The Moon Illusion

29 07 2008

On our first night in the huts in Colorado, we watched an enormous full moon rise in the sky. It was a cloudless night, full of stars despite the bright moonlight. A couple of nights later, looking out the window of a different hut at the now waning moon, the three of us discussed what I now call the moon illusion (Thanks, Walt!). Using a saucepan lid as an example, Ray explained the theory having to do with light refraction, which Betsy understood easily and which only piqued my curiosity.

Basically, when the full moon is rising or setting, it appears larger than it does when viewed much higher in the sky. There are lots of theories about why this is, although everyone seems to agree that it is an optical illusion of some sort. Since the moon is actually one radius of the earth farther away when rising and setting, one theory is that the additional atmosphere between moon and observer causes light refraction and the illusion of a larger moon. However, physicists have shown that differential refraction would actually flatten the appearance, rather than magnifying it.

Another popular theory is that it’s all a matter of perspective, since we see the rising and setting moon relative to trees or buildings on the horizon. The Ponzo Illusion illustrates how our perception of object size is influenced by other objects in our field of view. This doesn’t seem to explain it though, since even people in airplanes too far above the earth’s surface to perceive objects on the horizon still report the illusion.

There are lots of theories about brain function, the shape of our eyeballs, etc., but the most popular explanation seems to be the sky bowl theory. This is sort of the opposite of the Ponzo Illusion, and holds that we perceive the sky at the horizon to be farther away than the sky above us.  The theory is that objects on the horizon actually make the sky seem farther away, while the absence of reference points above us make the sky seem closer. Our perception of the greater depth of the horizon sky tricks us into believing the moon is larger, even though it isn’t.

Have I put you to sleep yet? Well, it may not solve the problems of the day, but it gives you something to think about with the next full moon.




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