Photograph Circumpolar Stars
Project for Elementary Astronomy
For this project you will photograph the North Star (Polaris) and the stars around it
several times, to see how they appear to move. You can make your photographs either over the
course of one long night, or once each night, every week or two, over a period of at least
Follow this procedure:
Once you have taken all your photos, analyze and describe the results as follows:
- For this project you need a camera. A digital camera is best, so long as it can take time
exposures of at least one second (longer is better, up to about 15 seconds). Before committing
yourself to doing this project, you should test your camera to make sure it is capable of
photographing stars. Your instructor can help you find the right camera settings.
See the notes below if you are interested in using a film camera.
- You also need a tripod, or some equivalent way of holding your camera steady during a
- Decide whether you will photograph the stars over the course of one long night, or once each
night, every week or two, over a period of at least two months. Choose one version or the other--not both!
- You must make all your photographs from exactly the same place. For the one-night version of
this project, you should just set up the camera and leave it in place all night. (A plastic bag
may be needed to keep dew from condensing on the camera.) For the multiple-night
version, you should mark the camera location somehow, so you can find the exact location each time
- Choose a camera location where you have a clear view of the North Star (Polaris) and the
stars around it. Also make sure that some illuminated terrestrial object (a building or tree top)
will be visible at the bottom of your photos. In your observing notes, describe your location
and the source of foreground illumination, as well as the weather, sky conditions, and phase of
the moon (if visible).
- Set your camera to take a time exposure of at least one second and, if possible, as long as 15
seconds. Turn the flash off. If you can adjust the sensor's sensitivity ("ISO"), set it as high
as possible. If your camera has a zoom lens, zoom it out to the widest possible angle. If you can,
set the focus at infinity and use the self-timer (if any) so you won't have to be touching the
camera when the shutter opens. Again, consult your instructor regarding camera settings.
Take a few test shots to check all your camera settings. Even though your camera may record all
its settings, write them down in as much detail as you can.
- If you're doing the one-night version of this project, take a photo every two hours during the
course of the entire night. Continue to make notes on the weather and observing conditions. In between
photos, you may admire the rest of the sky or otherwise enjoy your all-nighter. Be sure to keep warm!
- If you're doing the multiple-night version of this project, you must take all your photos at
the same time of night. (If the switch to or from daylight savings time occurs during the course of the
project, you must compensate for this.) Try to take one photo each week, weather permitting. Watch
the weather forecasts and be sure that the interval between photos is never more than two weeks.
(This means that if a long period of cloudy weather is forecast, you should take a photo on the
last clear night before the clouds move in!) The time period between your first and last photo should
be at least two months.
Notes on using a film camera:
- You may either make direct photographic prints of your photos, or paste them digitally into
your report. Be sure to label each photo with the date, time, and complete information about the
camera settings used. If you're good with image manipulation software, you may wish to reverse
the photos (so the sky is light and the stars are dark) for easier printing and labeling. You could
even try to produce a composite image, overlaying all the separate photos. But if you do this,
you should still include the separate photos in your report.
- Measure the angles by which the stars seem to rotate between each photo and the next. You can
do this most easily by picking a single prominent star and measuring its angle from the North
Star on each photo. Be sure to mark or describe which star you used, and say exactly how the
angle was measured.
Your report should include a table of your measured angles, clearly labeled and explained. Describe
the direction (clockwise or counter-clockwise) in which the stars seem to rotate. Did the stars
rotate at a steady rate, or did they seem to rotate faster between some photos than others?
- Determine the total angle of rotation between your earliest and latest photo, and use this
to calculate the observed rate of rotation in either degrees per hour (for the one-night version) or degrees
per day (for the multiple-night version). Show your calculation clearly in your report.
- Based on your data, calculate the time that would be required (in hours for the one-night
version or days for the multiple-night version) for a full, 360-degree rotation of the stars.
Explain the physical interpretation of your result, according to our modern understanding of the
- Discuss the accuracy of your angle measurements and calculations. That is, were you able to measure
the angles to the nearest degree, or half degree, or only within two or three degrees? What effect
does this uncertainty have on your final results? How could you improve the accuracy of your
- Can you tell from your photos whether the North Star truly remains fixed as the sky rotates?
- Be sure to follow the general instructions applicable
to all projects.
- You can do this project with a film camera, so long as it is capable of taking long time
exposures. The most common such cameras are 35mm SLRs, which you or your parents or grandparents
may have gathering dust in a closet somewhere. (Also, some very nice used film cameras can be purchased
quite cheaply on the internet.) You'll still need a tripod or the equivalent, and you'll probably
need a cable release to lock the shutter open for several seconds. Use the fastest film you can
find, and the widest-angle lens you have. Use an exposure time of one minute with ISO 400 film,
or 30 seconds with ISO 800, or 15 seconds with ISO 1600. Unless you are very experienced, ask your instructor
for advice on other camera settings.
- Unless you want to use a separate roll of film for each photo, you shouldn't try to use a film
camera for the multiple-night version of this project. You can't wait two months to know whether
the photos turned out!
- Getting the film processed correctly may be a challenge. Be sure to explain, when you drop off
your film, that these are night photos that are supposed to be dark. Otherwise they'll probably try
to make the photos much lighter, and ruin them in the process. If they don't know what you're talking
about when you try to explain this, take your film elsewhere. You should include a
few ordinary daylight photos at the beginning of the film roll, so they'll know where the boundaries
are between one photo and the next.