What is the moon illusion?
The moon illusion or moon effect has perplexed people since earliest historical times, at least as early as the 7th century BCE. It is described in early Chinese and Greek writings. Aristotle mentions it in 350 BCE.
The moon seems larger in angular size when it is near the horizon than when it is high in the sky. Some people judge it to be as much as twice as large, but the average estimate is 50% to 75% larger.
It’s not a small effect. The Sun displays the same illusion, but viewing the Sun directly with the eye is hazardous without proper precautions, so let’s leave that out of this discussion. The same illusion is observed with any extended object in the sky, such as a constellation.
Instead, the explanations commonly found in textbooks & the popular media, including all ‘moon illusion’ discussions on the internet, do not mention the basic angular size illusion. Such articles repeat the ancient idea that the horizon moon looks the same angular size as the zenith moon, but looks farther away so it logically looks a larger physical size. It often is said to look farther away due to a “flattened sky dome” illusion or due to changes in ‘cues to distance’ as illustrated by the Ponzo Illusion. These explanations require that the horizon moon must first look farther away. But, all researchers know that very few people see it that way.
