The Fascinating Illusion of the Moon's Size at the Horizon
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Chapter 1: Understanding the Moon Illusion
When a full moon rises above the horizon, it often looks dramatically larger than when it is high in the sky. Many people believe it is genuinely bigger at that moment, while others attribute this phenomenon to some magical atmospheric effects (a theory even Aristotle considered). However, the truth is that this perception is merely an illusion created by our brains.
Chapter 1.1: The Science Behind the Illusion
To visualize this illusion, take a look at the graphic below, which shows two moons of identical size. At first glance, what do you perceive? Surprising, isn’t it?
To demonstrate this intriguing effect, you can conduct a simple experiment the next time the full moon appears. This phenomenon, known as the Ponzo illusion, relates to how we perceive perspective. For a clearer understanding, consider this simplified example:
If you find that the top horizontal line seems longer, you are also falling victim to the Ponzo effect. Both lines are actually identical in length. I can confirm this, as I created the graphic by copying the top line and moving it to the bottom.
However, the illusion of the moon's size is influenced by more than just perspective. Our brains are wired to interpret objects on the horizon as being further away than those directly above us. For instance, clouds above us are typically only a few miles away, while clouds near the horizon can be hundreds of miles distant. Thus, when we see the moon close to the horizon, our brains mistakenly interpret it as being larger due to this perceived distance.
Chapter 1.2: Conducting Your Own Experiment
For an entertaining experiment to share with family or friends during the next full moon, take a small object, like a pencil eraser, and hold it at arm's length as the moon rises. Compare the size of the moon to the small object. If your object is the right size, they will appear similar (you can adjust your arm's length if needed).
Then, repeat the same test when the moon is higher in the sky, and you will see that the moon remains the same size in relation to your held object.
If you want to get a bit more creative, you can craft a simple moon measurer. Roll up a piece of paper, placing the wider end to your eye, and tape it once you can barely see the moon through the smaller end. Here’s a photo of my own version:
While labeling your Moon Measurer isn't necessary, it does add a touch of flair to the experiment.
Chapter 2: Documenting the Illusion
If you want photographic evidence of the moon's size illusion, consider this method suggested by Harvey Leifert, a freelance science writer:
“Take a picture of the moon when it’s just above the horizon, then without adjusting the camera settings, take another when it’s overhead. Print both images and hold them up to a light to superimpose one over the other.”
This test will reveal that the moon appears the same size at both positions.
Alternatively, you can skip the scientific rigor and simply enjoy the spectacle of the full moon rising. Every 29.5 days, I find myself exclaiming, “Wow, that’s enormous!”
When is the next full moon? Check the Time and Date website for details in your area.
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Chapter 2.1: Video Resources
Explore the concept of the moon's size illusion with this enlightening video: Why does the moon look so much bigger when it's on the horizon?
Dive deeper into this fascinating topic with the video: Why Does The Moon Look So Big Tonight?