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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.

sgtgreeneusmc


quality posts: 5 Private Messages sgtgreeneusmc

Maria is latin for sea

srlagarto


quality posts: 11 Private Messages srlagarto
sgtgreeneusmc wrote:Maria is latin for sea



I thought Maria was my co-worker.

Waj


quality posts: 2 Private Messages Waj

Basalt could also be an answer, not really sure what the question is regarding though, is it the formations or the shape?

skispeakeasy


quality posts: 43 Private Messages skispeakeasy

My astronomy teacher in college said that he believes the illusion has something to do with the fluids in our heads.

The moon illusion can be replicated with an aspirin tablet. Hold the tablet above your head with your arm stretched out and look straight at it while you tilt your head and your arm. You can see the tablet grow and shrink before your eyes!

quantamm


quality posts: 85 Private Messages quantamm
skispeakeasy wrote:My astronomy teacher in college said that he believes the illusion has something to do with the fluids in our heads.

The moon illusion can be replicated with an aspirin tablet. Hold the tablet above your head with your arm stretched out and look straight at it while you tilt your head and your arm. You can see the tablet grow and shrink before your eyes!



Change 'aspirin' to 'LSD' and you begin to understand what your professor was really up to.

zxinfinity


quality posts: 18 Private Messages zxinfinity

I'd like to see a comparison of the moon sizes using photographs when it's up in the sky and when it's just above the horizon. Taking the human brain out of the equation, is it truly the same size?

This experiment would of course require the same settings on each image, i.e. focal length, aperture, etc.

e1hillma


quality posts: 0 Private Messages e1hillma

Phil Plait (a.k.a "The Bad Astronomer") has discussed/explained the "Moon Illusion" several times in his "Bad Astrinomy" blog. One of the more recent can be found here: http://blogs.discovermagazine.com/badastronomy/2011/11/14/in-which-i-disagree-with-cartoon-neil-tyson/.

e1hillma


quality posts: 0 Private Messages e1hillma
zxinfinity wrote:I'd like to see a comparison of the moon sizes using photographs when it's up in the sky and when it's just above the horizon. Taking the human brain out of the equation, is it truly the same size?

This experiment would of course require the same settings on each image, i.e. focal length, aperture, etc.



This should show what you want; it is a combined series of exposures of the moon rising over Seattle, WA. It was posted to the "Astronomy Picture of the Day" site on 11 Oct. 2003.
http://apod.nasa.gov/apod/ap031011.html

sgtgreeneusmc


quality posts: 5 Private Messages sgtgreeneusmc
zxinfinity wrote:I'd like to see a comparison of the moon sizes using photographs when it's up in the sky and when it's just above the horizon. Taking the human brain out of the equation, is it truly the same size?

This experiment would of course require the same settings on each image, i.e. focal length, aperture, etc.



Get an empty paper towel tube and look at the moon at different heights in the sky. Despite what it appears, it will always maintain the same ratio inside the tube.

imafungi


quality posts: 0 Private Messages imafungi
sgtgreeneusmc wrote:Get an empty paper towel tube and look at the moon at different heights in the sky. Despite what it appears, it will always maintain the same ratio inside the tube.



Bingo.

dmaz


quality posts: 12 Private Messages dmaz

Ken Jennings just made a summary of all the information found on the wiki here:
http://en.wikipedia.org/wiki/Moon_illusion

Right down to the numbers.

Also keep in mind that all of the explanations for why it's an illusion, Michael, are all based on hypothesis, as no scientist has been able to prove why it is an illusion, Michael.

dcigary


quality posts: 11 Private Messages dcigary

This one has bothered me since the first time I Googled it many years ago. I read the article, and shook my head, and read it again. This is one of those myths that I'd love to see de-debunked - the moon really IS bigger at the horizon!

tslothrop


quality posts: 12 Private Messages tslothrop

An interesting experiment during a full moon is to take out a quarter and show it to a friend. Hold it up to your eye, then extend it to arms length and back. Ask how far away they think they will have to hold the quarter to just cover the moon. Usually, people are surprised to find that their arms are not nearly long enough.

jcolag


quality posts: 8 Private Messages jcolag

Two issues come to mind.

First, illusion or not, it doesn't seem consistent at all, from night to night. Some nights the difference between moonrise and zenith appears to be a lot, sometimes almost nothing, right from my front steps. That's not to have an opinion either way, except to say that it makes a lot of evidence anecdotal on both sides of the argument.

Second, if all of us see the illusion at one time or another, then the Moon does, in fact, "look bigger" when it's near the horizon. That's what the phrase means, not that it necessarily takes up more of the sky. (It reminds me of a "science" book I read, years back, explaining that the sky isn't really blue, we just see it as blue because the other wavelengths aren't reflected to our eyes. Because our perception of photons apparently isn't how color works...)

Third (of two), if it's not an optical effect, then there's only one possible solution: Clearly, the Moon inflates itself during the day and rises through jet propulsion of air escaping out the bottom. Thus, as it rises, it looks smaller.

I wonder if it has more to do with the light being diffuse than "spread" in some way. A fuzzy boundary around a darker-looking body is going to look bigger against a darkening backdrop, and it'd be harder to pin down the "actual" boundary.

stinhoutx


quality posts: 11 Private Messages stinhoutx

Isn't there a similar effect with the sun? (Did I miss someone already mentioning that here?)

cccnoise


quality posts: 0 Private Messages cccnoise
sgtgreeneusmc wrote:Get an empty paper towel tube and look at the moon at different heights in the sky. Despite what it appears, it will always maintain the same ratio inside the tube.



This is great.