This activity is frequently used to show the causes of the lunar phases. Generally a teacher tells students how to position the ball to show a full moon, a crescent moon, etc. This particular lesson is a little different in that the students have to figure out on their own how the balls need to be positioned. This lesson makes a good follow-up to a lesson about the motions of the Earth, Moon and Sun (such as PUMAS Example 03_10_04_1), but it can also stand as an independent lesson.

The purpose of this lesson is for students to learn the relative motions of the Earth, Sun and Moon. Awareness of these different motions is needed to develop an understanding
of the causes of the day/night cycle, the seasons and the cycle of lunar phases.