The Moon (or Sun) Illusion

© 1997 by Billy E. Pennal, Ph.D.

Have you ever wondered why the full moon (or sun) looks so much larger when it is on the horizon than when it is overhead? I used to wonder about it a lot until I learned that some psychologists have researched the subject and have provided the answer. Perception is one of the subjects of Psychology and a lot of research has been done to understand how we perceive things. Perceptual psychologists are the ones to thank for this bit of understanding.

There are other theories to explain this illusion, but I like this one best. If you would like to read more about other theories, click here.

People have thought the earth's atmosphere acted as a magnifying glass to enlarge the image of the full moon when it is on the horizon. Measurements of the size of the image the moon casts at different places in the sky show that this is not so. The image the full moon casts on the retina is always the same size regardless of the moon's location in the sky, so that can't be it.

The answer involves the marvelous thing we all carry around inside our heads called the brain. Our brains cannot tolerate things that don't make sense to us. The moon illusion involves some apparent inconsistencies that our brains adjust to provide us with reasonable information.

When we look up into the sky, we see an apparent dome rather than empty space. It looks like we are inside a huge bowl and are looking up at the inside of the bottom of the bowl. Now, we know this is not true, but that is what we tend to see. It has been discovered by measurement that the apparent distance from the edge of the bowl (the horizon) is further away than the part of the bowl that is directly overhead. This is probably due to the things on the earth that we also see when we look toward the horizon.

When the full moon is near the horizon, therefore, we tend to see it as being further away than when it is overhead because it looks like it is on the inside surface of the big bowl. Of course it really isn't further away, it only appears to be. Since the moon casts exactly the same size image on our retinas no matter where it is located, we have an inconsistency to resolve. How can it be further away and still cast the same size image? The answer our brain gives us is that it is larger when it is on the horizon. If it casts the same size image when it is further away it must be larger; therefore, it appears to be larger, and fools us all.

I'll bet you never thought about it this way. You can use this at any party for inconsequential conversation anytime you wish.

In response to a reader's question, actually we see all kinds of objects in the same manner. The context in which the object appears influences our perception of the object. If we see a cow, or people, or anything we know about, there will be some distortion if that object is seen in an unusual context. If we could place a cow or people on the underside of an apparent dome that changed apparent distance as does the moon illusion, we would perceive them as distorted also. Having personal knowledge of the size of a cow or people will also be a factor. If we see a cow in some situation that is not normal we will alter our perceptions to fit the new context. We may even believe the cow is a midget or giant depending on the context. There is a current television commercial which shows a car in the distance with a boulder in the foreground. The bolder changes size to a dirt clod immediately when a car wheel drives over it and crushes it. That brought the context to a different distance for the boulder/clod and a different interpretation of it.

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