What is the mysterious Moon Illusion?
Why does the rising or setting Full Moon look so big? Spoiler alert: it's a trick of the mind!
Ever notice that when the Moon is hovering just over the horizon, it looks much larger than when it is high in the sky? There's a surprising reason for this phenomenon.
We experience supermoons and micromoons throughout the year, when the Full Moon is closer to us or farther away, respectively. However, on any night we can see the Moon near the horizon — whether it is rising or setting at the time — it looks exceptionally big.
So, what causes this effect?
It's not something the Moon itself is doing. In fact, due to the rotation of the Earth, when the Moon is just above the horizon it's actually thousands of kilometres farther away from you than when it is directly overhead. So, it's not the distance of the Moon.
Instead, the apparent size of the Moon when it is close to the horizon is due to a little trick of our mind known as The Moon Illusion.
This simulation reveals how the Full Moon is farther away from an observer in the Great Lakes region of Earth at moonrise than at midnight. Credit: Scott Sutherland/NASA's Scientific Visualization Studio
As our eyes take in the world around us, our brain knows from experience that objects close to us tend to appear larger and in focus. In contrast, distant objects tend to be tiny and blurry. From this, it also knows that for a distant object to appear in focus, it must be enormous.
This close-up of the September 2019 Full Moon was snapped in Calgary, AB, during the Chinese Moon Festival. Although the camera zoomed in on the scene, if viewed in person, the Full Moon would still look immense next to the city's downtown skyscrapers due to the effects of the Moon illusion. Credit: Siv Heang Tav/UGC
So, when we see a bright Full Moon hanging crisp and clear in the sky above the horizon, it is contrasted by all of the objects on the ground, which appear smaller and blurrier the closer they are to the horizon. This combination confuses the brain. So, to compensate, the brain interprets the Full Moon as much bigger than it truly is. To be clear, the Moon is certainly much larger than any of the objects on the horizon (it's 3,474 km across), but this 'illusion' gives us the impression that the Moon looks enormous!
Look up into the sky closer to the middle of the night, and the Moon will be high above our heads. Usually, it will be the only thing we see, other than the stars and maybe a few planets. At that time, the brain is focused only on the Moon, and without the other objects in the field of view to complicate matters, it is free to just 'see' its actual size.
This zoomed-in image of the Full Moon was captured from Salisbury, NB, on February 9, 2020, and uploaded into the Weather Network's UGC gallery. Credit: Darlene MacLeod/Smith
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READ MORE: Why is the supermoon so compelling to us?
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Mind tricks
We have a few tricks of our own that can cancel out the Moon illusion, though.
For the first one, we don't need technology. Just go outside after sunset and find the Moon near the horizon.
Stretch your arm towards it, and cover the Moon with your thumb or even your pinky finger. Note how big the Moon looks compared to the digit in question, and keep that in mind. Maybe even take a picture of it if you want.
Later in the night, check out the Moon again when it is high in the sky. It may appear smaller than when you saw it earlier, but repeat the step to cover it with your thumb or finger. Compare it with what you saw before, and you'll find that the Moon is precisely the same size at both times.
There is a way technology can help us, though.
When the Moon is low on the horizon, take out your cell phone, turn your camera on, and point it at the Moon. Chances are the Moon will appear much smaller in comparison to everything around it on your phone screen than it looks like in the sky.
Note: it is possible for the Moon illusion to still work on us when looking at a picture or video. This is because the brain will make the same judgments of distance, blurriness, and size as it did when looking at a 'live' scene.
Still, directly comparing what we see in the sky at that time to what is shown on our small cell phone screen can help put things into better perspective. Plus, you can also take a few pictures to upload into the Weather Network UGC Gallery while you're at it!
