In early October, thousands of people around the globe stepped outside to gaze at their local night sky. As part of an event called the Great World Wide Star Count, adults and kids looked for one of two constellations (星座) — Cygnus in the Northern Hemisphere and Sagittarius in the Southern Hemisphere. The observers recorded how many stars they saw and how bright the stars were, and then posted their results online.
The Star Count was part of a global effort to help scientists learn more about how light pollution affects our view of the night sky. Whether participants saw thousands of stars — or just a dozen — depended a lot on whether they were surrounded by darkness or by twinkling city lights.
On a clear night, with no clouds, moonlight, or artificial lights to block the view, people can see more than 14,000 stars in the sky, but when people are surrounded by city lights they're lucky to see 150 stars.
If you've ever driven toward a big city at night and seen its glow from afar, you've witnessed light pollution. It occurs when light from streetlights, office buildings, signs, and other sources streams into space and illuminates the night sky. This light makes many stars invisible to people on earth.
Dust and particles of pollution from factories and industries worsen the effects of light pollution. When these particles float into the atmosphere, light ricochets (跳飞) off of them and scatters even more.
Light pollution doesn't only affect star visibility. It can harm wildlife too. Scientists don't entirely understand how animals navigate at night, but it's clear that artificial light can attract them, making them go off course. There's increasing evidence, for example, that migrating birds use sunsets and sunrises to help find their way.
