The sun is subject of the same effect. When the sun is low on the horizont it may some times look big, and even huge. Making a picture with a camera of it shows

The sun is subject of the same effect. When the sun is low on the horizont it may some times look big, and even huge. Making a picture with a camera of it shows only a small dot which is disapointing. Another data is that the percieved diameter vary from day to day when looking at the moon or sun from the smae spot. This excludes a psychological or perspective effect. The last relevant experimental data is that when the moon or sun appear big and when you look at them through a small hole, they appear small again. It works also when looking through a small hole made whith your fist. My idea is that this effect is due to the combination of two factors. The first one is strong humidity gradients of the air which can vary from day to day. Living at the sea border, these gradients may have bigger amplitudes. The same magnifying effect can be experienced when looking at the same mountains on a long period. I once saw a big low depth aquarium (20cmx1.5m). I could see the back of it through the water or by looking just above it. There was a huge magnifying effect when looking through the water giving the impression that the things in the back were much closer and bigger then they really were. The effect was still visible with only one eye open but with a slightly smaller magnitude. Looking through a small hole I made with my fist, the magnification nearly disapeared. The second factor is due to the eye apperture and how our brain process Input information. The exact mechanism is still unclear. This would explain that cameras don't see the effect or that it vanishes when looking through a small hole. I suspect that our eye and/or brain is able to restore focus of images by deconvolution to compensate out of focus images due to big apperture which we need in low light. Our eye doesn't work exactly like a camera. It has a powergul image processing unit behind the captor. The wide opening of our pupil would allow to capture light of the moon from a much larger surface than a conventional camera. And this would make our eye more subject to magnifying effect resulting from humid air refraction. looking though a small hole reduces the surface (apperture angle) of our pupil and reduces the effect. This idea should be tested. The first test should be to verify the correlation of humidity gradient of the air with the magnifying effect. This could be done in a lab. the second test should verify the correlation of pupil apperture with the magnifying effect. Changing the ambient light in combination of a fixed but significant humidity gradient should do the trick. The fact that not everybody see the magnification effect could be due to varing pupil apperture.