Why Nobody Can Explain the 'Moon Illusion'

A full moon rises over San Francisco. (Photo: SF Brit/Flickr)

When this month's full moon arrives, it will perform an optical illusion that has baffled onlookers since Aristotle. As with many moonrises — but especially full moons — it will look bizarrely large when it's near the horizon, then seem to shrink as it ascends.

This is the "moon illusion," and it's all in your head. The moon isn't changing sizes, and while its distance from Earth does change slightly over time — producing an occasional "supermoon," which really does appear up to 14% larger than usual — that happens too slowly to yield such a dramatic transformation in one night.

Early attempts to explain the moon illusion blamed the atmosphere, assuming the moon's image is magnified by airborne dust near Earth's surface. Dust particles are known to affect the color of sunsets and sunrises, after all, and can even cast an orange hue on full moons. But scientists later realized atmospheric distortion isn't the culprit; if anything, suspended dust should make the moon look slightly smaller when it's low in the sky.

If you want proof the moon illusion is purely psychological, just hold a ruler up to the moon when it's near the horizon and again when it's high in the sky. The lower moon may have seemed significantly larger, but a ruler will reveal its diameter hasn't changed. Cameras can also expose the moon's mendacity: This multiple-exposure image, for example, tracks the rocky satellite's consistent size as it rises over Seattle.

So what's going on? When we look at the moon, rays of reflected sunlight produce a roughly 0.15 millimeter-wide image on our retinas. "High moons and low moons make the same-sized spot," NASA Science's Tony Phillips writes in an explainer about the moon illusion, "yet the brain insists one is bigger than the other."

Visual artists have long used perspective to portray 3-D space on a 2-D canvas, and psychologist Mario Ponzo showed a century ago how our brains can misjudge an object's true size based on its background. Known as the "Ponzo illusion," this is caused by our knowledge that distant objects appear smaller than they actually are. In the animated GIF at right, the upper yellow bar seems wider than the lower one because it's "farther away" on the 2-D railroad tracks, prompting our brains to compensate for an expected distortion. Much like a high and low moon, however, they're both the same width, as the vertical red lines illustrate.

Surface features like trees and buildings might mimic this effect with the moon, along with another trick called the "Ebbinghaus illusion," which can make objects seem artificially large by juxtaposing them with smaller objects. But there's a problem with those theories, too. Pilots and sailors often see the moon illusion even when the horizon is virtually empty, suggesting foreground objects alone don't produce the phenomenon.

flat sky

Plenty of other explanations have been floated over the years, including the "flattened sky" model (pictured at right) and a size illusion known as "oculomotor micropsia." Although many of these theories are plausible — and more than one may offer the answer — science has yet to fully explain the millennia-old mystery.

For an enlightening, animated overview of our efforts to understand the moon illusion, check out this new TED-Ed video by science educator Andrew Vanden Heuvel:

And to see footage of the moon illusion at work, check out this stirring moonrise video filmed in January 2013 by New Zealand photographer Mark Gee: