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When the Moon Hits Your Eye

Have you ever seen a photo of a full moon, usually near the horizon where it looks HUGE.  As a young kid, scenes like that made me think it might crash into the earth.  But then when it gets higher in the sky it looks much smaller.

 The size of the moon doesn’t actually change.  What you see is an optical illusion.  And humans have been observing and debating this illusion for centuries!

The cause of this illusion is still debated.  Here are a couple theories.

  1. Refraction
    The first explanation was suggested back in ancient Greece by  Ptolemy.  He said that the atmosphere bends, or refracts, the light to make the moon look bigger like a magnifying glass.  It didn’t take long for physicists to prove this wrong.  The atmosphere does affect the color of the moon, but not its size.
  2. Apparent Distance
    This explanation puts the illusion in our brains.  That is because our eyeballs can’t make sense of how close or far away the moon is near the horizon, we think it is bigger than it really is. When it’s up high in the large wide-open sky it appears much smaller.
  3. Relative Size
    Finally, some think that near the horizon, we have other objects like trees, mountains, and buildings to compare the moon too making it look closer and larger.  But up in the sky there is nothing nearby to judge the size of the moon and we see its true size.  This is similar to the Ebbinghaus illusion – both of the orange circles below are the same size but they appear larger and smaller because of the different sized circles around them.

Learn more about the moon illusion here and here.

You can test the size of the moon with some straws!

Gather a coffee stirrer straw, regular drinking straw and a super size milkshake straw.

Which one do you think will be completely filled by the moon when you look through it?

Try it out and let me know here. Post a photo with and without the straw!

Next, look through the straw at the full moon when it is near the horizon just after sunset.   

Look again a couple of hours later when it is higher in the sky.  Has the size of the moon changed?  Does the size of the moon look different without the straw?

Speaking of straws, have you ever wondered if they would work in outer space?

Photo credit Tim McCord