Counting the Stars

Project for Elementary Astronomy

We have become acutely aware of the scarcity of natural resources during recent decades, and efforts abound to preserve many aspects of the natural world around us. One aspect that is often overlooked, however, is our night sky. Urbanization and the continual lighting of our cities at night threatens our view of the Universe. People who live, work and grow up in urban areas of the world can't even see many of the constellations, let alone fainter wonders of the night sky such as the Milky Way. In this project, you will explore the impact of light pollution in our local area by counting how many stars you can see from different locations.

To conduct this observing project, follow this procedure:

Your primary scientific instrument for this project will be a small cardboard tube. The center tube of a roll of toilet paper, or a paper towel tube cut in half, should work well.
On a clear, dark, moonless night, hold the tube up to your eye. Count and record the number of stars that you can see through the tube. Hold the tube steady, with your eye at the center of the tube's opening, during each star count.
Make sure you allow time for your eyes to adapt to the darkness. This is especially important for the darker observing sites. Allow a minimum of 15 to 20 minutes at the darker sites for your eyes to adjust before you make any measurements. Make a note of how long you waited for your eyes to adapt.
Repeat the tube count eight times (it must be exactly 8), each time looking in a different direction in the sky. Make sure your eight directions sample all directions of the sky about equally; don't do all eight facing south (for example), or all eight at the same angle above the horizon. For each star count, record the number of stars and the approximate compass direction and angle (above the horizon) toward which you were looking.
Repeat this procedure at three different locations: once in a brightly lit urban area, once in a very dark location away from all artificial lights, and once in a location of your choosing (perhaps your home). For your dark location, you'll need to drive at least half an hour from the cities of the Wasatch Front. From Ogden I suggest taking Highway 39 across Ogden Valley and up toward Monte Cristo. Remote parts of Morgan and Box Elder Counties are also suitable. Make sure you record the exact location, date, and time of each of your observations!

For your report, complete the following analysis of your data:

Make a table that neatly summarizes all of your observations, including your individual star counts, directions, and locations.
For each of the three locations, add your eight star counts and record the total number of stars that you counted.
You are viewing the night sky through the small portal of your tube, which covers only a fraction of the sky. To estimate the total number of stars visible in they sky, you will multiply the total number of stars from your eight sightings by a factor that relates the size of your viewing tube to the size of the entire sky. Measure the length L and the diameter D of the tube, using the same units (I recommend centimeters or millimeters) for each. Be sure to record these individual measurements. The total number of stars visible in the sky at a location is then approximately (This formula is not particularly easy to derive, but you should at least try to understand and explain why it is reasonable. For instance, would a longer tube show you more stars or fewer?) Calculate the total number of stars for each of your three locations, showing your calculations and results in your report.
In which location can you see the most stars? Explain (in some detail) why. Were there a lot more stars visible in some directions than others? Why are astronomers concerned about the effect of light pollution from cities near their telescopes? How big of a problem is light pollution, and what could be done to alleviate the problem? (Answering these questions should be a major part of your report.)
Be sure to follow the general instructions applicable to all projects.

A great deal of useful information about light pollution can be found at the website for the International Dark Sky Association (IDA). Be sure to cite this and/or any other sources of information that you use in your report.