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A full moon seems massive when it appears just over the horizon. This occurrence is peculiar, since in this position, the moon is at its most distant from us and thus should look a bit tinier compared to when it is at its peak.
“You essentially need to gaze across the Earth’s width, therefore [the moon is] a single Earth radius more distant than when it is positioned straight up,” Susanna Kohler, a specialist in astronomy and representative for the American Astronomical Society, mentioned to Live Science.
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This puzzle, known as the “moon illusion,” has bewildered stargazers for ages — and still today, “we don’t totally grasp its mechanism,” Kohler explained. Earlier justifications, including viewpoints from Aristotle, linked the visual trick to the amplifying qualities of fog or the bending of light within the environment. Yet, recent photographic evidence challenges this notion, revealing how refraction instead causes the moon to seem compressed as opposed to magnified.
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So instead, the moon illusion is likely “something occurring in the mind” as we piece together our size awareness, stated Bart Borghuis, a neuroscientist at the University of Louisville who composed a thesis on the subject throughout his undergraduate studies and presently studies visual processing.
Researchers have suggested multiple descriptions for how the moon’s dimension fools our brains, Kohler indicated. One concept is that the moon, nearer to the horizon, contrasts against the smaller objects on Earth’s face, such as trees and structures. Nevertheless, Kohler noted that the moon still looks more substantial even on a “featureless area,” such as the ocean, implying more contributing factors exist.
The theory with the most backup, often referenced in textbooks, emphasizes errors in how we commonly employ range to judge size. Size awareness, according to Borghuis, is a “two-part process.” Initially, our retinas document the object’s size. Secondly, we gauge its dimension by considering its perceived distance, a visual perception rule recognized as Emmert’s Law.
As stated by a study in the journal Science from 1962, this principle is applicable regarding the moon. The research determined that when a simulated moon is displayed at the end of a horizon, viewers perceive it as larger because the landscape gives the impression that it is more remote. Conversely, the illusion disappears when a moon appears devoid of terrain, lacking visual range indications.
The Ponzo illusion arises when identically sized lines seem different in length depending on their placement across different perspectives.
This was a finding “duplicated many times throughout psychophysics research”: A space that is filled in is understood to be broader and more extended than empty space,” Borghuis mentioned to Live Science.
Consequently, “most of us see the sky like a somewhat crushed bowl,” Kohler specified, even though the sky is technically a half sphere. This is another way of interpreting the moon illusion. Because the sky seems flattened, items on the horizon are thought to be more distant than what is directly overhead, leading us to mistakenly believe identically sized objects lower in the sky are larger.
This theory shares similarities with the framework of the Ponzo illusion, where identical lines appear of different lengths due to differing perspectives.
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Furthermore, you can experience this illusion firsthand using a home experiment. Stare at a radiant object, like a bulb, briefly, then shift your gaze to a plain wall. You are likely to detect a darker shadow, which should remain the same irrespective of what you see. However, switching from a remote wall to a nearby one might alter the spot’s size. “It’s the most telling miniature test or study one can do,” Borghuis claimed.
These optical tricks remain compelling even when conscious of our minds’ limits in size estimation. Nevertheless, “looking at the moon throughout its various phases is an outstanding practice, because it is genuinely fascinating,” Kohler said. “Being able to scrutinize the brain science involved simultaneously is also very cool.”
Moon quiz: What is your lunar knowledge?TOPICSLife’s Little Mysteries
Alice SunLive Science Contributor
Alice Sun is a science journalist based in Brooklyn. She covers a wide spectrum of topics, including ecology, neuroscience, social science and technology. Her published works have appeared in Audubon, Sierra, Inverse and others. She studied environmental biology at McGill University in Canada for her undergraduate studies. She also possesses a master’s degree in science, health and environmental journalism from NYU.
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