Lunar Halo

 
I happened to look out the window and caught a glimpse of a huge ring in the sky.
My first thoughts: 1-Moonbow! 2-Get camera 3-Get Tripod

It turns out it's not actually a "moonbow" it has the slightly less catchy title of a 22° halo.
The halo is caused by sunlight being refracted by the thin clouds. With a lunar halo, the sunlight is first reflected off the moon. Thin clouds, very high in the atmosphere (Cirro-stratus) are composed of ice crystals. They bend light at a 22° angle, which creates a solar halo or lunar halo that is 44° in diameter.
(paraphrased from Light and Color in the Outdoors, M. Minnaert)
Physics and cloud types aside, it was the first one I've seen so clearly and complete, and it was beautiful.  (Photo ©Lee Irvine www.pelicanimages.com )