Moon Phases

Target concept: Phases of the moon are caused by changes in how much of the illuminated portion of the moon is visible from the Earth.

CCC: Systems and system models

Practice: Developing and using models


  1. Set up the model:
    a. Have students stand in a circle or semi-circle around an exposed light bulb, which is the only light source in the room (windows should be darkened, all other lights off)
    b. Hand each student a small sphere on a stick
  2. Present the question they’ll be thinking about: Why does the moon change appearance?
  3. Introduce the model to students:
    a. The light bulb is the Sun. It’s way closer and smaller than it would be if things were to scale.
    b. Your head is the Earth. You are looking out into space from the Earth, like you were looking up at the sky. Tell them to make the Earth spin (they’ll have to spin their whole bodies).
    c. The sphere is the Moon. Where does the moon go? (Around the Earth—they should hold it at arms’ length in front of them.) Explain that the moon does not rotate at the same speed the Earth revolves (or else it would stay in the same spot in the sky), but we’ll keep it there or else we won’t be able to see it.
  4. Initial use of model. Tell students to watch the moon as it rotates around the Earth. What do they notice about how the moon looks? Make sure they notice that half of the moon is illuminated and that we see different portions in shade.
  5. Ask students to make 2 or 3 different phases of the moon (not full or new) by drawing it on the board and asking students to make their moon look like this.
  6. Have students explain to each other (in small groups or the whole class) why the appearance of the moon changes.
  7. Now, ask them to make their moon look like the new moon (no visible illumination). Some students will position themselves as the new moon and some will position themselves as a lunar eclipse. This will allow for a discussion on what makes an eclipse unusual and for emphasizing that the changes in the moon’s appearance are not the result of the Earth’s shadow falling on the moon.
  8. Other extensions including solar eclipses and the idea of a “dark side” of the moon (tidal locking) could then be pursued.


• Light bulb on a stand or clamp (or lamp with its shade removed)
• Extension cord
• Styrofoam balls or light-colored spheres
• Pencils

Similar to other sources, including, (Kavanaugh, Agan, & Sneider, 2005; Trundle et al., 2006).