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 two months. Follow this procedure:

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 time exposure.
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 you return.
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.

Once you have taken all your photos, analyze and describe the results as follows:

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 earth's motions.
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 measurements?
Can you tell from your photos whether the North Star truly remains fixed as the sky rotates? Explain carefully.
Be sure to follow the general instructions applicable to all projects.

Notes on using a film camera:

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.