When our school implemented AP Physics 1, I wanted to include a project that would extend over time, use more advanced data analysis, and teach students about handling experimental error. Using a donated 5-inch Newtonian telescope and an entry-level digital camera, the students gathered data from digital images of the four Galilean moons, Io, Europa, Ganymede, and Callisto (see Fig. 1), to verify Kepler’s third law of planetary motion (T2=Kr3). Simulation software could have been used. Among others, Project CLEA (Contemporary Laboratory Experiences in Astronomy) seems to be a favorite that includes verifying Kepler’s third law using the orbits of the Galilean moons. Galileo measured the moons’ periods with his telescope. My students could do the same with modern equipment. The investigation started with a purely Keplerian analysis of the data, then to a Newtonian analysis leading to accurate orbital properties of the Galilean moons and Jupiter’s mass. Once the images have been obtained, they can be used each year.

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