The Moon (Surprisingly) Doesn't Change Size

People tend to forget how much the brain manipulates vision. What one sees is not objective but rather a product of light hitting the pupil and being processed by the occipital lobe—a part of the brain near the back that is used for visual processing. This means that tricking the brain with stimuli can create various optical illusions, such as the Ponzo illusion, where objects seem distorted by other objects in an image. 

One of the most obscure examples of this can be found in the night sky. When people look at the moon on the horizon, it appears much larger than when viewed at the zenith—the point directly above them. This has been known since ancient times like when famous philosopher and scientist Aristotle and “the father of modern optics” Ibn al-Haytham observed this effect and mused over a possible solution.

Aristotle believed that this was the work of Earth’s atmosphere. He figured that the moon’s light was traveling through the atmosphere, and since it was on the horizon, it was traveling through more atmosphere than it would if directly overhead. He thought this caused a kind of lensing effect, such as that created by a magnifying glass, to make the moon look larger. However, this was disproved. The moon still seems larger when over mountains or tall buildings than at the zenith, meaning that the amount of atmosphere cannot be at fault.

Al-Haytham believed that when the moon is at the zenith, we perceive it to be closer and therefore relatively. For a constant visual size, closer objects would be smaller relative to farther objects. This is part of the case but not the entire reason.

In 1813, German philosopher Schopenhauer wrote that the moon illusion was “purely intellectual or cerebral and not optical or sensuous,” meaning the brain is entirely responsible for the effect. This helped other scientists orient their theories towards psychology and neuroscience rather than atmospheric science. 

A second hypothesis, formed in the late 1900s by multiple different scientists, was that foreground objects were responsible. Scientists hypothesized that objects in front of the moon, such as trees and buildings, made it seem bigger in comparison. However, according to NASA, astronauts also observe this effect from space, and it still happens over an empty horizon. This was a predicted result based on other theories at the time, such as Al-Haytham’s, because astronauts also see the horizon line as much closer than the far-away moon and assume it to be larger.

The current consensus among scientists is that the so-called “moon effect” is caused by multiple different phenomena. The main cause is the brain’s susceptibility to trickery through optical illusions. In this case, the Ponzo illusion is responsible. The most common example of this is two lines converging, seemingly going into the distance. When two bars of the same size are overlaid on these lines, the bar above seems longer because the tracks are shorter. Another example is that of two circles a few inches apart and two lines tangent to one circle. That circle will appear bigger due to the brain’s attempts to normalize the image and understand depth, even when it isn’t there. The key to this illusion is the fact that the two lines are the same length. The brain will perceive the line above as farther away, so it will correct the image to be consistent with that depth, making the object seem larger to make it seem closer to the farther line.

Another contributing factor is al-Haytham’s theory of relative size. He was correct in thinking that we perceive the moon at its zenith as smaller, but he was missing an important part. Much of the disparity in relative size perception is due to surrounding settings but not in the way we thought. It is instead due to the visible detail around the moon when it is on the horizon. When viewing the moon at that point, we can perceive clouds, other objects, and the horizon. Being within this context makes it seem larger than when surrounded by an empty night sky, as is shown in the Ebbinghaus illusion, where two circles of equal size are each put in the center of a ring of other circles. One of these rings is made up of bigger circles, making the central circle seem smaller. We perceive the night sky as very, very large, so it would make the moon seem smaller.

This is part of what makes the moon seem bigger. Additionally, when the moon is on the horizon, the comparison to the horizon makes it seem bigger. However, this doesn’t hold true when one observes the moon through a viewing device such as a telescope or even a paper towel roll, as the context of the view is removed, and the brain can’t play tricks on the viewer anymore.

Scientists are still not sure of the exact effects responsible for this illusion, but they do know that the brain and senses, especially the eyes, are very susceptible to tricks. We can observe other illusions in day-to-day life, such as the infamous “dress,” an internet-viral illusion based on how we perceive colors. Another common illusion is that of striped clothes—vertical stripes appear to slim the wearer while horizontal stripes appear to widen them. Most of these illusions developed as evolutionarily helpful tools to help humans with perception of the world. While illusions may make perception less accurate, they must have aided in our recognition of danger, heights, and depth. Many are still not fully understood—both in their purpose and how they work. Whether or not we fully understand the moon illusion, we can all agree that the moon on the night sky’s horizon is never a sight to be missed.