When a full moon peeks over the horizon, it looks very large. This is an interesting phenomenon; in this position, the moon is furthest from us, so it should appear slightly smaller than when it is at its zenith.
“You actually have to look far beyond the earth, so [the moon is] It’s one Earth radius further away than when it’s directly overhead,” Susannah Kohler, an astronomer and spokesperson for the American Astronomical Society, told Live Science.
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The mystery, called the “moon illusion,” has puzzled sky-gazers for thousands of years, but to this day, “we still don’t fully understand how it works,” Kohler said. Early explanations, including ideas from Aristotle, attributed the illusion to the expanding properties of fog and the refraction of light in the atmosphere. However, modern photographs have debunked this theory, showing how the moon appears to be squashed rather than enlarged by refraction.
So instead, the moon illusion is probably “something that happens in our brains” as we build our perception of size, said neuroscientist Bart Borgeis of the University of Louisville. He wrote a paper on this topic as an undergraduate and is currently researching visual processing.
Kohler points out that researchers have proposed many explanations for how the moon’s size tricks our brains. One idea is that as the moon approaches the horizon, it contrasts with small objects on the Earth’s surface, such as trees and buildings. But Kohler added that even on a “featureless plane” like the ocean, the moon still appears large, suggesting more factors are at play.
This theory, supported by a large body of evidence and frequently cited in textbooks, focuses on a fallacy in how we use distance to perceive size. According to Borgeis, size perception is a “two-step process.” First, our retina records the size of objects. And second, we judge its size by considering its perceived distance from us, a principle of visual perception known as Emmert’s law.
According to a study published in Science in 1962, this principle is at work on the moon. The study found that when a simulated moon appears at the edge of the horizon, people perceive the moon to be larger because the topography makes it appear farther away. In contrast, when the Moon is displayed without terrain and lacks visual distance indicators, the growing illusion disappears.
This is an observation “repeated many times in psychophysical experiments” that filled spaces are perceived as longer and longer than empty spaces, Borgeis told Live Science.
As a result, “most of us perceive the sky as something like a flat bowl,” Kohler said, even though the sky is technically a hemisphere. This is another way to think about the moon illusion. A flat sky makes us think that something on the horizon is further away than when it’s overhead, making us think that objects of the same size are larger if they’re lower in the sky.
This idea is similar to the basis of the Ponzo illusion, in which lines of the same size appear to be of different lengths because they are placed in different perspectives.
You can also see this illusion in action through experiments at home. If you stare at a bright object, such as a light bulb, for a few seconds and then look at a blank wall, you may see a darker shadow, but it should remain the same size no matter what you look at. However, you may notice that the spot size changes when you switch from looking at a far wall to a near wall. “This is one of the most obvious little tests and experiments that humans can do,” Borgeis says.
Such illusions persist even when we are aware of the limitations of our brain size estimates. In any case, “it’s always a great idea to check all the phases of the moon, because it’s really amazing,” Kohler said. “But it’s great to be able to see the cool side of brain science at the same time.”
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