For millennia, a perplexing sight has materialized in the evening sky: Sometimes, a gigantic moon appears to hover near the horizon, but as that overgrown orb climbs overhead, it shrinks to a fraction of its moonrise size.

This striking phenomenon is known as the moon illusion, and the fact that it happens has been well documented since at least the fourth century B.C. Obviously the moon isn’t actually changing size—but the precise reason it seems to be expanding and contracting continues to puzzle scientists today. (See 11 striking pictures of November’s record-setting supermoon.)

Here are the various explanations for the moon illusion and its enduring mystery.

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Lunar Trickery

Back in the fourth century B.C., Greek philosopher Aristotle suggested that Earth’s atmosphere might be enlarging the image of the horizon moon, just as water can make immersed objects seem magnified to our eyes.

The Greco-Egyptian astronomer Ptolemy suggested something similar in his famous treatise Almagest, published during the second century A.D., as did Greek astronomer Cleomedes around the same time, though they both also ascribed the phenomenon to a change in the moon’s apparent distance.

However, we now know that Earth’s atmosphere does nothing of the sort. It may change the color of the moon, depending on how particles bend and filter moonlight, but that’s about all it does.

More recently, psychologists began to appreciate that the huge horizon moon is a true trick of the imagination—and it’s relatively easy to see for yourself that things are not as they seem.

Set a camera on a tripod and snap multiple images of the huge moon rising. When the moon is high in the sky, go back and compare the size of the moon’s disk in your photos. You’ll see that there is exactly zero difference.

Or, when the moon rises, roll a piece of paper so that when you look through the tube, the paper just hugs the humongous moon. Tape the rolled-up paper at that size and use it to view the rising moon. You’ll find that the moon never shrinks or expands inside that circle.

The moon always occupies roughly 0.52 angular degrees on the sky, or about the size of a thumb tip held at arm’s length. That changes by a minuscule amount between lunar cycles, with the moon’s apparent size getting up to 14 percent larger than normal during its closest approach to Earth. Still, the actual angular size of the lunar disk during a single moonrise will always be the same.

All in Your Head

In the 11th century, Arab mathematician Ibn Al-Haytham developed the first plausible theory for how the moon illusion works, suggesting that the size difference has to do with how our brains perceive distance, and then how we automatically adjust an object’s apparent size to match.

Al-Haytham suggested that when the moon is overhead, we perceive it to be closer and therefore smaller. But when the moon is rising over a distant horizon, we perceive it to be farther away and therefore larger.

One reason why the horizon might appear more distant than the sky overhead is that our brains perceive the shape of “space” as a gently flattened dome rather than a perfectly round sphere. That means we judge celestial objects that are overhead to be closer than celestial objects on the horizon.

In general, humans are terrible at estimating vertical distance; next time you’re gazing at a mountainous ridge, try and guess how high it rises above you.

View Images The Ebbinghaus illusion is one optical effect that might help explain the moon illusion. In this illustration, the blue circles are exactly the same size, but the ones on the right appear larger than those on the left. Illustration by Peter Hermes Furian, Alamy

“The idea of the perceived flattened dome of the sky is more promising, based on the apparent shape of the sky rather than the apparent distance to the moon,” says University of Oxford psychologist Brian Rogers, who experiments with the moon illusion in a planetarium.

Another possibility has to do with the way surrounding visual cues can trick our brains. This idea is best illustrated by the Ponzo illusion, in which two identical objects appear to be vastly different sizes based on the visual cues provided by their surroundings.

Trouble is, according to the apparent distance hypothesis, which psychologist Don McCready calls “popular but inadequate,” the horizon moon should appear to be both larger and farther away. But when people are asked how far away they think the moon is, they say the horizon moon appears larger and closer.

So, that doesn’t exactly work.

Universe of Theories

Another idea, called the relative size hypothesis, suggests that the illusion’s roots are in the sizes of objects in the foreground terrain, and that without houses, mountains, or trees, the lunar disk would not appear as large. This particular idea is similar to the well-known Ebbinghaus illusion, which demonstrates that two identical objects can appear radically different in size depending on what surrounds them.

A more recent idea even suggests that binocular vision could be the culprit, with our brains trying to compensate for the moon’s perceived position in front of a flat, distant sky by distorting the size of the moon.

The truth is, no one yet agrees about what’s going on.

“Although there have been written books and hundreds of articles on the topic, the issue is not fully resolved. Some factors are clear, other not,” says psychologist Claus-Christian Carbon of Germany’s University of Bamberg, who also studies the moon illusion using planetariums.