Measure the Moon

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Updated on Jul 31, 2014

Has your kid noticed how much bigger the moon looks when it's low in the sky? What's going on? The moon isn't changing size or getting closer; it's actually just an optical illusion. The moon takes up just about the same proportion of our field of vision no matter how high or low it is relative to the horizon.

If your kid doesn't believe you, here's a simple activity you can do to show him that the moon is always the same size. All you need is a paper clip to get started.

What You Need:

  • Paper clip
  • Clear night when the moon is low in the sky

What You Do:

  1. Help your child unbend the paper clip, then twist it into a U shape.
  2. When the moon is low in the sky, go outside and have him hold the paper clip at arm’s length in the direction of the low, large-looking moon. Show him how to use the paper clip like a pair of calipers, bending it so the tips are touching the top and bottom of the moon to record its apparent diameter.
  3. Wait an hour or more for the moon to climb higher in the sky.
  4. Go back outside and have your child hold her paper clip calipers at arm’s length again. What does she see? The moon should fit within the paper clip exactly.

If your kid liked this activity, try exploring other optical illusions by checking out a book on optical illusions from the library or researching them online.