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Please note: Many of the everyday things around us are poorly understood.
One such thing is the Moon Illusion. I intended to post this question on Maximaria's Venus Rising thread but time slipped away so I ask it here instead. The rising moon appears three times as large
as when it is directly overhead. Why?
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Dear milum:I remember an article about this in Scientific American a long time ago. No way
of finding that. All I remember is that it is an illusion, which I have noticed many times.
I'll try searching Internet, but am not confident of finding anything good.
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Dear milum: I'll be double dipped in doo-doo, on my siearch for psychology moon size illusion I found this URL http://facstaff.uww.edu/mccreadd/
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Same with the sun...and with setting. Though I think the size at setting is smaller than the size at rising. Interesting, milo...something so obvious, and yet I never gave it any real thought until this.
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Yes Doc Bill, the folks behind that URL might have hit upon the mechanics of the lllusion. Now we should look for an evolutionary advantage for this deceptive portrayal of the external world. I will quote from books and summarize and bring WO'N up to speed.
The rising full moon appears huge on the horizon, although an hour later, when well up, it seems normal in size. This is the moon illusion. Aristotle, Ptolemy, and Confucius all commented on it. At present there exists more than a dozen solutions to the size-distance paradox with each theory having a musing of advocates but so far no consensus.
Most of the distance objects that we see in this world are closer when seen overhead than when seen in the far distance. Clouds are closer than mountains, so it seems natural to assign greater size to the visible objects on the horizon than to those overhead. But the sun and the moon are not in this group, they are exta-terrestrial and are virtually identical in size when on the horizon and when at their zenith. So why does the eye's mind see them as such?
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Dear milum: my vision just won't take reading that entire URL carefully. One thing I didn't see
mentioned was comparison of photograph at moonrise, and same lens photo at moon zenith.
I'm also surprised that a small percentage of people have a different illusion.
I cannot think of any evolutionary tie-in to the illusion.
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an article about this in Scientific American a long time ago. No way of finding that
I remember that article too, can't locate it immediately. The most recent mention of it is in
"...In Brief; March 2000; by Collins, Musser, Martindale,Yam; 3 Page(s)
Heart of Darkness; Superbug Cleans Up; Moon Illusion Explained; Lou Gehrig's Virus?; Surrogate Cat; One Last Stretch; Shrinking to Survive; Organic Space"
located through SciAm.com Archives. Only goes back to 1993, though.
A few years ago Scientific American offered a complete index of lhe last fifty years of so on a floppy disc; I don't know whether they still do. By now they could have a CD with the complete library on it, as like National Geographic does. I should look into it again; I have all the issues (just about) back to 1949 or so when the New Management took over (Gerald Piel, Dennis Flanagan, etc.) and it would be handy!
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Fascinating stuff, milum!...Thanks! 
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Pretty impressive collection of SciAm magazines wofahulicodoc, I couldn't pull up the 2000 issue that you referred to, but if you remember the point of it, it would be nice to know.
The following observations were lifted from the book The Story of Light by Ben Bova (remember him?) They are in no marching order but sorta dance up to the question, Why did Mother Evolution make the sun and moon appear so fat when they are on the horizon?
***Flatworms, like every other animal, tend to keep their eyes (two, four, or six, depending on the species) close to their brains. Even though their eyes are little more than little cups of light-sensitive material, being close to the brain shortens reaction time. And if you make your living preying on the microscopic jungle fauna of protozans, tiny snails, and worms; milliseconds count.
***The molluscs developed true vision. Snails, scallops, the squid, and the octopus all have developed eyes with real lenses and sophisticated retina. Still, it is rather startling to see a row of bright blue eyes peeking out from the edges of a scallop shell.
*** Spiders have eyes remarkably like those of the most advanced snails, a good example of convergent evolution. But like a cheap camera that doesn't have automatic focus, the spider must move back or creep closer to the object of his desire in order to get a clear look.
*** Insects, of course, developed compound eyes. Thousands of individual lenses, packed close together, are individually connected to protorecptor cells by tubes rather like a miniature version of the tube of a telescope. With thousands of individual images being carried to the brain, insects are very sensitive to motion in their field of view. Ever tried to stomp a roach, swat a fly, or slap a mosquito?
*** The human eye is among the best of the image-forming sort that biologists have taken to calling "camera-type" eyes in a sort of reverse wordplay that delights etymologists.
( to be continued in PM )
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Jumping from eyes-close-to-brains-in-lower-animals to brains themselves, I recall a little gem of a puzzle published by John de Cuevas in October 1994, whose solution read: The juvenile sea squirt wanders through the sea
searching for a suitable rock or hunk of coral
to cling to and make its home for life
When it finds its spot and takes root
it doesn’t need its brain any more
so it eats it
It’s rather like getting tenure -- Daniel Dennett, Consciousness
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