View Full Version : the moon illusion: could it be localized and magnified?

10-19-2009, 06:03 PM
Everyone is familiar with the optical illusion that objects such as the moon and the sun appear larger at the horizon. According to wikipedia, this is known as the moon illusion.

I've been bored at work lately(at times, anyway) and have been pondering the hypothetical: Is it possible that, given the right conditions(atmospheric or otherwise), that the moon illusion would be magnified at a given spot on earth, and in a significantly larger portion of the sky(not just the horizon)? In effect, the moon and stars would appear brighter and closer at that particular spot during certain regular atmospheric/seasonal cycles. If it's possible, what kind of conditions would be required?

I can't remember enough of my college physics classes (the parts that talked about light and optics) to remember if light in the atmosphere was even addressed, or if any of this is possible.

10-19-2009, 06:17 PM
The Moon illusion (http://antwrp.gsfc.nasa.gov/apod/ap020130.html) is entirely a matter of human perception, not any properties of the atmosphere or celestial bodies. There may be some foregrounds that would affect our sense of "how big things look", and hence enhance the illusion, but it'd be very tricky to pin them down, since it's very hard to measure such things.

10-19-2009, 07:27 PM
In my hometown in SC there was one street I knew of where on certain nights the moon would be low on the horizon along its axis (the street's axis, not the moon's). There were low hanging hanging Live Oak limbs framing the moon from the last yard, overlooking the river. As you drove away from the moon, it would appear to increase drastically in size for about two blocks. Freaked out visitors.

10-19-2009, 07:35 PM
I think you're confusing the optical illusion with atmospheric lensing, where celestial bodies (the sun and moon, mainly) can appear distorted or even reflected as they sink below the horizon.

10-21-2009, 06:02 PM
On a related note, how does one increase how large the moon (or sun) looks compared to foreground objects when taking photographs or shooting video?

10-21-2009, 08:17 PM
By changing how big the foreground objects look. No matter what you do with your camera, the Moon always spans a half a degree in the sky, but the angle taken up by a tree (as in Mindfield's example) or a pair of lovers or whatever depends on how far away you are. So if you want, for instance, a picture of two lovers silhouetted in front of the rising full Moon, you set up your camera a few hundred yards away from the lovers, and zoom in until you get as much detail as you want.

10-22-2009, 07:12 AM
What Chronos said: use a telephoto lens.

Here's (http://blogs.fayobserver.com/backyarduniverse/2008/12/12/tonights-big-bright-eastover-moon/) a good example. You can see that if you cropped even tighter (equivalent to using a longer lens), you could get the moon to almost fill the width of the frame while the vehicles below still appeared pretty small.

10-22-2009, 09:00 AM
I think the Ebbinghaus illusion (http://www.illusionism.org/Shape-Distortion/Ebbinghaus+illusion/) is essentially the same thing.