Big full Moon tonight and it's a good night to test the horizon illusion. When the Moon is on the horizon, cover one eye, uncover it, cover it, uncover it. Notice the Moon instantly changes size. Your brain, despite being aware consciously of the horizon illusion, cannot shut the illusion off. But if you block the 3D image from your retina, it can. In 2D, the Moon looks its natural size. In 3D your brain adjusts the size you perceive (for all intents and purposes the size you are sure you see) in relation to as if it were an object on Earth on the horizon. Your brain says, this is the size of the Moon based on the evidence hitting the retina. You believe your brain is saying, this is the size of the Moon based on the light entering the eye. And here you thought you were in control.
An interesting corollary is that your brain knows you are switching to "2D" when you cover one eye. The parallax effect at the root of 3D vision is negligible for objects at the distance of the moon, or even the horizon, given an approximate 3" pupillary separation. So even breaking the illusion depends on illusion.
I'm not sure what your hypothesis is there, , but I suggest you try covering one eye and let us know what you saw.
For those interested: Why does the Moon look so huge on the horizon? I've met Phil Plait a few times. He knows his stuff.
Unfortunately, my wife and I went to the movies tonight (saw "Much Ado About Nothing" - I always find it somewhat jarring to hear Elizabethan era English spoken in a contemporary setting), so by the time we saw the moon, it was well off the horizon.
I have just thought of an issue with my covering an eye effect. In the link I posted above, "Why does the Moon look so huge on the horizon?", look at the hallway image example in the article. When you see the narrowing black lines in the 2D image that give us the perception of depth, covering one eye doesn't change how we perceive the red lines because the brain is using the lines, and not our stereoscopic vision to determine distance. On the horizon, it's your eye parallax that perceives the distance. Take that away, and unless you have narrowing lines, like say a straight highway that goes to the horizon, and you see the effect. This made me think maybe other cues in the scene make a difference. If so, try the test with a nearby horizon like buildings or trees. I'm going to have to try this with a far-off-in-the-distance horizon and see if it changes the degree of the effect.
Ah, I see. But it's the parallax effect of the nearby horizon, not of the Moon that causes the effect that changes when you cover and uncover one eye. Your brain interprets the size of the Moon based on assuming the Moon is the same distance as the horizon. When you remove the parallax, the brain removes the adjusted size interpretation and the Moon looks the actual size of the image reaching the retina. It's hard to imagine but there are many other illusions which rely on the same principle. The brain doesn't present the actual image to one's consciousness, it presents the interpreted image. It's a fascinating thing to contemplate.
Stereopsis (3D effect from distinct images from two points), for the separation of eyes in a human head (about 3 inches) is negligible at the normal distance of the horizon. It's negligible for anything further than a couple hundred yards. Line up a star with a tree tip on the horizon, using one eye. Close that eye and open the other and the tree tip will remain aligned to the same star. There is no perceptible stereopsis at the distance of the horizon.
Are you denying the Moon looks larger on the horizon than overhead, or saying there's another reason why the Moon looks larger on the horizon? And are you refusing to try the test and see for yourself? I've done it many times, the illusion changes when you cover an eye. It's a fact. And, one can find other people have repeated the experiment: About the Moon illusion I find that particular quirk about upside down perception fascinating.
As I said in my original response, stereopsis isn't the source of the illusion, because stereopsis is negligible at those distances. But somehow cvering one eye tells the brain to interpret the scene differently. Therefore a new illusion takes over in dispelling the relative size illusion. Its interesting that covering one eye ma,es the brain interpret the scene efifferently even though stereopsis isn't what fooled the brain in the first place. The entirety of 3D perception includes factors other than the parallax effect, but only the parallax effect is directly removed by switching to monocular vision. It's a fascinating insight into perceptual psychology.
Riiiight. Care to hypothesize how the brain perceives something other than parallax when you go from monocular to binocular vision and why the same effect doesn't happen when you look at a 2D image of the same thing? Doesn't this contradict what you just said?
No. sigh. Nevermind, I thought it was an interesting subtlety, nof fodder for another argument. Have a nice day.
No hypothesis or explanation then? Good thing I enjoy investigating on my own. Stereoscopic perception of real depths at large distances What these researchers found was stereoscopic depth perception was detectable without other cues at a distance of 40 meters and when one added additional cues, the depth perception was more acute. So both, parallax and additional visual cues appear to affect the illusion. It's best seen with this graph where monocular vision negates depth perception even with other visual cues. If not parallax, then it's difficult to explain how the effect was detected in the dark without additional visual cues, and it is difficult to explain why when relying solely on the visual cues on a 2D image there is no difference between monocular and binocular vision. If you have an hypothesis that would explain these findings other than parallax, I'd be willing to entertain it. But just declaring it's not parallax isn't enough. I plan to see how monocular vs binocular vision affects the Moon-horizon illusion when the horizon is a greater distance than the trees and buildings the Moon rises over as seen from my yard. But it's cloudy here, so it'll have to wait.
I'm constantly surprised by how the brain effects vision and, even though you can be aware of the trick, you can't unsee it! It's frustrating but fascinating. It's like this litte picture, where the top and bottom fields are exactly the same shade of grey. You've no doubt seen it. But for others, cover the shaded space between the two surfaces and you'll see they are exactly the same colour.
