Moon Illusion: New Theory Reignites Debate
One of the classic optical illusions involves the Moon, which appears larger near the horizon than overhead. This illusion has been known and discussed for centuries and yet its explanation is still hotly contested. Today, the debate is set to reignite thanks to the work of Joseph Antonides and Toshiro Kubota at Susquehanna University in Pennsylvania.
Perhaps the most well known explanation is the Size-Contrast theory. Near the horizon, the moon is close to objects of a size that we know, such as trees, buildings and so on. And since it is comparable in size to these familiar objects, it appears larger.
Antonides and Kubota say there are two problems with this theory.
The first is that it does not explain the degree of expansion. Some observers report the moon appearing twice as large near the horizon and yet in experiments with the Ebbinghaus illusion, observers typically report an increase of only about 10 per cent. The second is that it does not explain why the effect disappears in photographs and videos. By contrast the Ebbinghaus illusion is easy to reproduce.
The new theory is based on the idea that the brain judges distance in two different ways. The first is with binocular vision. When the image from each eye is the same, an object must be distant. The second is our built-in model of the world in which we perceive the sky to be a certain finite distance away and the Sun, moon and stars to be in front of it (rather than appearing through a hole, for example).
This results in a contradiction. Our perceptual model of the world suggests the moon is closer than the sky while our binocular vision suggest it is not. Antonides and Kubota’s theory is that the illusion is the result of the way the brain handles this contradiction. “We hypothesize that the brain resolves this contradiction by distorting the visual projections of the moon resulting in an increase in angular size,” they say.
Image by ~Nate-Zeman