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A Halo Around the Moon
http://antwrp.gsfc.nasa.gov/apod/ap081207.html

Explanation: Have you ever seen a halo around the Moon? 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. The 
town in the foreground of the above picture is San Sebastian, Spain. The 
distant planet Jupiter appears by chance on the halo's upper right. 
Exactly how ice-crystals form in clouds remains under investigation.


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