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Lunar craters inverting illusion

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#1 russell23

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Posted 22 February 2010 - 06:04 PM

I've had this happen at times over the years but I've never bothered to find out why it happens. Last night when I was observing the Moon I was getting tired and suddenly the craters would pop so that the crater floor would appear as a plateau and the various peaks in the craters would appear as depressions.

What causes this phenomenon. It was an all or nothing thing. As soon as one crater reversed they all did and when they reverted back to normal they all did.

Dave

#2 Kutno

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Posted 22 February 2010 - 07:42 PM

David,

I do not know why it happens; but it occurred last night, when I started to look at the Moon. While I have experienced this with my reflectors, it was the first time I saw it in a refractor. I had to keep telling myself that the "pits" in the middle of the "plateaus" were really central peaks in craters; then, the image would revert to normal.

While I do not why this optical illusion occurs, what I am about to say next is an anecdotal theory about its frequency of occurrence: Its triggering may have something to do with that fact that it was the first night I was out under the stars, with a scope, this year. I do not recall this happening last year, once my viewing season really began, during the spring. If expectations play a role, and being able to "talk" myself into the proper image suggests that this is so, then perhaps the "novelty" of viewing the Moon's black-gray-white landscape makes it conducive for the illusion to occur.

#3 ColoHank

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Posted 22 February 2010 - 09:33 PM

Maybe it's just me, but it seems that the eye is conditioned to interpret aerial images of terrain most faithfully when scenes are illuminated from the upper left (from the northwest if a terrestrial image). I've never experienced the illusion when observing the Moon through a telescope, but have noticed the effect when viewing satellite imagery on Google Earth or good old aerial photos. In those instances, it's easy to remedy things by rotating the photo or the image until it appears illuminated from the upper left. A young Moon would be illuminated from the right as viewed naked-eye, so lunar terrains might appear inverted in depth unless, of course, the telescope reverses images right-to-left. I'd imagine that would tend to negate the illusion. Confused? So am I.

#4 russell23

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Posted 22 February 2010 - 10:05 PM

The thing I noticed is that the effect really seemed to take hold if my eyes relaxed so that I was looking, but not really looking -- kind of like those computer generated posters that were big in the 90's in which it looked like a bunch of colors but if you let your eyes relax a 3-D image popped out.

I could make the effect go away if I wanted. But when I tried to keep the effect it would go back and forth between normal and inverted.

Dave

#5 ColoHank

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Posted 23 February 2010 - 12:28 AM

I'm familiar with those novelty 3D posters, and used the same technique to study stereo pairs of aerial photos back in the day. But that takes two eyes. Are you using a binoviewer with your scope?

#6 russell23

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Posted 23 February 2010 - 07:54 AM

I'm familiar with those novelty 3D posters, and used the same technique to study stereo pairs of aerial photos back in the day. But that takes two eyes. Are you using a binoviewer with your scope?


No, I'm not using binoviewers. I was using that example because I noticed it starting to happen when my eyes got a little tired and relaxed - almost like I was staring through the Moon instead of at it. OTOH I could directly look at the Moon and force it to happen too and then move my eyes around to different craters while it was happening.

I don't know if it makes a difference, but I always observe with my left eye even though I'm right handed. I don't know what is normal in that regard because I generally don't on my own.

Dave

#7 desertstars

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Posted 23 February 2010 - 08:13 AM

I'm not sure what the technical explanation might be for the illusion, but I've experienced it many times myself. It often (but not always) comes late in an observing session so fatigue may play into it. I've also experienced it while looking at photos of craters, most recently in an image of a Saturnian moon. (Don't remember which one.)

#8 jim_m

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Posted 23 February 2010 - 08:35 AM

A bit of a strange sensation, eye/brain confusion. I do not see it as much on the Moon as on prited material.On the Moon, I seldom look straight down into craters, usually at a sligth to deep angle. Kinda helps to keep your orientation.
Jim

#9 photonovore

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Posted 23 February 2010 - 11:02 AM

This is an example of an optical illusion called "reversible perspective" typified by what is called a "Necker Cube". Other examples are 'reversible staircases'', inverting 'hollow masks' etc. It seems that lighting direction and degree of relief combined with degree of familiarity of the object itself are all factors. The effect is also associated with 2-d presentations; I have not heard of real objects reversing their perspective when viewed by the naked eye (real objects in real space), although i think if one were tired enough it could happen momentarily. I have always noticed, myself, that craters are more likely to pop, or reverse perspective, when the sunangle is higher and the cues of lighting less intense--and concentrating on the high relief of the terminator will cause things to pop back into proper perspective. I have heard that the direction of lighting is also a contributing factor, higher angles of lighting (from viewer's perspective, lighting sourced from the top of an image) cause less reversal problems than lighting coming from below the horizontal median line of the image. (crater illusion)

Posted Image


I also think this illusion is more common when fatigued-- and points to this illusion being more psychological than physiological in origin.

#10 Kutno

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Posted 23 February 2010 - 11:57 AM

Great illustration, Mardi!

#11 ColoHank

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Posted 23 February 2010 - 03:09 PM

This is an example of an optical illusion called "reversible perspective" typified by what is called a "Necker Cube".



Thanks. It's nice to know there's a name for that phenomenon. The whole business brings to mind the drawings of M.C. Escher.

#12 Rick Woods

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Posted 24 February 2010 - 02:59 PM

Mardi: You're back! YAY!!!!
I'm familiar with the illusion, but I've never actually seen it. Not on the moon, and not in your photo. And I've tried! But, I *have* seen it in Mad Magazine, when they used to have the "Mad Poiuit", a 2-prong - no, 3-prong optical illusion.
Which makes me agree with Hank about Escher's work.

#13 Jim Rosenstock

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Posted 24 February 2010 - 06:43 PM

This illusion happens to me often while observing the Moon.

For me, it happens more often near first quarter Moon; it also happens more often with binoviewers than in monovision. I haven't correlated it with fatigue, though.

When I get one of these "field reversals", I can't just "stare it down" to make the image switch back. Either I must remove my eye from the eyepiece for a moment, or--more effective--pan my scope down the terminator....at some pont the craters will look right again, and I can pan back to where the image reversed.

It's mostly a cool illusion--viewing domes when you know they're craters!--but sometimes it gets a little annoying when the illusion is persistent.

Jim

Oh, BTW, the Mad Magazine "impossible object" illusion was called the "Poiuyt". Try typing it, and you'll see how they came up with the name! :cool:

#14 photonovore

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Posted 24 February 2010 - 08:53 PM

Mardi: You're back! YAY!!!!
I'm familiar with the illusion, but I've never actually seen it. Not on the moon, and not in your photo. And I've tried! But, I *have* seen it in Mad Magazine, when they used to have the "Mad Poiuit", a 2-prong - no, 3-prong optical illusion.
Which makes me agree with Hank about Escher's work.


Rick, count yourself fortunate! It can be really annoying, trust me... :p

#15 russell23

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Posted 24 February 2010 - 11:58 PM

Jim, Interesting you mention 1st quarter because I haven't seen it in a while until I noticed it at 1st quarter Moon the other night.

Dave

#16 photonovore

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Posted 25 February 2010 - 10:28 AM

This is exactly when I noticed the effect (stronger than usual)--this first quarter. The telescope i was using delivers a mirror image (diagonal) which preserves the upward angle to the illumination seen with the naked eye at this phase. Perhaps users of scopes that simply invert the image (and see an illumination angle coming from above as a result) were not so commonly affected?

#17 ColoHank

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Posted 25 February 2010 - 08:21 PM

I recently finished reading a little book by R.A. Proctor entitled Half Hours with the Telescope . My thirteenth-edition copy was published in 1902, but the original dates back to 1868. Proctor devotes only part of one very short chapter to the Moon, which he describes as being one of the easiest and yet most disappointing objects to view. His principle complaint is that the Moon is boring because it never changes.

If only he had seen topographically inverted views of the Moon, with depressed mountains and mounded craters to break the monotony! No doubt he would have been enthralled.

#18 calibos

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Posted 27 February 2010 - 08:58 AM

I'd be exactly the same as Jim. See it nearly every time I view the moon. Gets annoying and I have to look away to get he view to switch back. Can't force it by concentrating or staring. Have to reboot my vision by looking away as it were :D I also see it happen all the time when looking at the moon in Starry Night Pro and Moon Atlas on my iPhone.

#19 BSJ

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Posted 27 February 2010 - 12:35 PM

Pay close attention to any relief map. One that uses shading to represent elevation.

The "shadows" will always be towards the bottom of the map. Natural illumination will never allow that to happen but the mind is tricked to perceive the intended effect.

If you're ever looking at an aerial photo, position it so the shadows are towards you. It's easier to see things as they really are.

Some of the things I learned as an imagery analyst in the Army…

#20 photonovore

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Posted 27 February 2010 - 01:55 PM

"Shadows towards you" works if the native elevations are primarily positive (as on Earth). If the elevations are primarily negative (as in the case of lunar crater fields) the opposite would apply--then you would position "shadows *away* from you (if possible) for a more "natural" perspective view.

#21 Scott Watson

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Posted 27 February 2010 - 08:48 PM

It isn't so much an optical illusion as an optical ambiguity. I studied this phenomenon in graduate school and found that if you take an image (like a crater) that can be interpreted as either concave or convex and you tell the subject that it is one of the other, they will usually interpret that way. If you don't tell them anything, they will usually interpret it as a crater. It seems we default to the most common interpretation from our previous experience.

#22 Carol L

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Posted 03 March 2010 - 10:47 PM

Here's another graphic which illustrates the "reversible perspective" illusion (great to see you again, Mardi!).
blister -vs- crater

They look like blisters until you turn the image upside down. :)

#23 photonovore

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Posted 04 March 2010 - 10:36 AM

Hi Carol! Just like old times, mm? :) That is an excellent illustration of the effect. I made an animated gif of the opposite images just for fun... Crater-dome Illusion

#24 calibos

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Posted 14 March 2010 - 07:22 AM

If only I could switch views as easy on the moon in real life. I was able to switch from blisters to craters very easily without even looking at the image upside down. Pity its not that easy on the moon.

#25 revans

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Posted 20 March 2010 - 08:28 PM

When I have a hard time deciding if I'm looking at a crater or a dome in a photo or on the moon, I usually check the bright areas of the crater rims to see if they are on the right or left. The opposite side will usually be in shadow. The lighting pattern on raised structures like domes is usually the opposite of what it is on adjacent craters. So if craters are bright on the left and dark on the right, then domes or raised structures will be dark on the left and bright on the right. I don't know if this is always true, but I find it to be a help...

Rick






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