Moon Size

Have you noticed how much larger the moon appears when it is near the horizon than when it is high overhead? Have you heard the explanation for this phenomenon? Is it an optical illusion, or refractive effects in the Earth's atmosphere? Why not measure the true sizes yourself? This might make a good science fair project!

One simple method would be to look at the moon through a cardboard tube (like those found inside rolls of paper towels). Viewing the moon this way isolates it from the surrounding reference frame of trees and houses.

  • Stand the cardboard tube on end and trace a circle around the base of the tube onto a piece of paper.
  • Label the circle with the time of your observation.
  • Make your first observation of the moon when it is near the horizon. Look at the moon through the cardboard tube and then make a sketch in the circle you traced showing how much of the tube is filled by the image of the moon.

    Repeat the above steps every hour or two for about 6 hours (when the moon is overhead). Compare your sketches.

    A better way to carry out this experiment might be to actually photograph the moon at various times, beginning when the full moon rises. Try to shift the position of the moon so it appears first on the left side of the picture and finally on the right side of the picture. This way you can keep the frames in order. Record the times that the photographs were taken. After the pictures are developed, measure the diameter of the moon with a micrometer. Measurement errors will be smaller for larger images of the moon than for smaller images, so use a telescope or telescopic lens on the end of your camera if possible. Consider having the photographs enlarged, but the enlarging scale must be the same for all photographs.

    Do you see any differences in the measured size of the moon? What do you conclude? Do you have an explanation for your result? If you try this experiment, I would love to hear how it goes.

    Debra Fischer, Debra's Homepage