Some people think they are rare, but a ring around the Moon - a Moon halo - is not especially. However, they are beautiful and uncommon enough that its fun to get other folks to come out of the house to see them. The ring is made of Moon light that is refracted (bent) as it passes thru an ice crystal. Moon rings only occur if there are relatively high (and thus cold with ice crystals) wispy clouds. The common hexagonal shaped crystals bend the light by 22 degrees (two fist widths, if you want to measure it), so the entire ring is 44 degrees wide. I have only seen rings when the Moon was near full, and that seems to also be true for the photos I've found on the web. Any phase Moon should be able to generate a ring, but they are probably too faint except near full Moon. Perhaps Moon rings only exist because of the surge in brightness at full Moon! Will we now see a competition to detect the halo furthest in time from full Moon?
"I used a Pentax ME Super 35 mm camera with a 28mm lens set on auto to capture these photos. I under and over exposed a few of the shots by one notch. Running on auto the exposure seemed to be about 4 to 6 seconds. My camera was on one of Walmarts el-cheapo tripods, and I used Fuji 400 speed film. Photos were taken around 10:30 EST 01 04 04. More photos of this can be seen at my Moon_Watchers Yahoo group web page."