Urban streetlamps, floodlights, neon signs, and lighted billboards, which cast as much as half of their light upwards, create a glow that can make invisible all but the brightest stars in the sky. Fewer than one in 10 Americans are able to see all of the 2,500 stars that should be visible at night. In northern Europe, visibility is even worse. Given current rates of concentrated urban growth, astronomers estimate, by 2020 no stars will be visible in northern European cities.
Sources: John Mason, British Astronomical Association (London); Time, 16 December 1996; Dark Sky Initiative Against Light Pollution