When light from the moon hits ice crystals in our upper atmosphere, the light diffracts in much the same way as sunlight does to create rainbows. Since these ice crystals typically have the same structure (a hexagonal shape) the size of the halo is always the same size, 46˚ in diameter.
Accompanying this Lunar Halo is also a Moon Corona (the smaller, brighter circle). Moon Coronas are caused by a thin layer of clouds present in the upper atmosphere. These objects are only a few degrees in size.
Looking closely at the very center of the image, there is an extremely bright and small circle. Zooming in on this area reveals the full moon, only 0.5˚ in size.
In this image, the moon is in the constellation of Gemini. Within the lunar halo (to the lower left of the moon) is Mars.
Location: Friend's Creek, IL
Camera: Canon XSi (Hap Griffin modified)
Lens: Sigma 10mm f/2.8 Fisheye
Mount: Still Tripod
Shot: 1x15 seconds, 1x6 seconds, 1x2.5 seconds, 1x1/1600th second. All at f/4 @ ISO 200