Understanding the Moon's Own Shadow

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Fall 2009-Day 13
Sun-Earth-Moon Activities

Class began with a trip to the roof! The moon was out and the sky was clear for once. We caught it! The students each pointed one of their arms at the sun and one of their arms at the moon. There was an angle greater than 45 degrees, but less than 90 degrees. This meant that the moon is in its crescent phase. It was lit on the left side, which also meant that the moon is waning (getting smaller). We had the students pull out their ping pong balls and hold them up so that they are making the same lit shape of the moon with their models. Half of the moon (their ping pong balls) was shaded. Why is half of the moon shaded? The students discussed this and talked amongst themselves. When we heard answers being mumbled, we asked them to share their thinking. The moon is blocking itself. The back half of the moon is not receiving any sunlight because the moon casts its own shadow on itself. A common misconception is that the reason there is a shadow on the moon is because the earth is creating it. This is false information. We know this is so because nowhere on the ping pong balls were the students’ shadows. Therefore, the moon must be causing its own shadow!