Moon Illusion
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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" is something you may have noticed - this is when the moon appears to be larger near the horizon than when it's high in the sky. You can prove it for your self. Hold a dime at arm's length and depending on the length of your arm the dime will cover some or all of the disc of the moon. Do this at moon rise and again near midnight and see that size does not change.

Since way back in Aristotle's time a significant amount of scientific solutions have been proposed, like the illusionary optics of refracted light, but in the last hundred years there's been agreement that the moon illusion explanation lies in psychology, 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.

One theory says that our minds unconsciously percieve the sky as being a dome with the horizon farther away than the the top. As the moon slides across the dome our brains tell us it's shrinking only to grow again as it approches the western horizon. But psychologists have not yet worked out the details of these tricks of perception, and most think several are involved.

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

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