Meteorological Satellite Program provides global images of city lights that clearly reveal the widespread presence of light pollution. This is spectacularly shown in the image Earth at Night (seen above), which was created by Craig Mayhew and Robert Simmon (both NASA) using data provided by Marc Imhoff (NASA) and Christopher Elvidge (NOAA).
Light pollution is a major problem for professional astronomers, which is why they spend large amounts of money to build their observatories under the darkest skies they can find, including mountaintops in Hawaii and Chile. Amateur astronomers have the same problem, especially in the Eastern United States. Some have moved to Western states to better practice their night viewing.
Years ago I experienced a week of very dark skies while leading a dozen teenagers from my church on a 16-day, 1,000-mile bicycle trip from Albuquerque to Padre Island, Texas. In Eastern New Mexico, every night we had a magnificent view of the Milky Way and the meteors that occasionally flashed overhead. Distant towns revealed their presence by small domes of light.