When the full moon peeks out just above the horizon, it appears huge. It’s a strange phenomenon because when it’s in this position, the Moon is farthest from us, so it should appear about a little smaller than when it’s beyond the zenith.
“Actually, you have to look across the distance of Earth, so [the moon is] one Earth radius farther than when directly overhead,” Susanna Kohlerastronomer and spokesman for the American Astronomical Society, told Live Science.
Dubbed the “moon illusion,” the mystery has puzzled sky watchers for millennia — and to this day, “we don’t fully understand how it works,” Kohler said. Early explanations, including ideas from Aristotle, blamed illusion magnifying properties of fog or refraction of light in the atmosphere. However, photographs taken in modern times disprove this theory, showing how the refraction instead makes the Moon appear squished rather than enlarged.
So instead, the moon illusion is likely “something that happens inside the brain” as we create our perception of size, he said Bart Borghuisneuroscientist at the University of Louisville, who wrote literary thesis on this topic during his undergraduate studies and is now researching visual processing.
Researchers have proposed many explanations for how the moon’s size tricks our brains, Kohler noted. One idea is that when the Moon is closer to the horizon, it stands in contrast to smaller objects, such as trees and buildings, on Earth’s surface. However, Kohler added that the moon still appears larger even on a “featureless plane” such as the ocean, suggesting more factors are at play.
Theory supported most evidenceand often cited in textbooks, focuses on the fallacies of how we often use distance to perceive size. According to Borghuis, the perception of size is a “two-step process”. First, our retinas register the size of the object. And second, we judge its size by its perceived distance from us, a principle of visual perception known as Emmert’s Law.
This principle is at play when it comes to the Moon, according to research published in the journal Science in 1962. A study found that when a simulated moon is shown at the end of the horizon, people perceive it as larger because the terrain appears to be further away. In contrast, when the moon is shown without any terrain, where visual distance indicators are absent, the magnifying illusion disappears.
This was an observation “that has been repeated many times in psychophysical experiments: A filled space is perceived as larger and longer than an empty space,” Borghuis told Live Science.
As a result, “most of us see the sky as a flattened bowl,” Kohler said, even though the sky is technically a half-sphere. This is another way to think about the moon illusion. With a flat sky, when something is on the horizon, we think it is further away than when it is above us, leading us to believe that an object of the same size is larger when it is lower in the sky.
This idea is similar to the foundation The Ponza Illusionin which lines of the same size appear to differ in length because they are placed in different perspectives.
You can also see this illusion in action through a home experiment. If you look at a bright object like a light bulb for a few seconds and then look at a blank wall, you’ll likely see a darker shadow that should stay the same size no matter what you’re looking at. However, when you switch from looking at the far wall to a closer one, you may notice that the size of the dot changes. “It’s the most illustrative little test or experiment you can do,” Borghuis said.
These illusions persist even though we know the limits of our brain in estimating size. Regardless, “checking the moon in all its phases is always a great idea because it’s really cool,” Kohler said. “But being able to look at the cool side of brain science at the same time is great.”
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