What a beautiful moon last night…

by Stephanie Chasteen on June 19, 2008

Edmund E. Kasaitis.

Photo credit: The full moon rising over Manchester, Maryland. Credit: Edmund E. Kasaitis.

I went hiking under the full moon last night, without even knowing that it was something special (other than a beautiful big pink full moon over the lights of Boulder). Last night was the solstice moon, as one of my fellow hikers tried to inform me. I say “try” because he was struggling to explain why it was that the moon looked abnormally large last night. It turns out that it’s just an exaggerated moon illusion. The moon illusion refers to the fact that the moon looks much larger at the horizon than it does overhead.

Back when I was studying psychology (gosh, in the early 1990’s, I feel *old*) the accepted explanation was that it was due to the Ponzo effect. That refers to the fact that we know that if something is far away, but still looks big, it must be *really* big. Like, if you see an ant lumbering on the horizon, but your depth cues tell you the ant is about a mile away, but it still looks big to you? Time to run, you’re in a bad B movie.

And when the moon is on the horizon, we have a lot of depth cues (like trees and such) that tell our brain the moon is really far away. And so since it still looks pretty big, our brains conclude that it must be really REALLY big — like the ant Godzilla above. And when the moon is overhead, there’s nothing to tell us that it’s far away, so we assume that (for its size) it’s just a puny regular sized moon.

But there’s a problem with that, since airline pilots still see the moon illusion when all they can see around them is clouds. Some people played around with atmospheric effects (there’s more atmosphere between you and the moon when you’re looking across the earth at the horizon instead of straight up into space) but that doesn’t explain it.

The more recent explanation, which I kind of like, says that we actually have different conceptions of the horizon versus the sky overhead. We think of the horizon as far away (as far as the eye can see!) and the sky overhead as being just about as tall as the clouds. So, it’s not so much that the depth cues at the horizon tell us that the moon is far away, but that we just have this erroneous impression that the sky overhead is really close by.

It’s amazing the ways our eyes and brains can trick us. That’s why it’s important to recognize how our perceptual systems work when trying to understand what we see, especially when trying to evaluate what seem like outlandish scientific claims (like faces on mars for example!)

Photo credit: The full moon rising over Manchester, Maryland. Credit: Edmund E. Kasaitis.

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