The Moon rises over the mountains of Utah. Credit: NASA/Bill Dunford.
Why does the Moon sometimes appear so large when it rises on the horizon?
Ryan Fraz
Buffalo, North Dakota
Ryan refers to the so-called Moon IllusionSometimes, when the Moon is just peeking over the horizon, it appears truly enormous. Some people see it, others don't, and no one knows the final answer about the Moon illusion. What causes it? Some may incorrectly assume that Earth's atmosphere acts as a sort of lens when the Moon is low in the sky and its light passes through a thicker lens of air. This is not true.
The Moon illusion is actually more of a trick of perception, an optical illusion. For some reason, the brain misinterprets the Moon as being larger when it is closer to the horizon than when it is high in the sky. One well-known theory is the so-called reference point or context illusion.
Basically, it says that the Moon looks bigger near the horizon because of the way the brain misinterprets the sizes of objects when they share the same field of view. When the Moon is near the horizon, we see it next to familiar objects like trees and buildings, and we know deep down that trees and buildings are big objects. So the brain can also misperceive the Moon on the horizon as being big.
Related: Full Moon Calendar: When is the next full moon?
In contrast, when we see the Moon in the open sky, with no large objects to provide context and comparison, the brain may perceive it as relatively small. This explanation is just one of many. It is a controversial topic and Entire books have been written about this mysterious question..
Daniel Pendick
Associated Editioneither
Editor's note: This article was first published as an audio file in 2009. It has been transcribed and republished.
