In 1994, an earthquake knocked out power in Los Angeles, Calif. In the days that followed, concerned residents reported a giant cloud of silvery light stretching across the night sky. Some worried that the mysterious sight was somehow related to the earthquake. These people had lived in the large, brightly lit city their entire lives. Many had rarely seen stars. They certainly had never before seen this night-sky cloud of light.
It was the Milky Way — our galaxy!
The continental United States at night, as seen from a NASA satellite in 2012. This image shows where there is a lot of artificial light at night.
NASA Earth Observatory/NOAA NGDC
Until this blackout, city lights had for decades blocked such dim light arriving from the distant reaches of space.
When artificial (human-made) light spills into areas where it's not wanted, it's called light pollution. Indeed, it is considered one of the biggest forms of pollution. It prevents two-thirds of U.S. residents and half of those in Europe from viewing the Milky Way. And light pollution increases every year, as cities continue to grow.
That doesn’t mean electric lights are bad. They have allowed people to work and play beyond sundown for roughly a century. Lights along roadways can make driving safer. In many ways, lighting up the night has been a good thing.
But there can be drawbacks. Sometimes lights that are meant to be helpful pose unintended harms. And this problem goes beyond our ability to enjoy the night sky.
Research is revealing that light pollution can alter the behavior of plants, animals and people. Some unnatural light-dark cycles may even make us sick.
