Developing an Explanation for the Moon's Phases

If we want to draw a diagram of the sun/earth/moon system, we need to think about: how are these bodies arranged? An easy phase to think about is the third quarter moon. We had one last Monday, October 8.

• When we look at the sun and the moon, how do their sizes compare? (circles of roughly the same diameter)

–This suggests they might be roughly the same distance away from Earth

• How can we tell which of these is the case?
  

Let’s try to reproduce here in the lab what some of us have seen in the sky:

• How can you move to make the ball look half lit when hold arms at right angle? (volunteer has to move far away from lamp)
• From this, we INFER THAT THE SUN IS FAR AWAY FROM EARTH AND THE MOON IS CLOSE.
• Have each student stand near the lamp, hold arms at right angle, see that ball is more than half lit, move away until see half lit.
  

What about the relative size of the sun and the moon?

• They look about the same size. However, we have just inferred that the sun is far away.

• Have everyone stand at the back of the room and hold thumb up and compare it to wall clock, they look roughly the same size. Have everyone walk up close to wall clock and compare thumb, wall clock is very big.

• FROM THIS WE INFER THAT THE SUN IS VERY BIG, MOON SMALL.
  

Small groups: Now use this information to draw the arrangement of the sun, earth, and moon for the full moon, waning gibbous, third quarter, waning crescent, new positions as seen here on earth and seen if one is looking down on the sun/earth/system from above.