The Moon Illusion

Cecil's Columns/Staff Reports
1

Hello,

I’m confused by your explanation of the so-called Moon Illusion:

http://www.straightdope.com/classics/a2_110.html

My understanding has always been that the explanation you provide is merely one of a few theories, and that no reliable consensus has been reached by astronomers or anyone else. I would love your thoughts on the following.

NASA: http://www.nasa.gov/vision/universe/watchtheskies/20jun_moonillusion.html

"When you look at the moon, rays of moonlight converge and form an image about 0.15 mm wide in the back of your eye. High moons and low moons make the same sized spot. So why does your brain think one is bigger than the other? After all these years, scientists still aren’t sure why.

“…A similar illusion was discovered in 1913 by Mario Ponzo, who drew two identical bars across a pair of converging lines… The upper bar looks wider because it spans a greater apparent distance between the rails. This is the ‘Ponzo Illusion.’ Some researchers believe that the Moon Illusion is Ponzo’s Illusion, with trees and houses playing the role of Ponzo’s converging lines. Foreground objects trick your brain into thinking the moon is bigger than it really is… There are other explanations, too.”

BBC.com: BBC NEWS | UK | Magazine | Why does the moon look so big now?

“It is the world’s largest optical illusion, and one of its most enduring mysteries.
…Nasa can’t explain why the moon appears bigger when it’s on the horizon than when it’s high in the night sky. The mystery of the Moon Illusion, witnessed by millions of people this week [June 24 2005], has puzzled great thinkers for centuries. There have even been books devoted to the matter… But opinion differs on why there is such an apparent discrepancy in size between a moon on the horizon and one in the distant sky. Two main theories dominate. The first, known as the Ponzo Illusion…”

Wikipedia (contains a helpful link to The Straight Dope!): Moon illusion - Wikipedia

“After reviewing the many different explanations in their 2002 book The Mystery of the Moon Illusion, Ross and Plug conclude ‘No single theory has emerged victorious’ (p 180).”

Thanks for your time.

Rich

2

Hmm, the explanation Cecil offered about the sky being shaped like a flattened bowl, seems to be pretty much the same thing as the Ponzo illusion, and the other two explanations on the Wikipedia page. To me, they seem like three different ways of describing the same thing.

3

I suppose, and Wikipedia, for its part, says that perhaps the answer is some combination of the different theories.

What strikes me are the quotes “…why does your brain think one is bigger than the other? After all these years, scientists still aren’t sure why,” “Nasa can’t explain why the moon appears bigger when it’s on the horizon than when it’s high in the night sky,” and “No single theory has emerged victorious.”

To me, those are in direct opposition to The Straight Dope’s claim that their is one objectively correct explanation.