Big Dipper I

Follow this procedure:

• Pick a starting time after it is completely dark, when the Big Dipper is clearly visible. (If it's very low in the sky, you may be able to see only part of it.) Try a pay attention to the weather, and do the project on a night with no clouds in the forecast.
• Your observing location should have a good clear view of the northern horizon, without any tall obstacles in the way (trees, buildings, etc.). You will make all your observations from the same location.
• Download and print the file PolarPlot.pdf, and use this to record your sketches. The North Star should be plotted in the middle of the circles. The interval between one circle and the next is 10° (one fist).
• Each time you observe the Big Dipper, carefully plot the position of the seven stars that make up the dipper. Use your hands/fists to measure angles in the sky. Be very careful to plot each star correctly in relation to the others and in relation to the North Star. Also record the date and time of your observations.
• Repeat the observation/sketch procedure every 2 hours all night long, until just before sunrise. You may either make all sketches on the same plot (perhaps in different colors), or use a different plot for each. If unexpected clouds keep you from finishing your observations, try again on the next clear night.
• Note that over the course of your observations, the stars should not change their distance away from one another or away from the North Star. Rather, they should seem to rotate around together in a circle.

• Using your sketches, measure the total number of degrees that the Big Dipper has rotated around the North Star.
• Divide the total number of degrees by the total time in hours between your first and last observations. This is the average number of degrees of motion per hour. If you've been careful, it should be close to 15° per hour.
• Multiply your result by 24 hours to predict the angle that the Big Dipper rotates through in one day.
• It is highly unlikely that a single nightÕs measurements will be accurate enough to get exactly the correct answer. The correct answer is actually 361°, not 360°. Discuss why the correct answer is 361°. (Hint: think about all of Earth's motions.)
• Discuss your results and why you think they might be off from the expected results.
• Be sure to follow the general instructions applicable to all projects.

• If you have a suitable camera with a tripod and know how to use it, you can take photographs of the stars instead of making sketches. The camera must be capable of making time exposures of at least several seconds. Almost any digital SLR camera can do this, as can some digital point-and-shoot cameras (including most Canon models). A film camera with a locking shutter or cable release will also work. Before attempting to do this project with a camera, please bring the camera to your instructor to determine whether it is suitable, and to discuss what settings to use.
• A wide angle lens (or a zoom lens at its widest setting) is best. Even so, you may not be able to fit all of the Big Dipper into the field of view. That's ok as long as enough other stars are visible. Aim the camera so the North Star is at the center of the field of view, with at least one terrestrial object (perhaps a tree or rooftop) also visible. Once the camera is set up and aimed correctly, don't move it again for the rest of the night. Take a photo every two hours. (In between photos, you may need to cover the camera loosely with a sack to prevent dew or frost from forming.)
• Before printing your photos, it's best to invert them (switching black and white) in Photoshop or a similar program. Your instructor can help with this.