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Posted by Richard D. Stafford, Ph.D. on July 27, 2001 19:49:06 UTC


I loved it -- to the point and on the mark! I thought I might mention something to you that I noticed long ago and have never really been able to communicate. (Another case where people think I'm nuts.) In a nutshell, our brains think we live inside a rough sphere of about 100 feet in radius (I say a rough sphere because it's not really spherical). I suspect the actual distance is approximately the distance we can throw a rock.

Why do I say that? Consider the moon or the sun. Most people will agree that they appear (visually) to be roughly the size of a basketball. What does that mean? What it means is that an object which can be covered with your thumb at arms length (1/2 inch at 36 inches) is perceived to be about one foot in diameter. That would imply the brain is putting it about 70 feet away.

Why is the moon perceived as larger when it is near the horizon? Because the "Brain's" world is squashed down a bit; things can be further away in a horizontal direction then they can in a vertical. Notice also that when the moon is behind an object who's size is known, (which implies it is bigger than the object), the brain will allow it to be perceived bigger but, no bigger than is absolutely necessary: i.e., it will be perceived to be *exactly* behind the object and the brain will allow no separation at all. (The brain makes it as small as it possibly can!)

Or let's talk about flying in a plane. As you leave the ground, people will agree that things on the ground look smaller. Ask them how large they appear to be. Just how wide does the road appear to be? People will quite often give an answer -- Oh, about a 1/2 inch (at a couple thousand feet) or Oh, about 1/16 inch (when you get a little higher). Or get up to 30,000 feet and they will estimate a 5,000 ft mountain to look like it is only a couple of feet high. I am convinced that the brain can't conceive of being more than about 20 feet off the ground.

Try it next time you happen to fly. Ask someone to tell you how big something looks and then back calculate the distance it would have to be away at that size to give the image size you see on your retina. I think you will find a rather interesting illusion exists there.

Have fun -- Dick

PS The next time you see sun beams comming through the clouds, try to percieve them as parallel. The brain will not allow it because that would put the sun too large and too far away (outside the brain's universe).

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