The Debunker: Is The Moon Bigger Nearer The Horizon?

by Ken Jennings

In honor of the late Neil Armstrong and the Apollo astronauts, we’ve asked Jeopardy! champ Ken Jennings to run down some common misconceptions that lots of people still believe about the moon. Did you know it’s not even made out of green cheese? Science ruins everything.

Moon Myth #3: The Moon Is Larger In The Sky When It's Nearer The Horizon.

This impression is so convincing that it can be hard to convince people that it is, in fact, a psychological phenomenon and not an astronomical one. Don’t feel bad: even Aristotle was so suckered by the illusion that he thought it must be caused by atmospheric effects or something. But no: if we go by physical factors alone (refraction and actual distance), the Moon actually appears 1.5 percent smaller when it’s rising or setting! So why does it look so much bigger?

No one’s quite sure. The “Moon illusion,” as it’s called in the literature, has been batted around since the times of the Greeks and Romans, and the reasons for it appear to be so complex and intermingled that they’re hard to summarize. It’s logical to assume that it might be a contextual effect: the moon on the horizon is dwarfing things like distant trees and houses, so it looks big by comparison, but when it’s high in the sky, surrounded by empty space, it looks small. But that doesn’t seem to be true: we know that the moon illusion tends to persist even at sea or on an empty plain, where there are no reference objects at all, and yet it disappears when people look at a horizon moon upside down, even if the same nearby objects are present. Something a bit more spatially complicated is evidently going on.

The root of the problem seems to be the fact that the human brain is really, really easy to fool when it comes to things like size and distance, as you know if you remember the optical illusions that tend to show up in kiddie puzzle pages: lines that are the same length but look very different, that kind of thing. We also have a weird understanding of how the night sky works: distance cues may give our visual cortex the mistaken idea that the sky is a flat “ceiling” above us, not (essentially) the inside of a sphere. As a result, we have a hard time comparing sizes between horizon objects and overhead ones. Psychologists have yet to nail down how this perception creates the “Moon illusion,” but it appears to be at the heart of the problem. But rest assured that it is an illusion—the Moon’s not shrinking and growing every evening just for your amusement. You just think it is.

Quick Quiz:The lunar illusion of a “Man in the Moon” face is caused by dark volcanic maria on the Moon’s surface. What word do we use for these maria in English?

Ken Jennings is the author of Brainiac, Ken Jennings's Trivia Almanac, and Maphead. He's also the proud owner of an underwhelming Bag o' Crap. Follow him at ken-jennings.com or on Twitter as @KenJennings.

Photo by Flickr member the bridge. Used under a Creative Commons License.