WHY does the Moon seem so much bigger when it鈥檚 near the horizon than when it鈥檚 high in the sky? Although scientists have known since ancient times that this is an illusion, they have not agreed on an explanation. Now, an experiment with fake moons may have settled the debate.
The traditional explanation of the Moon illusion is that objects on the horizon give the brain points of reference that suggest the Moon is very distant. But when the Moon is high in the sky, the brain reacts to the lack of visual cues by thinking it is nearer. Because a distant object that covers the same area on the retina must be bigger, the brain concludes that the horizon Moon is huge.
But that theory fails to explain why the big low-lying Moon looks closer, not farther away. In the 1960s, psychologists explained this by suggesting that the eyes tend to focus on foreground objects when viewing a horizon Moon, but relax towards infinite focus when looking at the Moon in an empty sky. The brain interprets the relaxed focus as meaning that the Moon is very distant鈥攁nd because the most distant objects look small, perceives the Moon as being small too.
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To resolve the issue, Lloyd Kaufman, a psychologist at Long Island University in Brookville, New York, and his son James, a researcher at IBM鈥檚 Almaden Research Center in San Jose, California, took five volunteers up a Long Island hill with a broad view of the horizon. They stood their volunteers in front of a large semi-reflecting mirror through which they could see the horizon. In the mirror they could also see the reflected images of four fake moons from a projector.
When the volunteers focused through the mirror on the horizon, the four moons merged into two, creating two apparently three-dimensional moons. In one set of trials, the two moons appeared on the horizon; in the others, they were high in the sky.
By pressing a key, the volunteers could make one moon of the pair appear to move closer or farther away. The computer actually shifted one of the four projected images slightly sideways. This created an apparent change in distance without changing the size of the image on the retina.
For each trial, the volunteers adjusted the position of the movable moon until it appeared to be halfway between them and the fixed moon. It turned out that they placed a horizon moon 鈥渇arther away鈥� than an elevated moon to achieve this. On average, they took the horizon moons to be four times as distant as the elevated moons.
鈥淭hat means that the visual system is responding to the horizon moon as if it were much farther away than the elevated moon,鈥� says Kaufman, just as the older explanation of the illusion predicts.
So why does the real horizon Moon look closer? Aries Arditi, a vision researcher at Lighthouse International in New York City, says this stems from a logical thought process that occurs long after our unconscious mind has estimated the Moon鈥檚 distance, and hence its size. Because the Moon looks bigger, it must be closer. 鈥淲e can register apparent distance unconsciously in direct contradiction to our conscious experience,鈥� he says. 鈥淚t鈥檚 a very decisive and convincing experiment.鈥�