The students agree that comparing the order of phases in their model to the order of moon phases shown on a calendar helps them assess the apparent relationship between the earth, sun, and moon. Mr. Gilbert asks what evidence seems to be most helpful in testing the different explanations. Some of the groups agree that the position of the earth’s shadow during the month is critical evidence. Mr. Gilbert asks them to explain why.

The students explain that the orientation of the earth’s shadow brings it in contact with the moon in various ways during the month. One team points out that, during the first quarter phase of the moon, the earth’s shadow would have to turn a right corner in order to fall on the moon. “That is not the way that light and shadows work.” Based upon such evidence, even the students who proposed the “earth’s shadow” model decide to reject it. To check for understanding, Mr. Gilbert asks, “How would the sequence of moon phases be affected if the moon moved around the earth in the opposite direction?” The investigations raise a problem for several groups. Students are confused because, in some of the drawings, it looks like there should be an eclipse of the moon and an eclipse of the sun every month. “Something must be wrong with our model because we know that doesn’t happen.” “Good observation,” remarks Mr. Gilbert “What modifications would you need to make in your models so that the cycle of moon phases does not produce these eclipses every month? What additional information might help you? What reference materials might you use?” The class decides to consult their textbook and references from the media resource center.

As the class discusses their readings, Mr. Gilbert questions them about the plane of the moon’s orbit around the earth, compared to the plane of earth’s orbit around the sun, and how it changes during the year. The student teams then modify their earth, sun, and moon models and alter their drawings to apply this new information. At this point Mr. Gilbert asks them to step back from their work to reflect on the models of the balls and light source they are using, as they had with the beads on the toothpicks. Again he poses the questions, “What features of the



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