Lunar Corona ~ An image from Canada by Lauri Kangas.

"We had the temperature plummet last night and an interesting ground fog formed.  I have attached a photo of the lunar corona.  What was really interesting was the beautiful colour of the large blue ring to the naked eye.  I could see the small water particles with my LED flashlight swirling around the camera equipment.  I had to centre the moon in my camera viewfinder to avoid lens flare.  The lens was a Canon 17-40mm set at 27mm, f/4.0, 1 second, ISO 2500 using a Canon 5D MKII."

 ©Jim Grant, shown with permission
IRIS Mie scattering calculations for monosized drops at left and ones with a 20% standard deviation Gaussian distribution of sizes at right.

Monosized drops from freshly formed fog or cloud give the purest - many ringed - coronae.
Atmospheric
Optics
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In old fog the drop size distribution widens and the mean size increases. Coronae from the different drops have different sizes in the sky. Their overlap can give a ring less blur.
                
Coronae arise from diffraction of sunlight or moonlight by clouds of randomly spaced cloud or fog droplets.

Each droplet produces its own diffraction pattern. And what we see in the sky is a sky transform - the collective diffraction glints from millions of drops.