View Full Version : Moon optical illusion

2011-Aug-20, 04:21 AM
Why are those optical illusions on the Moon craters? I forced my eyes to not see bumps on Copernicus,but I couldn't.Only if I looked again,I saw the deep crater,but only for a few moments. :) If you believe that sunlight comes from the southwest, it is seen as a deep crater.If you think the sunlight comes from the northeast, it is no longer seen as a crater, but as a swelling . This is my Copernicus thtough 8" reflector telescope

Copernicus crater optical illusion

2011-Aug-20, 04:37 AM
With a single lens shot and with only one light source. No depth of field is apparent or possible...
Under different lighting conditions the depth of field can be perceived.. Lower, lesser angles can show the depth and hight of features. That image provided is a excellent case study.

2011-Aug-20, 04:41 AM
Some people have this directional illusion. Rotate the picture 180 degrees and holes are domes. I've had the illusion with smooth mounds or craters but never with those with features but many do. As you say, the brain can assume the light is coming from the reverse direction and when it does the illusion is striking.

Good moon shot though I'd reduce the size a bit. It's pretty soft at this size. This also helps reduce the illusion I've found. At least for me.


2011-Aug-21, 12:07 AM
May I suggest that one of the reasons we have problems with seeing them as craters is simply that it isn't something we see here on earth. At least not very frequently. Hills, domes and even just small bumps are more common to our eyes than round depressions, so when we see such an image without any clues as to light direction, our experience tells us that the light is coming from the wrong direction..

And yup, that's just a wild guess!

2011-Aug-21, 02:13 AM
Maybe in some cases that's true. May vary by the individual But for me rotating the image 180 degrees always removes or sometimes creates the illusion. Since I see it only with smooth features it is far more common for me to see mounds as holes since they are more often featureless with smooth edges. I've had the illusion from the air with prehistoric mounds. As we approached they were hills but get over them and suddenly they were trenches. Fly in from the other side and no illusion. My dad, who was doing the flying never saw the illusion either way nor did I the second pass from the "trench" direction but by then sun angle was higher and shadows mostly gone. That might have something to do with it as well. Lots of possibilities.


2011-Aug-21, 08:27 AM
Post edited to reflect an additional ten minutes of experimenting!!

It's interesting if somewhat inexplicable.. As an aside, when I rotated *this* one 180 degrees it initially worked extremely well for me, and the crater instantly appeared in place of the mound. HOWEVER... when I then started experimenting with flipping it and mirroring it, my brain just decided at some point that it was indeed a mound, and once that decision was made :) it wouldn't allow me to 'flip' my perception. A bit like the related optical illusions - like that spinning dancer thingummy.

Quite disturbing when it happens suddenly and you can't easily reverse the perception...

I'd invite others to try it and report back..

2011-Aug-21, 08:27 PM
Since I never see this illusion with detailed features no amount of twisting could make it a mound for me. Even with featureless ones the illusion is rare. Rotating gets rid of it every time. Usually rotating back keeps the correct interpretation. If not another round of rotation does the trick. Then I've been in this hobby for nearly 60 years so that may be part of it. Back then I used a home made 6" f/12 scope. I never saw craters as mounds but my dad often did. He knew it was wrong but couldn't shake it. Optically the scope was excellent but mount was horrid. I put it on a better mount and my dad no longer had the issue. Maybe he never found focus due to the shaking and the soft image caused it. Fits my inability to see the illusion when detail is present. Our eye/brain combination is really a rather poor tool compared to what we assume it capable of. Dr. Richard Wiseman has done a lot of investigating in this area.


2011-Aug-22, 01:20 AM
The solitary crater, like above, usually appears concave to me. But a field of several, esp similarly-sized ones, then I see mounds.

Nowhere Man
2011-Aug-22, 04:02 AM
Looking at the image right now, I see a flat plateau surrounded by a moat. If I rotate it so that the lit edge is in the lower half of the image, I get a crater. This has always bugged me. Many of the crater pictures in Astronomy mag do this to me.


2011-Aug-22, 06:10 PM
When we are observing a live outdoor scene in nature, the light source is assumed to be from the upper portion of the image (the Sun never shines on us from below the horizon). That is why flipping the image vertically changes how we interpret shadows. We are wired to assume light comes from above.

2011-Aug-23, 05:00 AM
I find turning the image 90 degree turns the mound into a crater, a 180 degree doesn't work for me.
Oddly, if I cover either eye, I see the crater and continue to see a crater when I uncover my eye. If I close my eyes or look away for a moment, the mound is back.

2011-Aug-24, 10:10 AM
Thank you for seeing the illusion. I hope to not get used to this view in the future and see always the craters in this way. :)