|
We grieve to report that there is no single satisfactory explanation for this storied phenomenon, otherwise known as the "moon illusion." The apparent size difference in the moon was noted in the 7th century B.C. by Chinese and Greek astronomers, and has been puzzling heavy intellectual hitters ever since: Aristotle, Ptolemy, Roger Bacon, Leonardo da Vinci, Johannes Kepler, Rene Descartes, and Karl Gauss. As of last count, there are at least eight competing theories and one whole book devoted to the moon illusion. What do we know as fact? The moon illusion is a psychological, not a physical phenomenon. The moon always subtends (i.e., takes up) an angle of 0.5 degrees in the sky, so something must be going on between our retina and our cortex. The more popular theories have involved
comparisons with buildings and other objects on the horizon, differences between apparent size when looking horizontally and looking vertically, and the physical position of eyeballs in the cranium. The first theory seems to have the most popular support, even though the moon illusion occurs on water as well as land. As you might imagine, there are several scientific web sites devoted to the phenomenon. Donald Simanek's overview contains the interesting observation that if you compare pictures of the moon at various stages in the sky, the moon looks exactly the same size. Carl Wenning's page features some other fun optical illusions like the famous "Ponzo Illusion"
involving the apparent convergence of parallel lines. And Hendrick Ball presents a heroically simple (if probably incomplete) explanation.
|
