"The Moon Illusion"
A Brief Explanation !!!

This "moon illusion" is mind-baffling since the horizon and elevated moons are actually the same size and distance away from earthly viewers. Others who have written about the moon illusion over the years include such notable scientists as: Aristotle, Leonardo da Vinci, Johann Kepler and Rene Descartes.

A renewed interest in this subject has sparked new research in recent years. The debate is divided among many theories, two of which are: "The Apparent Distance" theory and the "Visual Angle Illusions" theory. This page deals only with the theory of "Apparent Distance". An excellent article on the "Visual Angle" theory can be found by visiting a site entitled "The Moon Illusion Explained" . Since this is a complicated and confusing subject I suggest that you read articles on all of the theories and then decide for yourself.

Many people think this is a real phenomenon, perhaps some sort of magnification caused by the atmosphere. The eye is tricked into measuring the moon against nearby objects such as buildings trees and hills. These types of objects, which are located on most horizons, create the illusion of increased size. The scientific term for this is called oculomotor macropsia. The fact is, this is strictly an optical illusion!

Illustration Of An Optical Illusion:
Proposed by Mario Ponzo in 1913.
Also called the Railway Lines Illusion.

The Ponzo perspective illusion dating from 1913 in which two same-length lines are drawn between or across a pair of converging lines resembling railroad tracks going off into the distance. The upper line appears much larger because it spans a greater apparent distance between the rails, which our mind assumes are parallel. Place your mouse over the buttons below to view this effect !!!

This "moon illusion" optical trick can best be demonstrated by performing a couple of simple experiments. All you will need is a pencil, a tube of some type and your hand. Wait until a night when a full or near full moon is expected. Try and catch the moon just as it clears the horizon and again a little later when it's high in the sky.
Experiment No. 1:
The celestial bodies are actually the same angular size at all times, whether they are near the horizon or high in the sky. The Sun and Moon are always degree in angular diameter. You can prove this for yourself! The eraser on a common #2 pencil is about degree when held at arm's length. If you extend a pencil while looking at the Moon, you can see that the eraser is about as wide as the Moon whether it's near the horizon or high in the sky! No one can prove for sure why we observe the Moon Illusion, but it is definitely a trick of the eye!
Experiment No. 2:
Viewing the moon through an aperture, such as pinching it between your thumb and forefinger or viewing it through a tube, which hides the the terrain leading up to the moon. The moon suddenly appears to be small because our brain locates it at the nearby distance of the edges of the aperture. Removing the aperture restores the terrain's distance cues and the moon springs back to its large, illusory size.
The debate continues on this age old question. Total acceptance by the scientific community of any one theory won't happen any time soon. So my suggestion is simply to enjoy this wonderful illusion. I hope this simple explanation, along with the two experiments and links, gives you a better understanding of what the "moon illusion" is all about. If you want a more in depth explanation then visit the links I've included below.
Moon Illusion Links:
- The Moon Illusion Explained
- Explaining The Moon Illusion
- An Article From Sandlot Science
- Links and Bibliographies
- Carl J. Wenning - Illinois State University
- Bad Astronomy
- More Moon Illusion Theory

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Copyright © 2000 By Keith Cooley