The Weather Notebook: Brainstorm Answer: A Large Moon Rising

Brainstorm Answer: A Large Moon Rising
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Dave Thurlow, Host

What's the oldest unsolved scientific problem? It just might be the moon illusion, which has stubbornly held out for almost three thousand years. Hi, I'm Dave Thurlow and this is The Weather Notebook.

The Moon Illusion
The "moon illusion" is something you may have noticed - this is when the moon appear to be larger near the horizon than when it's high in the sky. The same goes for the sun and constellations too, but it's easier to spot with the moon.

Now, some of the best minds in history have tackled the moon illusion, applying the scientific ideas of their time. Aristotle attributed it to the refraction, or bending, of light from the moon as it passes through the Earth's atmosphere, somewhat like the angular size of an object underwater appears larger than it actually is. Ptolemy developed this idea in detail, and though there is a small refractive effect, detailed astronomical measurements finally showed that it was not the cause of the illusion.

Since then a Big Dipper's worth of solutions have been proposed, but in the last hundred years there's been agreement that the explanation lies in the field of psychology, and not physics. Many of the arguments rely on theories of visual perception; the one you hear most commonly is that the moon appears larger near the horizon because we can compare it to distant objects there. But the moon illusion also appears over open water, so this explanation isn't the full story.

In a way, our minds think the moon near the horizon is relatively closer, and since the moon stays the same brightness we're tricked into thinking the moon there is larger. But psychologists have not yet worked out the details of all these tricks of perception, and most think several are probably involved.

Until then, stand on a hilltop and enjoy the mystery.

Today's show was written by David Appell. The Weather Notebook is underwritten by Subaru with major support provided by the National Science Foundation.