Paper Plate Education
"Serving the Universe on a Paper Plate"

Activity: Retrograde Motion

The following text is excerpted from GLPA Proceedings, 1992, p. 84. Used with permission:

Material: 2 paper plates

scissors

pencil or marking pen

Goal: Student builds a model to observe retrograde motion.

Background: Retrograde motion occurs as we observe planets in the sky. They seem to move toward the east compared to the background of stars...then stop and move west for a period of time, then resume the east motion. This was a puzzle for early observers. For 1500 years, the answer was "known", but with increasing accuracy of measurement, the "known" answer didn't predict well enough, until Kepler combined the idea of a sun-centered solar system with the planets traveling on elliptical orbits (instead of circular ones), and now we know why retrograde motion occurs.

Activity: Cut one paper plate around the edge so the plate is about 1" smaller than the second plate. Cut in from the edge of both plates making a small tab. Turn it up and draw a circle on the tab representing a planet. Push the pencil into the center of both plates making a small hole. Stack the plates with the pencil point up through the center so they can turn. Look across the plates as you turn them. Turn them so the inner plate (planet) goes faster than the outer one. Watch it against the backdrop of a distant wall. You will see the outer plate appear to "back-up" as the inner one passes it. This is retrograde motion. It is the same thing you see when you are riding in a car, looking out the side window, and a big truck passes you. It feels like you are moving backwards. With the planets this takes much longer (different times for different planets) but it is the same thing.

Assessment: Check student model to see if they followed directions. Ask if they see the motion. Have the student demonstrate it to another student or to the class. Assign a grade based on points covered...in building the model or explaining the motion.

Extension: Student research earlier "known" reason for retrograde motion. Have student build a paper plate model to demonstrate it. (Little paper plates mounted on the edge of the big one...observer at the center instead of on the inner plate.)

Library Ref: Hipparchus; Ptolemy, Claudius; Copernicus, Nicolaus; Brahe, Tycho; Kepler, Johannes

Contributed by Wayne James.

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