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# the mass and velocity of the impactor

1. Distribute one Styrofoam ball and one pencil to each student, and instruct them to place the pencil into the ball.

2. Have students stand in a semicircle or circle facing the lamp. Explain that the lamp represents the Sun and that each of their heads represents Earth, with their noses being their hometown.

3. Ask students to hold the model Moon at arm’s length. Allow time for them to explore how the model Sun’s light reflects off the model Moon as they place it in different positions around their heads. (Students usually observe that their own shadows cover the model Moon when it is opposite the light source, simulating a moon eclipse during the full moon phase. Ask students to hold the model above or below the shadow of their heads, and ignore the eclipse for the time being.)

4. Ask each student to position themselves such that they can view the full moon (facing away from the sun).
• How much of the moon does the Earth see illuminated? [the entire circle]
• How much of the moon is actually being hit by the sun’s rays [the front half]
• Remind me, which direction does the Earth rotate on its axis? [counterclockwise]

5. Instruct students to rotate counterclockwise like Earth, very slowly, noticing how the light illuminating the moon changes. Stop at key points (gibbous, quarter, new, etc.) but encourage students to observe and describe, rather than shout vocabulary words. At each point, ask questions such as:
• How much of the moon does the Earth see illuminated? How would you describe the shape?
• Which side of the moon appears to grow brighter/darker? [right side for Northern Hemisphere]
• How much of the moon is actually being hit by the sun’s rays? [half]
• Why do astronomer’s call one of the phases “quarter moon” when half of the surface appears lit up? [Tricky answer: Because the moon has completed or has left to complete ¼ of its orbit.]

6. Repeat step 5, this time introducing or reviewing vocabulary.

7. Earthlings see many phases as the moon orbits the Earth.
• How long do you think it takes for the moon to fully revolve around our planet? Think about how often you might see a full moon. [approximately 1 month; 29.5 days]
• If the orbital period of the moon is 29.5 days, can we sometimes see more than one full moon each month? [Yep! It is possible for a calendar month to have two full moons — one at the beginning and one at the end of the month. When this happens, the second full moon is called a “blue moon,” although its color isn’t really affected at all. These coincidences happen every few years.]

8. Allow time for students to continue to experiment with the movement of the Moon. Consider having them work together to draw a diagram of the Moon’s changing position in order to create each phase. You may want to give them the included diagram with empty spaces to fill in drawings and phase names.