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Doing Science

Unit Two: Investigating the Sun-Earth-Moon System
Doing Science

4. The Moon's Orbit: An Observational Test

In this module, you have explored the nature of the moon's orbit. Like all good scientists, you should test your model against observation. A simple observational test comes from the prediction of your model that the moon will be at different distances from the Earth during the month. If the moon changes its distance during its orbit, its apparent diameter should also change accordingly: larger when nearer, and smaller when farther. By taking an image of the moon every night for a month, you should be able to see it change in apparent size, and the changes should correspond with your model: smallest when at apogee, and largest when at perigee. In this Doing Science, you will take daily images of the moon to test the lunar orbit model in the textbook.

Take images of the moon every night for a full lunar cycle (weather permitting). If you have your own telescope and camera (preferable digital), take the images using your own equipment. Be sure to keep the focus and magnification/zoom the same for every image.

If you do not have your own equipment, you may obtain images at the MicroObservatory remote-control telescope site at:

Click on the Guest Observer Portal, then click on Telescope as Time Machine, and follow the instructions to set up and request your image. Be sure to set a short exposure time, or your image will be overexposed.

You may be able to shorten the time to attain your set of images by clicking on get images in the menu at upper left. This give you access to all the images taken on MicroObservatory telescopes for the previous two weeks. Since the moon is one of a limited number of Guest Observer objects, you will usually find several images of the moon for any given date. You may browse through and download any images. Click first on the Info link for each image: that will bring up an information page with thumbnail. Check the thumbnail to make sure the image contains a complete view of the moon, and is neither overexposed nor obscured by clouds. You will need images with a well-defined edge all around the Moon to make your measurements.

Make a table with each image's name, date of exposure, and center coordinates (Right Ascension and Declination). Leave a blank column for the lunar diameter. That information is found in the image's header information. The information is available through the Info link, or you may access it by reading the file in your computer's text program.

Open each images in a program like ImageJ, one that has a linear measurement tool. All measurements will be in pixels, so you will not need to scale the image. Measure the diameter of each lunar image, and record the result in your table. (That may require some ingenuity for images near the New Moon.)

Make a graph of your data. Place the date on the horizontal axis, and the lunar diameter on the y-axis. If you have access to a star map with right ascension and declination, plot the location of the moon on each night. Or, use Starry Night or a similar program to take position measurements.

Your Report:
Make a poster or PowerPoint presentation to display your results. Include your data table and graphs, and a few sample images. From your data table and plots, you should be able to estimate the current direction of the lunar perigee and apogee. Compare your observed results with your model. Explain any discrepancies. How does the change in lunar diameter compare with the change in distance between Earth and the moon?