Moon Illusion: Why a Full Moon Looks Huge on the Horizon

supermoon dec. 3, 2017
The full moon on Dec. 3, 2017 seen from Washington, D.C. looks huge on the horizon. And while it was a supermoon (larger than other full moons) the effect on the horizon is just an illusion rooted in how our brains work. With photographs, the effect can be magnified with telephoto lenses. Photo: NASA/Bill Ingalls

When you see the moon just as it rises, it can seem much larger compared to later, when it’s higher in the sky. Many swear it actually is larger on the horizon. But it’s not. It’s just an illusion, and you can do a simple test to prove it.

First, the reason for the illusion:

Scientists say our brains get trained to assume objects on the horizon are farther away than things overhead, because, for example when we see clouds overhead, they are in fact just a few miles away, whereas clouds near the horizon can be hundreds of miles away. When your brain assumes something is farther away—such as the moon on the horizon—but it really is not, then you’ll interpret it as being bigger.

The effect is known as the Ponzo illusion, illustrated in the graphic below. Both moons are the same size, but if you’re like most people, one will seem bigger at first glance.

moon illusion ponzo

You can prove this illusion to yourself (or impress someone else). Take a small object, such as a pencil eraser or the tip of a key, and hold it out at arm’s length as the moon rises. Compare the size of the moon and the small object. If you’ve chosen a right-sized object, they’ll be the same (adjust the distance of the object to make it so).

Then do the same test later when the moon is higher up and appears smaller. You’ll see, if you do the test accurately, that the moon is still the exact same size in relation to the object in your hand.

You can do this with an impromptu moon measurer, too, by rolling up a piece of paper as shown below and taping it off when you get it to the right size to just barely reveal the moon’s full glory. Either way, the test will show that the supposedly giant moon on the horizon is all in your head.

Related: What’s a Supermoon?

Robert Roy Britt
NoPho resident Robert Roy Britt has written for In&Out publications since its inception in 2005. Britt began his journalism career in New Jersey newspapers in the early 1990s. He later became a science writer and was editor-in-chief of the online media sites Space.com and Live Science. He has written four novels. Email the author.
Robert Roy Britt on Email

Robert Roy Britt

NoPho resident Robert Roy Britt has written for In&Out publications since its inception in 2005. Britt began his journalism career in New Jersey newspapers in the early 1990s. He later became a science writer and was editor-in-chief of the online media sites Space.com and Live Science. He has written four novels. Email the author.

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