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Complex Numbers #1078956 added October 26, 2024 at 9:26am
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Moon Illusion
From SciAm, and an author I hung out with for a week a few years ago:
I can smugly assert that this is something I'd already figured out.
I remember watching the full moon rise one early evening a while back.
You'd think astronomers would spend more time watching moonrises.
As it cleared the horizon, the moon looked huge!
Eastern Colorado, where this occurred, is basically a continuation of Kansas: Flat as the proverbial pancake. There's an actual horizon there.
Anyone who is capable of seeing the moon (or the sun) near the horizon has experienced this effect.
I don't have an unobstructed view of the horizon here, but I've spent time on the coasts, so yes.
But it’s not real. Simple measurements of the moon show it’s essentially the same size on the horizon as when it’s overhead. This really is an illusion.
As opposed to time, which really is not (yes, another time article is in the queue).
Attempts to explain it are as old as the illusion itself, and most come up short. Aristotle wrote about it, for example, attributing it to the effects of mist.
As smart as Aristotle was, he got lots of stuff wrong. This was one of those things.
A related idea, still common today, is that Earth’s air acts like a lens, refracting (bending) the light from the moon and magnifying it.
The "simple measurements" noted in an earlier quote refute that bit instantly. Refraction does make it appear redder than usual, though, just like it does with the rising or setting sun.
Another common but mistaken explanation is that when the moon’s on the horizon, you’re subconsciously comparing it with nearby objects such as trees and buildings, making it look bigger. But that can’t be right; the illusion still occurs when the horizon is empty, such as at sea or on the plains.
But, see, coming up with explanations like that, even when they can be disproven... that's part of science.
So what’s the cause? Like so many things in science, there are two effects at play here.
Phil indeed goes on to explain the cause, but no need for me to quote it word-for-word. The first part of the answer is related to this well-known optical illusion. 
The second part is devoted to showing that we don't see the sky as a hemisphere, but as a nearly-flat surface. This is the harder part to accept, I think, but next time you're out on the plains or the ocean on a partly cloudy day, note how the bottoms of the clouds make the sky seem flat. This is for the same reason that the Earth itself appears (but only appears, dammit) flat in such locations.
So you've got two (nearly) planes, earth and sky (the temptation exists to make a plains/planes pun, but I'm not in the mood), but only one moon. Still, presumably, you've seen the moon high in the sky quite often, so then when you see it near the horizon, your brain goes back to remembering seeing that.
Moon illusion misconceptions still abound, and like so many myths, they likely won’t go away no matter how much someone like me writes about them.
Yes, this is, indeed, the curse of fact-checkers. |
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