same. Mr. Gilbert knows that this experience will give students an opportunity to get a sense of what causes the phases of the moon. He also knows that it will help them understand something about the use and limits of models, helping them not only to learn about the moon, but to understand that models are tools that scientists often use to build and test new knowledge.

Constructing a Model. The next day, the end of the observations, Mr. Gilbert asks his students to look closely at their posted charts of the moon’s phases over the past five weeks. Mr. Gilbert asks: “What do you think causes this repeated monthly pattern of moon phases?” He

asks the students to work in groups of three and after about 10 minutes, two different explanations emerge. Some of the students suggest that the earth’s shadow covers different amounts of the moon’s surface at different times of the month, resulting in the moon’s pattern of phases. Others propose that as the moon moves through its orbit around the earth, we see different amounts of the side of the moon that is lighted by the sun. Next Mr. Gilbert asks the students to form small groups based upon the different explanations. He asks each group to make a labeled drawing that would support its explanation for why the moon changes shape. Mr. Gilbert can tell from the discussion of their drawings that many of the students are not particularly confident about their explanations. For some, different explanations seem to make sense. Before dismissing them, Mr. Gilbert asks the students to think about how they might use models to test the two different explanations.

The next day, the students design an investigation to test each explanation. Using globes for the earth, tennis balls for the moon, and the light from an overhead projector for the sun, each group is ready to manipulate the materials in a darkened room to explore relationships between the relative positions and motions of the objects and the resulting pattern of phases. The exploration gives stu-

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