MYSTERIES OF THE BRAIN Part 23: What size is the moon?

Curious friends,

                Lets start with one of my favourite poems: “To the Moon” by Shelley

     Art thou pale for weariness

       Of climbing heaven, and gazing on the earth

       Wandering companionless

       Among the stars that have a different birth,-

       And ever-changing, like a joyless eye

       That finds no object worth its constancy?


                We have all noticed that the moon appears larger near the horizon and smaller when it is high in the sky. In this short VERITAS, I will talk about the reason for this phenomenon.

                The ancient Greeks had observed this and Aristotle had tried to explain this in terms of refraction and magnification caused by the Earth’s atmosphere. However the problem with the Greeks was that they never bothered experimenting. They would just make a theory and then assume that it is correct.

                In the 11th century the Arab/Persian scientist Ibn al-Haytham did a huge number of experiments on light and how we see. He wrote his findings in the “Book of Optics”( Kitab al-Manazir). Some scientists consider this one of the most influential books in the history of Physics along with Newton’s Principia Mathematica. This book set optics on a firm scientific foundation. The book studied reflection, refraction, visual perception, how the eye works. There is even a description of the pinhole camera! Al-Haytham even noted that the speed of light is finite and the speed changes in different mediums. He was also the first to apply geometry to the study of light forming the subject of geometrical optics. This book influenced a lot of research on light and its properties in renaissance Europe several centuries later.

                Ibn Al-Haytham also observed that the moon appears much bigger near the horizon and smaller high in the sky. He knew Aristotle’s theory also. Al-Haytham was a real scientist and experimented and immediately found that Aristotle was wrong. Using experiments and geometry Al-Haytham was able to show that if atmospheric refraction/magnification were to be taken into account, then the moon high in the sky would appear slightly larger than the moon at the horizon. However the moon at the horizon appears larger. So atmospheric refraction cannot explain this. Aristotle was wrong( as he has been on so many other occasions as well).

                Al-Haytham did many more experiments and geometrical calculations before coming to the conclusion that this phenomenon is actually an illusion! The brain is playing tricks on us. Al-Haytham was also the first to explain how or why this illusion actually occurs.

                You can yourself verify that this is an illusion by taking pictures of the moon at the horizon and high in the sky. They will have the same diameter!

                When we look at an object at a distance, it appears smaller but our brain knows that it distant and that is why it appears small. Our brain can judge the size of the object by judging its distance. That is how artists give an impression of distance on a flat canvas: as things move away, they appear smaller. This is called Linear Perspective. And in our minds the horizon is always a distant place. We see a mountain that appears so small at the horizon but our brain knows that it is much larger than it appears. Now the moon is at equal distance whether it is high in the sky or at the horizon. But our brain thinks that since it is at the horizon it is much farther away and it should be larger than it appears. So the brain compensates and makes the moon look bigger than it is.

                Note that the above is just one possible explanation of the moon illusion. There are more. Many more. There is a book on the Moon illusion in which 24 chapters are written by 24 different illusion researchers and they do not agree with each other. So this is not a closed topic!

                Another explanation is that when the moon is high in the sky there are no other objects to compare its size against. When it is at the horizon there are many things that we can compare against: the trees, the mountains, the houses and the image of the moon is bigger at the horizon than any of these objects. The human mind bases a lot of its perception on comparison. A thing placed next to larger thing appears smaller than it really is. However if the same object was placed with smaller things, it would appear bigger. The brain compares the size of the moon at the horizon with the distant and small objects at the horizon and gives us the impression that the moon is larger.

                There are more explanations as well. You can find them on the internet. Ibn Al-Haytham did a lot of experiments on illusions. In fact he was the first to show/suggest that the eye is just the place where the image is formed. The ultimate perception of seeing takes place in the brain.

                Now, you may ask, why is this still a open topic even a 1000 years after Al-Haytham first suggested that it is an illusion. Why are different theories of how this illusion works still around and there is no one explanation? The answer is simple: the illusion is not something that we can recreate outside the brain. The illusion is in the brain, caused by the brain. And what goes inside the brain is still an unsolved mystery. See the rest of the VERITAS articles on the mysteries of the brain at: VERITAS blog site(  Look at the “Mysteries of the Brain” tag).


Let me now end this VERITAS just as I started it: with a poem. This one is by Robert Frost:

I stole forth dimly in the dripping pause

Between two downpours to see what there was.

And a masked moon had spread down compass rays

To a cone mountain in the midnight haze,

As if the final estimate were hers,

And as it measured in her calipers,

The mountain stood exalted in its place.

So love will take between the hands a face….





Go wondrous creature, mount where science guides

go measure earth, weigh air, state the tides,

instruct the planets in what orbs to run

correct old time, regulate the sun


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