The Moon Illusion
Picture those moments at dusk when, standing alongside a friend or family member—whether driving, on a rooftop, or by the beach—you point out the moon on the horizon, yelling out loud about its enormous size. Wow, look at the moon tonight!
You’ve likely marveled at the moon’s seemingly exaggerated appearance and wondered why it looks huge. But think about it. In a few hours it will shrink down to its normal size in the sky. Right?
Wrong. You’ve been deceived. You are experiencing what has been known for thousands of years as the moon illusion. Contrary to your perception, the moon doesn’t actually change size whether it hangs low on the horizon or sits high above in the sky. This fact has been tested and confirmed through photography, measuring instruments, and simple observation.
Consider some common sense: try the thumbnail test. Hold your thumb out at the horizon, noting that the moon takes up roughly the size of your thumbnail. As the moon ascends to the top of the sky, perform the same gesture—your thumb still covers approximately the same portion of the moon. The size remains consistent, yet it inexplicably appears larger near the horizon.
You’re in good company though. Philosophers, scientists, and scholars spanning the last two millennia, from Aristotle and Ptolemy to Leonardo da Vinci and Descartes, have been stumped and failed to explain why.
This illusion occurs due to the human brain’s handling of incoming visual stimuli. Let’s look back at the history of this phenomenon:
-The beginning: dating back to the 7th century BC, records on a stone tablet in Nineveh documented the moon’s apparent and puzzling enlargement on the horizon, predicting an eclipse.
-Lei Chu in 2nd century AD China noted a similar illusion with the sun on the horizon. He wondered why its increased heat intensity occurred at noon when appearing more distant. He could not determine what constitutes the actual closeness of the planet. He brought the problem to Confucius, who could not solve it.
-At the same time, Ptolemy championed the magnification theory. The common belief was that the atmosphere magnifies our view, similar to how the surface of water causes a refraction of the contents underneath the water. This is still believed by many lay people today: that there is a physical magnifying taking place outside of the human body. This has since been disproved by the actual measurements that have been taken of the moon in studies using photography.
-Galileo’s student, Castelli, investigated why the Big Dipper and other constellations appear larger on the horizon. But he couldn’t figure it out. Leonardo da Vinci speculated about objects appearing larger at midnight due to pupil contraction, and objects in the sky appearing smaller at mid-day because of the abundance of light. But he didn’t get anywhere regarding the moon.
-In 1899, German scientist Oskar Zoth theorized about gravity’s role and eye distortion when looking skyward. He also thought your visual axes tend to converge when the eyes turn upward due to the muscle controlling the movement, and this would cause items to appear closer. This effect is known as convergence micropsia. However, contradictory results persisted in modern studies.
Interest in the moon illusion resurged in the 1960s and 70s, despite continued debate and inconclusive findings.
Bruce Walker’s experiments published in Optical Spectra in 1978 used photos to show that the size of the moon is 100% completely the same regardless of where it is in the sky.
This was big. It ruled out all of the optics theories, all of the refraction theories, anything outside of the human brain.
The illusion now became a psychological issue.
Questions started to emerge:
Is the image hitting the retina actually larger and thus entering the brain this way?
Or is this something happening in the occipital lobe far beyond the optic nerve?
A theory called the size-contrast theory entered the discussion. It suggested that our perception of the moon’s size is influenced by the association of nearby large objects, like trees or buildings. Thus, our brains create an illusion of increased size when seen and associated with other large objects. This theory gets shaky though, as the moon illusion is still reported when there are no cues on the horizon.
Brightness was also mentioned as a possible culprit. Due to pollution, the moon is not as bright. Therefore the pupils dilate to take in more light. This sharpens and enlarges the focus on a darker object. This dilation causes a larger retinal image which is shipped onto the brain. Unfortunately, this theory was discredited across multiple studies in the 1960s where researchers controlled for the brightness of the moon. The sun also appears larger on the horizon which ruled out a lot of pupil-based theories having to do with light and dilation.
Other strange problems continued to puzzle scientists:
a) Airline pilots and sailors, devoid of surrounding references, still report experiencing the illusion.
b) The moon illusion doesn’t manifest in planetariums where conditions closely mimic the outdoors.
c) Bizarrely, for most people, the illusion disappears when they view the moon upside down.
Alternate hypotheses also emerged, such as the relative distance hypothesis, attributing the illusion to our brain’s evolutionary tendency to perceive objects near the horizon as farther away and, consequently, larger. Yet, this contradicts the common perception that the moon appears closer when on the horizon.
So what’s the answer moving forward?
What does the current body of knowledge in the 21st century say about this?
The answer is: nobody knows.
There are a few ideas that have since developed.
Number one is the relative distance hypothesis, which is based on a little shortcut or hack used by our brains. Over millions of years in human evolution, our visual systems developed to associate things near the horizon with being very far away. After all, when you’re standing on the ground, the horizon is the farthest thing you can see. Things out there might still be important and valuable for survival. For example, water, or food, or shelter. It’s possible your brain ‘cheats’ and makes that stuff seem bigger than it really is.
There is one thing that’s not an illusion though: the moon’s yellow color. The moon tends to have a more yellow or orangish hue compared to when it’s high overhead and more of a pure white. This happens because the moon’s light travels a longer distance through the atmosphere. As it travels a longer path, more of the shorter bluer wavelengths of light are scattered away, leaving more of the longer, redder wavelengths. This is amplified by the dust and pollution in the atmosphere that can also deepen that reddish color. Not everything we perceive is a lie.
The second modern theory is back to this convergence micropsia. That is, our brains judge distance in general based on how much our eyes must focus. In the moon example on the horizon, the brain knows you’re looking far away because it had to adjust the eye a certain amount to capture the detail of the moon.
But the brain at the same time thinks, “Man, look at all of that detail! And yet I’m sensing this object is so far away, because data from the eyes focusing are reporting it as so. So, the moon must be huge to be providing all that detail.” And that’s the story that hits our frontal lobes.
The interplay of these theories results in a contradiction within our perception: the moon seems both larger and closer than reality. So, something must give to resolve the contradiction. We land on the side of the coin where the moon looks monstrous at times.
Remarkably, some individuals don’t experience this illusion at all.
Maybe the moon shrinks overhead? It’s possible that our eyeballs lose their focus when looking upward and we see a more accurate size, which looks smaller to us. Who knows.
In essence, the moon illusion remains a total enigma. Our perception continues to bend.
For further exploration, you can read Maurice Hershenson’s book, “The Moon Illusion.” This was a fun deep dive and I think about it every time I see the moonrise on a clear evening.
Go outside tonight and see if you are deceived again.