protractor (the 0°–180° line). When they located an object on the horizon by sighting through the straw, the weighted string hung straight down the 90° line. As they rotated the straw to observe an object directly overhead, the weighted string hung along the 0°-180° line of the protractor. When the students sighted an object in the sky through the straw, the string would hang straight down and hit the protractor at a point that would indicate at what angle the object appeared above the horizon in the sky. For example, an object overhead would be 90° above the horizon. The students also learned to use a compass to measure an object’s “azimuth” — that is, its distance along the north/ south plane of the horizon, an orientation such as N 30 degrees E. With angular elevation plus azimuth, the students could completely describe an object’s location: azimuth told them what direction to look in and angular elevation told they how high above the horizon to look in that direction. Students had practiced using the sextant and compass by determining the angular elevation and azimuth of trees, the school flagpole, telephone poles, tops of buildings, and airplanes in the sky. Group data had been posted on a class data chart in order to identify outliers (data that don’t fit), as well as to determine the acceptable range of values (error bars) for measurements. Mr. Gilbert found that such inquiry lessons about the use of tools, coupled with a public sharing and discussion of data, was extremely helpful in getting students to evaluate data and to improve the accuracy of obtaining and reporting it.

Introductory Lesson. Today Mr. Gilbert plans to introduce his students to the study of the phases of the moon. He knows from conducting his own observations that tracking the moon’s phases can be challenging because of the possibility of occasional intervening clouds, but he feels that students will be able to learn more deeply from the opportunity to conduct an investigation of this phenomenon firsthand. He has decided to begin this lesson today because the moon is currently two days past new and, for the next two weeks, it will be visible in the afternoon and early evening.

He begins the lesson by asking his students to write down everything they know about the moon, together with the questions that they have about the moon. He then asks them to discuss their lists with a partner, making note of the items that are included on both lists. Following these discussions, Mr. Gilbert asks his students to compile their lists into one class list of what they know about the moon, and another class list of questions they have about the moon. Mr. Gilbert identifies six items on the students’ list that he knows are crucial to their understanding of the moon’s phases:

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