Mapping Light Pollution in Your Community

Initial Publication Date: May 13, 2020

Part 1: How Dark is the Sky?

To see this many stars surrounding Orion, you would need to have a very dark site to observe from. Image courtesy of NASA. Click image for larger view.

To collect the data you need to make your contour map, you will need to go out on one particular night to take measurements of the relative brightness of the night sky, using the constellation of Orion as an indicator. This activity is best done when Orion is near its highest point in the sky, sometime in February or March (between the hours of 7 pm and 9 pm), but it can be done anytime Orion is visible in the night sky. If you prefer to do this activity in the fall, use the constellation Cygnus instead of Orion as your light pollution indicator.

As a class, discuss your prior experiences with dark skies. Has anyone in the class ever seen the Milky Way? If so, where did they see it? Was it at their home, out in the country, or while on a ship in the ocean?
If recommended by your teacher, watch this International Dark Sky Association PowerPoint presentation (PowerPoint 13.7MB Nov19 08).
Before you go outside, examine the "General View of the Sky around Orion" section of the Orion Locator and Light Pollution Chart. Orion looks like a stick figure shaped like an hourglass. At the narrow part of the hourglass are three bright stars in a row of similar brightness making up Orion's belt. Locate the bright red star at the top left of the stick figure (Betelgeuse) and the bright blue-white star on the lower left side of the constellation (Rigel).
On a night without a moon and without clouds, search for a place near where you live that is fairly dark. If you stand in your yard or near your house, turn off your house lights. When visiting a dark location, be sure to keep safety in mind.
- Never go alone—always have a partner or an adult accompany you.
- Always carry a flashlight and only turn it off when you are ready to make your observations.
- Take notes if you need to, but record your data when you get home rather than spending unnecessary time at the dark site.
Use a compass or landmarks to help you determine which direction is south, and locate Orion in the sky.

Show me what to look for
Turn off your flashlight and wait for at least 10 minutes for your eyes to adjust to the darkness.
When your eyes are dark-adapted, hold the "General View of the Sky around Orion" chart over your head, orienting it so that Orion's knees (the two bright stars below Orion's belt) point toward South.
Using Star Maps #1-7 on the "Light Pollution Indicator" chart, identify which one of the star maps most closely matches what you are seeing in the sky. Star map #7 shows all the stars your eyes would see with no light pollution of any kind. As you move from Star Map #7 to Star Map# 1, the light pollution level increases. In Map #1, none of the stars in Orion can be seen.
On your Dark Sky Data Sheet, circle the number of the star map that matches most closely what you see in the sky. Also make a note of the phase of the Moon and the cloud coverage (if any).
If you have a GPS unit, write down the coordinates of your location. If you don't have one, take down the street address so you can locate your observation site on a map.
Back inside your home, fill in the rest of the information requested on your observation form, especially the date and start and end times for the observation.