The Moon rises over mountains in Utah. Credit: NASA/Bill Dunford.
Why does the Moon look so large sometimes as it rises on the horizon?
Ryan Fraz
Buffalo, North Dakota
Ryan is referring to the so-called Moon illusion. Sometimes as the Moon just climbs above the horizon, it looks positively enormous. Some people see it, others don’t, and nobody knows the final answer on the Moon illusion. What causes it?Some may incorrectly assume that Earth’s atmosphere acts like some 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 more likely a trick of perception, an optical illusion. For some reason, the brain misinterprets the Moon as being larger when it is nearer to the horizon than when it is high in the sky. A well-known theory is called the point of reference 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 close to the horizon, we see it as next to familiar objects like trees and buildings, and we know deep down that trees and buildings are large objects. So the brain may misperceive the horizon Moon as big too.
Related: Full Moon calendar: When’s the next Full Moon?
In contrast, when we see the Moon in the open sky devoid of large objects to provide context and comparison, the brain may perceive it as relatively small. This explanation is just one of many. This is a controversial topic and entire books are written about this mysterious question.
Daniel Pendick
Associate editor
Editor’s note: This article was first published as an audio file in 2009. It has been transcribed and republished.
