Abstract
In an effort to provide students with first-hand experience at explaining astronomical observations, an exercise on interpreting images of Jupiter and its Galilean satellites has been developed. The exercise first provides students with seven negative images of the same portion of the night sky taken five days apart and asks them to demonstrate that one of the nine points of light shown is a planet, whereas the other points are fixed stars. They are then given a magnified image on which the planet shows as a disk and there are four other small points of light, all of which fall on a straight line that passes through the center of the planet. The students are asked what is peculiar about that arrangement of the bodies and then are directed to two composite images that show how the relationships of the five bodies change night after night through a one-month period. They are led to interpret the meaning of the changing positions and to determine the period of one of the satellites. The exercise simulates Galileo's discovery of the four largest moons of Jupiter and leads the students through the logic of his interpretation that what he was seeing was four satellites orbiting Jupiter, a situation that directly contradicted the prevailing wisdom that all astronomical bodies orbit Earth. The objective of the exercise is to help students learn how observations are used to test scientific hypotheses and to give them some experience in using reason to interpret observations. The exercise can be done in class or out, as a group activity or individually, and as a very structured activity or as a completely open-ended one.
