A hundred years ago, you could go out on a clear night in the country and see two thousand stars with the unaided eye. Other objects like the Milky Way, planets, nebulae and star clusters were often visible. With binoculars or telescopes, more objects became visible.
In many locations, the number of visible stars has been reduced by half or more. Only the brightest objects are easy to see, the Milky Way and other dimmer objects are invisible.
These changes have occurred because of light pollution. The accumulated effect of countless lights in large cities (streetlights, car lights, lights from homes and businesses, etc.) drown out all but the brightest astronomical objects. Even hundreds of miles/kilometers from a big city or near smaller cities prominent glows appear near the horizon that block out the sky. Less obvious is a fainter glow over the entire sky that reduces the number of stars visible.
It is harder and harder to find isolated locations where dark skies still exist.
Some lights at night are essential; lights allow us to live and work when the sky is dark; appropriate lighting may reduce crime. However lights are often brighter than they need to be. Inefficient lighting results in high electric bills and contributes to air pollution and similar problems. While lights can help people see at night, glare from lights that are too bright make it difficult to see.
The trick is to use just enough light to serve a particular need and send that light in the direction needed. Anything more is wasteful. If you are considering installing lighting, consider using the most efficient kind.
Many communities (such as Tucson Arizona) have set up light pollution ordinances that allow lighting at night, but have guidelines that prevent wasteful lighting. However the problem has not been solved and greater awareness of light pollution is needed (a few organizations are attempting to educate the public on this issue).
Find out if your community has a light pollution ordinance. If not, bring this to the attention of your city government. The best approach is to mention the costs of electricity to city officials, you may be surprised the effect a single persistent individual can have.
Southeast Michigan Light Pollution Issues.
International Dark-Sky Association. (http://www.darksky.org/)
Astronomy Picture of the Day: Earth at Night. (http://antwrp.gsfc.nasa.gov/apod/ap001127.html)
Google Earth Light Pollution Overlay Files.