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Every 29.5 days, the Moon’s shape appears to change in a predictable cycle. We call the shapes phases of the Moon. This activity will show how the Moon’s orbit causes the Moon’s phases. Students use Ping-Pong or Styrofoam balls to model the way the Sun illuminates the Moon as the Moon orbits Earth. Then they model the orbit of the Moon around Earth using a painted ball to understand why the Moon shows phases.