OK - it's not "Amore". Not in this post anyway. It could well be history's oldest puzzle in optics - even in all of science. And it could also be history's oldest such unresolved puzzle.
Consider this text from 2005, which I lifted from a page on the UNESCO Natural Sciences Portal under the title "The Miracle of Light".
Have you ever wondered why the moon looks bigger when it's low in the sky and close to the horizon?
This familiar observation, known as the '"moon illusion" or alternatively, the "horizon illusion", is arguably the oldest unsolved scientific puzzle today. It extends to the rising and setting sun, and also to star constellations. The ancients mistakenly attributed it to what they thought were certain magnifying properties of the atmosphere. And just a day or two ago, that's what I would have thought.
The moon illusion. This image has been edited to exaggerate the effect.
Credit: http://slapnose.com/archives/2005/06...oon_illusion/.
But is this really a physical effect? Is it optics?
Surprisingly, the answer is no. The moon illusion was correctly diagnosed by the illustrious "father of optics" Alhazen (965-1040) as an aspect of the psychology of human visual perception. It's not a property of the light that enters the human eye.
The illusion has been tested by comparing photos of the moon when it's on the horizon to photos when it's directly overhead.
Photography reveals that to the camera, the angular diameter of the moon is always about 0.5 degrees, regardless of whether it's on the horizon or directly overhead. In this time-lapse multiple exposure, the moon was tracked across the sky above Seattle at 150 second intervals. Credit: Shay Stephens.
Most people are surprised to find that the size of the moon as recorded on film is almost exactly the same from one photo to the next.
The moon illusion: animated "gif".
Credit: http://www.muskingum.edu/~psych/psycfair/moon.htm.
A mind-bending explanation for the moon illusion was given by Alhazen in his Book of Optics.
First, he proposed what is now called the SDIH (Size-Distance Invariance Hypothesis), explaining why an object would appear to be larger if it is perceived to be further away: An effect of visual processing in the brain.
Most present-day explanations of the moon illusion are based on some version of SDIH.
Second, Alhazen explained why the dome of the sky appears flattened; i.e. why stars near the horizon seem to be further away than stars that are directly overhead. Paradoxically however, most people say that the "big" moon on the horizon actually seems closer than the smaller looking moon overhead.
It's precisely this paradox which certain present day researchers are still trying to resolve.
Moving on to other sources ...
Archaeologists found clay tablets from the royal library at Nineveh, dating to the 7th century BC, with references to the moon illusion inscribed in cuneiform.
It was remarked by the ancients in what is now China and by Aristotle ca. 350 BC.
Ptolemy considered the problem ca. 150 AD.
Roger Bacon, Leonardo da Vinci, Johann Kepler, Rene Descartes, Marin Mersenne, Christiaan Huygens, Leonard Euler, Alexander von Humboldt, Hermann von Helmholtz and Thomas Huxley II wrote discourses on the moon illusion.
Maurice Hershenson authored a 400 page book The Moon Illusion, published in 1989.
Internet searches keying on "moon illusion" have retrieved more than 2000 online reports.
One of the better (and longer) essays on this subject was posted online by Donald E. Simanek as The Moon Illusion: An Unsolved Mystery. It was revised as recently as 2002.
In 2000, PNAS (Proceedings of the National Academy of Sciences) featured Explaining the moon illusion from a father and son team:Father-Son Scientists Confirm Why Horizon Moon Appears Larger is a summary of the PNAS report that includes an animated binocular fusion experiment that you can try. It's based on the IBM moon simulations.The Kaufmans ... designed two experiments to measure directly the perceived distance to the moon. Both tests used an apparatus built at IBM Research to project stereoscopic images of artificial moons from an IBM ThinkPad computer display to optical infinity so viewers could see them against an actual sky. Professor Kaufman then took people to a Long Island hilltop, where he made hundreds of measurements of their perceptions of the distance to the moon.
Binocular fusion experiment. See Father-Son Scientists Confirm Why Horizon Moon Appears Larger for viewing instructions. Select the stereogram (above) for a high-resolution JPEG image.
Helen Ross and Cornelis Plug coauthored a 250 page book The Mystery of the Moon Illusion, published in 2002. It's said they researched the illusion for their entire working lives and published many smaller papers on the subject.
In 2005, BBC News took up the issue, opening with:They went on to report:It can put a man in space and land a probe on Mars, but NASA can't explain why the moon appears bigger when it's on the horizon than when it's high overhead.This is illustrated with four diagrams that you can view in sequence.Two main theories dominate. The first, known as the Ponzo Illusion - named after Mario Ponzo who demonstrated it in 1913 - suggests that the mind judges the size of an object based on its background.
Ponzo drew two identical bars across a picture of railway tracks which converge as they recede into the distance. The upper bar appears wider because it appears to span the rails, as opposed to the lower bar, which sits between the rails. In the same way, with a low-lying moon, trees and buildings in the distance, which are familiar foreground reference points, set up a contrast that makes the moon, as we perceive it in our brain, appear larger than the image of the moon that is projected by our eyes onto our retinas.
Skeptics of this theory point to sea and air-based observers (ship and aircraft crews) who also see the illusion, even in the absence of ground reference points (like trees and buildings) that are visible to land-based viewers.
Alternatively, there's a theory that the human brain perceives the sky as a flattened dome rather than the exact hemisphere that actually describes it. We perceive things that we see immediately overhead - flying birds for example - as closer to us than birds on the horizon. Birds flying overhead are closer to us than birds on the horizon. When the moon is near the horizon, the human brain, conditioned by its memories of birds, miscalculates the moon's true distance and size; i.e. the brain compensates for the low-lying moon's seemingly greater distance by perceiving it as larger than the image that is received by the eyes.
In 2006, another moon illusion "crusader" criticized the Kaufman father-and-son report in PNAS (cited above) as too simplistic. Don McCready, Psychology Professor Emeritus at the University of Wisconsin (Whitewater), brought oculomotor micropsia and macropsia front and center in his explanation. His closing paragraph is remarkable:Roscoe and his colleagues have conducted many experiments which clearly show that the moon illusion illustrates oculomotor micropsia.
Roscoe's many publications have emphasized the largely overlooked role that oculomotor micropsia can have in aviation accidents when, to land the aircraft in low visibility conditions (weather), the pilot depends upon a "heads up" viewing device that shows an image of the landing strip ... This screen, viewed by the pilot at close range, induces oculomotor micropsia, so the pilot consequently "sees" the runway as further ahead than it truly is, and so may land (i.e. crash) beyond the runway (and some have).
A similar condition with the eyes unwittingly adjusting to a near distance can occur for an automobile driver with a wet windshield, especially at night, or when driving in fog. The resulting oculomotor micropsia can make objects in the road ahead look deceptively far away, so the driver tends to underestimate the safe braking distance, and may discover, too late, that an object was actually much closer than it appeared.
In other words, trying to understand the causes of the moon illusion is more than just an idle academic pursuit.
In a sidebar on this webpage, McCready suggests that oculomotor micropsia and macropsia were selected in one of our evolutionary ancestors ala Darwin because they offer survivability advantages in the natural environments that predated civilization. But the Internet blogger "slapnose" (Anthony Hecht) was more succinct in his assessment:Other, more "sciencey" explanations are around, but they're too long and contain words like oculomotor micropsia ...
Additional sources:
Archimedes Lab: The Moon Illusion
Did Ptolemy understand the moon illusion?
Explanations of the Moon Illusion
Moon illusion (Wikipedia)
NASA: Summer Moon Illusion
New Thoughts on Understanding the Moon Illusion from Carl J. Wenning
Perception: Alhazen's contribution to the Moon Illusion
The Moon Illusion: A Literature Thesis by Bart Borghuis.
The Moon Illusion: The Internet Version by Paul Niquette.
Why does the moon appear bigger near the horizon?
Bookmarks