models work well? What features don’t?” Students respond that the model does not do a good job at explaining the changes in the height of the moon above the horizon, but it does show how the phases of the moon occur.

After this discussion, Mr. Gilbert notes that, historically, models have played a role in understanding the “heavens.” He asks them to recall

The Copernican Universe

“In the midst of all dwells the sun,” said Copernicus. To him, the universe was systematic, mathematical, knowable, and above all, simple. In this diagram from De revolutionibus the Earth is number v, “Telluris,” counting from the outermost ring of stars. Copernicus was not entirely liberated from Aristotle and Ptolemy, however, for he too believed the orbits to be circular and uniform.

—Bruno, L.C. (1987). The Tradition of Science.

what they remember about the early historical explanations for the motions of bodies in the night sky. Together, Mr. Gilbert and the students recall that, prior to the time of Copernicus and Galileo, the accepted model of the heavens was that all the planets and stars revolved around the earth, which was located in the center of the universe. They discuss how the predictable patterns of stars moving across the night sky were used as evidence to support this early explanation. “What evidence did Galileo uncover that caused him to question the earth-centered explanation?” Mr. Gilbert asks. The students use this question to focus their reading in their reference materials. During the ensuing discussion, Mr. Gilbert asks the students to compare the evidence-to-explanation thinking they used in their testing of the two different explanations for Moon phases to the scientific work that Galileo conducted – in which he observed the phases of the moons of Jupiter and then constructed an explanation to account for the evidence. For Galileo the explanation required placing the sun and not the earth to be at the center of the heavens. From their investigations, readings, and discussions, the students begin to understand how scientific explanations are formulated and evaluated with evidence, and to understand that the scientific community accepts and uses various explanations until they are displaced by better

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