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:
For your report, complete the following analysis of
- 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
- 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!
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.
- 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
(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.