QUESTION: Please explain why the sun and moon appear larger when they are near the horizon than when they are overhead. This appears to be an optical illusion since they subtend the same angle regardless of position.
– Bob Hahn,
Fallbrook
ANSWER: The Greek philosopher Aristotle about 350 B.C. ascribed the enlarged appearance of the horizon moon to magnification by the atmosphere. To this day, atmospheric magnification remains a popular, albeit incorrect, explanation.
Bending of light by the Earth's atmosphere does affect the way we see celestial bodies on the horizon. Bending of light can make the setting (or rising) sun or moon appear oval, because light from the bottom of the setting sun is bent through more of the atmosphere than light from the top of the sun. This slightly squashes the sun or moon vertically without changing its horizontal dimension – so this fails to explain the enlargement.
Scientists now agree that the enlargement is an illusion, but they are still arguing about how our perceptual systems play this trick on us. You can convince yourself that it is an illusion by measuring the raised and horizon moon with calipers or a ruler held at arm's length. For most people, viewing the horizon moon upside down also eliminates the illusion.
The authors of the book, “The Mystery of the Moon Illusion,” conclude that the illusion is probably the combined effect of several factors, including terrain.
On the horizon we mentally compare the size of the moon with trees and other landscape features. Experiments to block out or manipulate the appearance of the terrain using mirrors can change the perceived enlargement by 26 percent to 66 percent. However, the moon illusion can also occur when the horizon is featureless, such as over the ocean.
Simply looking upward by raising the eyes or tilting the head reduces the perceived size of artificial moons in experiments. Why our visual systems do this is uncertain, but it may cause the overhead moon to appear up to 10 percent smaller than the horizon moon.
Another factor that may contribute to the illusion is the difference in color between the horizon and overhead moon caused by the atmosphere.
“The Mystery of the Moon Illusion” rules out several explanations for the enlargement, including a popular one that it is because we perceive the sky as a flattened dome. The authors conclude that the sky does not always appear flattened, and that explaining one illusion with another illusion is dubious.
So after more than two millenniums of arguments and experiments, the moon illusion still retains some secrets.
Sherry Seethaler has a doctorate in science education and is a UCSD science writer and educator. Send your scientific questions to her at Quest, San Diego Union-Tribune, P.O. Box 120191, San Diego, CA 92112-0191. Or e-mail sseethaler@ucsd.edu. Please include your name, city of residence and phone number.
