Why does the moon look larger when it’s on the horizon?
When a full moon appears just above the horizon, it looks huge. This is a curious phenomenon, because when in this position the moon is furthest from us and should therefore appear slightly smaller than when it is at its zenith.
“In fact, you have to look far away from the Earth, so [the moon is] an Earth radius further than when directly overhead,” Suzanne Kohlerastronomer and spokesperson for the American Astronomical Society, told Live Science.
This mystery, nicknamed the “moon illusion,” has baffled skywatchers for millennia — and to this day, “we don’t really understand how it works,” Kohler said. Early explanations, including Aristotle’s ideas, attributed the illusion to magnifying properties of mist or the refraction of light in the atmosphere. However, photographs taken in modern times debunking this theory, showing how refraction makes the moon appear squashed rather than enlarged.
Instead, the moon illusion is likely “something that happens inside the brain” when we construct our perceptions of size, said Bart Borghuisneuroscientist at the University of Louisville who wrote a literature thesis on the topic during his undergraduate studies and is currently researching visual processing.
Researchers have proposed many explanations for how the size of the moon tricks our brains, Kohler noted. One idea is that when the Moon is closer to the horizon, it contrasts with 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” like the ocean, suggesting more factors are at play.
The theory supported by most evidenceand frequently cited in textbooks, focuses on the fallacy that we often use distance to perceive size. According to Borghuis, size perception is a “two-step process.” First, our retinas register the size of the object. And second, we judge its size by taking into account 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 a study published in the journal Science in 1962. The study found that when a simulated moon is shown at the end of a horizon, people perceive it as larger because the terrain makes it appear farther away. In contrast, when a moon is depicted without any relief, where it lacks indicators suggesting visual distance, the growing illusion disappears.
This is an observation “that has been repeated many times in psychophysical experiments”: a filled space is perceived as more extensive and longer than an empty space,” Borghuis told Live Science.
As a result, “most of us perceive the sky as sort of a flattened bowl,” Kohler said, even though the sky is technically a half-sphere. This is another way of thinking about the lunar illusion. With a flattened sky, when something is on the horizon, we think it is farther away than if it is above us, which makes us believe that an object of the same size is larger when it is lower in the sky.
This idea is similar to the basis of Ponzo Illusionin which lines of the same size appear to have different lengths because they are placed in different perspectives.
You can also see this illusion in action with an at-home experiment. If you stare at a bright object, like a light bulb, for a few seconds and then look at an empty wall, you’ll likely see a darker shadow, which should stay the same size no matter what you’re looking at. However, when you move from a far wall to a closer wall, 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 our brain’s limitations in estimating size. Regardless, “observing the moon in all its phases is always a great idea, because it’s really cool,” Kohler said. “But being able to also look at the cool side of brain science is a good thing.”
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