Why does the moon look larger when it's on the horizon?

When a full moon peeks just above the horizon, it appears enormous. This is a curious phenomenon, as when it is at this position, the moon is the furthest away from us and so should appear about slightly smaller than when it is at its zenith.

"You actually have to look across the distance of the Earth, so [the moon is] one Earth radius further away than when it's directly overhead," Susanna Kohler, an astronomer and spokesperson for the American Astronomical Society, told Live Science.

So why does the moon look larger when it's on the horizon?

This mystery, dubbed the "moon illusion," has baffled skywatchers for millennia — and to this day, "we don't fully understand how it works," Kohler said. Early explanations, including ideas from Aristotle, blamed the illusion on the magnifying properties of mist or the refraction of light in the atmosphere. However, photographs taken in modern times debunk this theory, showing how refraction instead makes the moon look squished rather than enlarged.

So instead, the moon illusion is probably "something that happens inside the brain" when we construct our perceptions of size, said Bart Borghuis, a neuroscientist at the University of Louisville who wrote a literature thesis on the subject during his undergraduate studies and now researches visual processing.

Researchers have proposed many explanations for how the moon size tricks our brains, Kohler noted. One idea is that when the moon is closer to the horizon, it is held in contrast with the 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, which suggests there are more factors at play.

The theory backed by the most evidence, and frequently cited in textbooks, focuses on the fallacies of how we often use distance to perceive size. According to Borghuis, size perception is a "two-step process." First, our retinas record the size of the object. And second, we judge its size by accounting for its perceived distance away from us, a visual perception principle 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. The study found that when a simulated moon is shown at the end of a horizon, people perceive it to be larger because the terrain makes it seem like it is farther away. In contrast, when a moon is shown without any terrain, where it lacks indicators suggesting visual distance, the augmenting illusion disappears.

This was an observation "that was repeated many times in psychophysics experiments": A filled-in space is perceived to be more extended and longer than an empty space," Borghuis told Live Science.

As a result, "most of us perceive the sky as kind of 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 flattened sky, when something is on the horizon, we think it is farther away than if it's overhead, which tricks us into thinking that a same-sized object is larger when it's lower in the sky.

This idea is similar to the basis of the Ponzo illusion, in which the same-sized lines appear to differ in length because they are placed at different perspectives.

You can also see this illusion in action through an at-home experiment. If you stare 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, which should stay the same size no matter what you look at. However, as you switch from looking at a faraway wall to a closer one, you may notice that the size of the spot changes. "It's the most illustrating little test or experiment that you can do," Borghuis said.

These illusions persist even if we know our brain's limits in estimating size. Regardless, "checking out the moon in all of 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 at the same time is neat."

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