Step outside on a clear night. How many stars do you see?

Dozens? Hundreds? Thousands?

Your answer depends on the amount of light pollution in your night sky. Light pollution is generated by street lights, billboards, car headlights and household lighting. Every night billions of bulbs send their energy skyward, lighting up the night sky instead of the ground below. The overall effect is sky glow – a layer of glowing air that obscures everything beyond it except for the brightest stars.

Last year, 20 percent of StarWatchers measured darker skies during Earth Hour. Here's what you had to say:

• Very few lights in the area. Saw 5 stars after 9:00. Saw 6 before 9:00 p.m.. - Athens, Ontario
• Observing from the parking lot outside my townhouse. All of the exterior orange lights were on creating a lot of light pollution. - London, Ontario
• Street lights made it impossible to see more than 3 stars! - East York, Ontario

This map compares the average darkness of the night sky measured by Citizen Scientists across North America during and after last year's Earth Hour event.

By participating in the 2009 StarWatch you will help us track the impact of Earth Hour from year to year.

Light pollution is an indicator of wasted energy. Save money, the climate and the starry skies - check out these resources to find out how you can reduce light pollution in your community.

For more information about Earth Hour visit earthhour.org or wwf.ca