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Halo Around A Full Moon

potd - 11/21/10 From The Astronomy Photo Of The Day:

“This fairly common sight occurs when high thin clouds containing millions of tiny ice crystals cover much of the sky. Each ice crystal acts like a miniature lens. Because most of the crystals have a similar elongated hexagonal shape, light entering one crystal face and exiting through the opposing face refracts 22 degrees, which corresponds to the radius of the Moon Halo. A similar Sun Halo may be visible during the day.”

Another common term is ‘Moon Dog’. I’ve seen this occurrence about three times :)