By Vince Condella Published May 21, 2003 at 5:10 AM

Nobody likes to be told they are "seeing things." It's just your imagination. No, we don't like to hear that. We tend to believe what we see. Our eyes seldom play tricks on us. At least that's what we think. But a common illusion proves otherwise. See what you think.

I'm referring to the so-called "moon illusion." You know this one pretty well. The moon looks huge when it rises or sets near the horizon, yet looks "normal" size when it is directly overhead. This has puzzled sky watchers for thousands of years. There is a popular theory that claims the size change near the horizon is due to atmospheric refraction, or the bending of light due to the moon shining through the thicker atmosphere near the horizon. In reality, the refraction causes the moon (and the sun) to look flattened on top and bottom, sort of a "squished" appearance. This would cause a smaller looking moon, not larger.

Over the years the optical illusion theory seems to win out. But which theory? One claims that the horizon moon appears huge because visual cues in the foreground landscape make the moon seem far away. The other theory claims the same cues make the moon seem closer. A father-son research team recently performed tests to explain the moon illusion. Lloyd Kaufman is a professor emeritus of psychology and neural science at New York University. His son James is a physicist at an IBM research center in San Jose, California. They claim that the illusion results because sky watchers judge a horizon moon to be much farther away than an overhead moon. The brain exaggerates the perceived size of the moon because the brain is thinking that the moon is so far away that it must be really huge to take up so much space in the sky.

Try this experiment to see if the moon changes size for you. Take a small tube (like the kind that remains after the paper towels run out) and view the horizon moon through the tube. View it with one eye looking through the tube and the other eye closed. Once you take the horizon and any intervening buildings or trees out of view, the moon appears normal size through the tube. Now open the other eye. The moon appears one size with the eye viewing through the tube and a larger size with the eye viewing it without the tube. How strange!


Another way to view this moon illusion is the Ponzo Illusion, first demonstrated by Mario Ponzo in 1913. Draw two lines that start wide at the bottom of the page but converge together at the top of the page. Now draw horizontal lines crossing the two lines. You have created the "railroad tracks" going off into the distance. Now draw two thick lines of equal length, one near the bottom of the page where the "railroad tracks" are wide, and the other near the top of the page where the tracks are narrow. Even though these new lines you have drawn are the same length, the one at the narrow part of the tracks appears longer. Our eyes perceive this line to be farther away, farther down the track, then the line that is at the bottom of the page.

Another way to look at this moon illusion is to realize that our minds perceive the sky as a flattened dome. The dome appears closer over our heads than it does near the horizon. After all, we see clouds and airplanes passing nearby overhead, but they travel off into the distance, far from us, when they reach the horizon. The moon is beyond our atmosphere, of course, and at a great distance, so it doesn't change size from horizon to zenith. Yet our minds think the moon is a flattened disc traveling across the dome of the sky. When the moon is near the horizon, our minds think it is far away even though its size doesn't decrease. Therefore it must be much larger than when it is directly overhead (when we think of it as being closer). In our mind's eye, the horizon moon is larger.

Has all of this illusion talk left you disillusioned? Don't despair about optical illusions. The next time the moon rises in the east (or the sun, too), just enjoy the awesome sight and the beauty of nature.