Rings around the moon

Aunt Simha and I were out for a walk tonight when we looked up and noticed this unusual phenomenon that we observe every few years, always in winter. After our walk I returned with a digital camera to take this picture. (I had to adjust the balance in Photoshop for the ring to appear in the photo, but I did not retouch the photo in any way. I only emphasized details that already existed in the photo, but weren't visible.)



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.

To the naked eye the moon actually looked smaller than it appears in this photo, and the halo was brighter. The effect was of a huge, cathedral-like dome in the sky, almost directly overhead, 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 illusion of a dome. Awe-inspiring.

In our experience the effect always happens at near full-moon (tonight is 14th Shevat).

Here's a profesisonal photo of a 22 degree halo from NASA:



Very occasionally there may be two rings, one within the other. I've never seen this in real life, but here's a nice picture I found on the Internet:



According to this site folklore has it that a ring around the moon signifies the approach of bad weather, and in many cases this may be true. You see, the ice crystals that cover the halo are actually part of thin, high-altitude cirrus clouds that generally precede a warm front by one or two days. A warm front is typically associated with a low pressure system, or in other words a storm.

When there's a ring around the moon
Rain or snow is coming soon

It is also sometimes said that the number of stars within a moon halo indicate the number days before the bad weather arrives. Well, ours had just one star (and another on the border) so let's see what happens in the next two days!