The "ring around the moon" effect (or "lunar halo") is caused by the refraction of moonlight by ice crystals in the upper atmosphere. The regular hexagonal shape of the crystals results in a focusing of the light into a ring. The angular size of a ring is generally either 22 or (more rarely) 46 degrees - ours tonight being the latter. (Very occasionally there may be two rings, one within the other.) In our experience the effect always happens at near full-moon (tonight is 14th Shevat).
To the naked eye the moon actually looked smaller than it appears in this photo, and the halo was brighter. As the moon was almost directly overhead, the effect was of a huge, cathedral-like dome in the sky, with the round moon being a Pantheon-like opening to a bright sky (or heaven) beyond. Note that the inner edge of the ring is distinct, while the outer edge is blurred, which contributes to the dome illusion. Magnificent.
I have written a little more about this effect on Uncle Jonathan's Blog.
