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Has the Moon created the world's most enduring optical illusion?

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Since the time of Aristotle, people have noticed that the moon looks a lot bigger just as it's rising than it does when it's high in the sky. We're now in the time of NASA, and we still don't exactly know what it is that causes a low moon to be a monster moon. We do have a few good ideas, though. Explore the enduring mystery of the Moon Illusion!

When I was a kid I used to get very annoyed when people told me that the moon on the horizon was the same size as the one high up in the sky. I had eyes! I could see the moon and it was far, far bigger sitting right on top of the roofs of houses than it was directly above us! It seemed to me that, when I compared the far-away moon to much-closer houses, it would seem smaller not bigger.


Well, it's been a while, and I've learned that eyes in general and my eyes in particular can be pretty stupid. But greater eyes, that were hooked up to greater minds, have also been fooled by the infamous Moon Illusion. In Aristotle's time they were so sure of the Moon Illusion that it was thought that the atmosphere created some kind of optical lensing effect and made it bigger when it was on the horizon. NASA, which has tackled this illusion too, has confirmed that there's no physical reason why the moon should look bigger on the horizon. No one has been able to conclusively prove why exactly the moon seems huge when it's just rising or setting but puny up in the sky, but there are a few theories.

The Ponzo Illusion


When I reasoned that, compared to the houses, the moon should look particularly teeny, I was getting it backwards. Twenty-nine-year-old psychologist Mario Ponzo let everyone know why in 1911, when he demonstrated a simple illusion. A woman lying on her back in a field turns her head and sees above the line of the grass a young male child walking nearby in a t-shirt and jeans. Fine. When the woman sits up, she gets a couple of visual clues and sees that the "child" figure is considerably farther away than she originally assumed. Immediately her mind kicks in and realizes that the "child" she saw is actually a full-grown man. He only appeared smaller because she assumed he was closer. That's not the illusion.

The Ponzo Illusion carries over from that. When we see two figures, one that appears to be close up, and one that appears to be farther away, we assume size difference. Like the woman in the field, we will see the farther-away one as bigger, because our brain understands that farther-away objects are bigger when we get up to them. Our brains, for whatever reason, go a little overboard on this, and have us perceive faraway objects as even bigger than they are. So the two yellow lines above, the one that appears to be farther away looks bigger. When the moon is close to the horizon, the objects in our visual field let us see it as far away, and we assume that it's bigger. When it's up in the sky, with no nearby objects to compare it to, it looks nearby and we assume it's smaller.

The Flattened Dome Illusion

The plain Ponzo Illusion sounds good, until people do experiments in which they blot out the horizon and people looking at a low moon will still see it as bigger than a high moon. Airline pilots also report seeing rising moons as bigger than zenith moons even if they can't effectively see the ground. What now?


Remember, the Ponzo Illusion isn't dependent on us seeing that objects are closer. It's just the way our brain reacts to thinking that objects are closer. We know that the "sky" isn't a physical thing, really. It's us staring out into a layer of gas and then out into space. The depth of both of those things is pretty much even everywhere you look. Still, people don't perceive it as an even distance. People see the sky as dome, but not a perfect dome. It's a wide, shallow dome, like an overturned mixing bowl. Our brains know the moon is floating in space, but our eyes interpret it as a paper circle sliding over this mixing bowl. Because the bowl is flattened directly above us, and wide at the edges, we think that moon directly above us is closer than the moon at the horizon, and see it as smaller.

Angles and Micropsia


As anyone who clicks the "optical illusions" tag on this blog knows, the eyes never get tired of playing us for suckers. It's like they know we haven't come up with suitable replacements for them yet, and so they can get away with anything. One of the ways they distort the world depends on how you focus them.

Turn towards a light fixture, or other distance object. Hold your finger up in front of your face and focus on the finger. You should see the object in the background. When you focus on the finger, the object will look a little smaller. Look at the object, and it will look a little wider. When we are looking up into the sky and seeing it as 180 degrees, the moon will take up a certain number of those degrees. Focusing close by will cause the moon to look smaller. Focusing into the far distance will make the moon look bigger.


When we look to the horizon, with houses and trees, our eyes adjust for far away viewing, and we see the moon as wide. If we look up into the sky, we have no visual cues, and our eyes adjust to only a few meters ahead of us. As a result, the moon looks shrimpy up there in the sky

Whatever the reason, I've never met anyone who thought the moon looked just as big on the horizon than it did high up in the sky. Is there anyone that's particularly clear-eyed reading today?


Top Image: Ed Yourdon

High Moon Image: Thomas Quine

Via PNAS, University of Wisconsin, and NASA.