Seeing Science: The Size of the Full Moon Rising

• A sheet of blue construction paper.  A blue pen or pencil can be used instead, but is less preferable.
• Scissors
• Glue or tape
• A sheet of yellow construction paper.  A white sheet of paper can be used instead, but is less preferable.
• A timer or a clock that shows seconds and a helper
• An area with a clear view of the horizon and the zenith (which is the region of the sky directly overhead).  Also, this part of the activity should be done mid-morning or in the evening (to avoid looking at the sun), and on a day that is fairly cloudless with a lot of blue sky.

1. Cut a square out of the blue construction paper, about 1 to 2 inches on each side.  
2. Lay the yellow sheet of construction paper down in the landscape position.  Fold the paper in half, hamburger style (taking the right side of the paper and folding it over the left side).  Then unfold the paper.  
3. Using glue or tape, attach the blue square to the yellow sheet of construction paper, in the middle of the left or right half of the yellow sheet.  Be careful not to cover the top of the square or yellow sheet with the tape or glue.
4. If you do not have construction paper, you may draw a solid blue square on the right or left side of a plain white sheet of paper, as described.

1. Hold the yellow paper with the blue square in front of you.  Stare at the blue square for 30 seconds.  (Use a timer or have a helper watch a clock for you.) Without changing the distance between your head and the yellow paper, switch from looking at the blue square to looking at the empty half of the yellow paper.  Do you see the afterimage of the square? If you cannot clearly see an afterimage, try repeating this step until you can.
2. Once the afterimage fades, keep your head the same distance from the paper as it was before and again stare at the blue square for 30 seconds. Then look for the afterimage on the empty half of the yellow paper, but this time try moving your head away from, or closer to, the yellow paper.  How does the size of the afterimage change as you change the distance between your head and the yellow sheet of paper?  If you could not clearly see a change, try repeating this step until you can.
3. Overall, how did the size of the afterimage change as you changed the distance between your head and the yellow paper?
4. Next, go outside to an area with a clear view of the horizon and zenith.  Do not do this at noon, when the sun is directly overhead, since this interferes with your observation of the afterimage at the zenith (the sky directly overhead).  Mid-morning or evening is probably the best time to do this, and the day should be fairly cloudless with a lot of blue sky to look at.  
5. Stare at the blue square on the yellow sheet for 30 seconds and then look at the horizon.  Is there an apparent change in the size of the afterimage?  In other words, does the afterimage look smaller, larger, or the same size as the blue square? You can repeat this step a few times if you are unsure of your observations.
6. Next stare at the blue square for 30 seconds and then look at the zenith.  Is there an apparent change in the size of the afterimage?  Does the afterimage look smaller, larger, or the same size as the blue square?  Again, you can repeat this step if you are unsure of your observations.
7. Overall, did the afterimage appear smaller, larger, or the same size at the horizon compared to at the zenith?

Extra: Try the first part of this activity again (before you go outside) but this time try to quantify how the size of the afterimage changes depending on the distance between your head and the yellow sheet.  (Be sure to always stare at the blue square from the same distance.)  You could put the yellow sheet on a wall next to a ruler to estimate the size of the afterimage.  Alternatively you could draw several differently sized squares (some bigger and some smaller than the blue square) nested together on the yellow sheet and try to see which square the afterimage best fits in. How does the distance between your head and the afterimage quantitatively correlate to the size of the afterimage?

Extra: Try to estimate the change in the size of the afterimage at the zenith compared to at the horizon.  To do this you could cut out several squares (on a white sheet of paper would work), some smaller and some larger than the blue square, and hold them out at arm’s length when looking at the afterimages.  Try to find the squares that are most similar in size to the afterimages.  Based on your findings, what is roughly the magnitude of the change of the full moon’s apparent size, between the horizon and the zenith?

Extra: Try to estimate the change in the size of the afterimage at 45 degrees above the horizon; that is, halfway between the horizon and the zenith.  What is the relative size of the afterimage at 45 degrees above the horizon, and how does it compare to the afterimage’s size at the zenith and the horizon?