What is “moon illusion” mystery ?

Have you ever noticed the moon at horizon looks much bigger than moon at zenith ? If your answer is yes , then you are not alone . Most of people have seen such illusion . But why is it illusion ?  Actually moon mustn’t look bigger at horizon that what appears at zenith because horizon moon is much farther to observer’s eyes than moon at zenith .

Moon looks bigger at horizon

observers who see moon at horizon perceive that moon is much nearer to him but it is contradiction of  what is reality .  Moon illusion is very famous and long debated problem among scientists . People from centuries tried to find explanation of such Nature’s illusion . It is known from 2000 years . Many explanation had been given in old text books of astronomy , although all explanation are wrong ( we will see later ). Scientists say it is just celestial illusion that perceive  by human beings and our mind plays tricks  . This illusion is not associated with moon only but many astronomers claim that  same illusion takes place with our Sun and constellation . It has been calculated that moon at horizon looks bigger than moon at zenith by factor 2 . It is much differences . Let’s see what did historical philosophers and scientists says about moon illusion .

Ancient philosophers’ arguments about moon illusion 

Ancient Greek philosopher and scientist Aristotle attempted to find explanation of this illusion in around 4 BC .He argued that it might be because of atmospheric refraction , light coming from horizon moon has to travel long distance via Earth’s atmosphere and hence it magnifies Moon  . But Aristotle was wrong  because as we know that Earth’s atmosphere neither magnifies nor diminishes the celestial object rather it only changes wavelength of light that enters into observer’s eyes . That’s why moon which just rises looks reddish  because when moon lies at horizon , light has to travel through greater thickness of atmosphere so reddish colour of moon at horizon is not illusion but it is a physical true phenomenon .

Ptolemy was also curious to solve this illusion . Claudius Ptolemy was well know astronomer of city of Alexandria . Ptolemy was interested in Aristotle’s argument . He used various ways to explain this illusion such as atmospheric refraction , optical illusion etc. According to him , moon looks larger at horizon than moon at zenith because between observers’ eyes and moon , lights have to travel long way through terrestrial objects filled between it .  But his argument was totally invalid according to current explanation of atmospheric properties . Physics professor Edwin C. Boring rejects his theory that was believed to be true . Boring states , “…..yet the argument is wrong for simple reason that moon in elevation looks farther away than moon on horizon – to all who have observed the moon much and thought about matter. If asked away , they say , ‘the moon in elevation ;of course , because it looks farther away – thus investing Ptolemy’s logic “.

Greek astronomer Cleomedes tried to explain this illusion in his astronomy book around 200 AD . He used the argument that moon looks bigger because it is at farther distance. He used the  statement of size – distance invariance. He is the earliest extant author to offer apparent distance as a clear explanation of the celestial illusion.

Does it all  our mind’s trick ? 

Well, scientist as well as psychologist are in same arena to find the answer and give satisfactory explanation of moon illusion . Let me explain plausible explanation proposed by scientists .

The Ponzo  illusion is kind of optical illusion discovered by by Mario Ponzo in 1911 .Scientists say that Ponzo illusion may be strong logic behind moon illusion mystery at much extent . Ponzo suggested that human mind judge the size of object by taking surrounding objects as reference .

This picture shows Ponzo illusion . Both moon which is of same size , but one looks bigger and other smaller . credit :- NASA

This figure is depicting moon illusion shows everything occurs in mind
This picture shows Ponzo illusion . Both moon which is of same size , but one looks bigger and other smaller . credit :- NASA
It is to be  see in above  picture that  moon at farther distance  looks bigger than nearer one although both are at same size . This is because our mind is forced to judge the size of moon according to size of track which gets converge at farther distance .

“Ebbinghaus illusion” is another explanation that might help to solve the moon illusion problem.

Ebbinghaus illusion : both middle orange  circles are identical in size but circle which is in left ,is surrounded by bigger circle hence it looks comparatively smaller than circle (right one ) which is surrounded by smaller circle .

Despite of many explanation , none of them gives satisfactory explanation of moon illusion and it is mystery .


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I am science enthusiast and I love to write blogs for students and youths who love science. I have deep interest in astronomy and cosmology. I believe that science is most effective tool to serve humankind. .