Have you ever seen an enormous hazy ring surrounding the Moon? This is an atmospheric phenomenon that goes by a number of names including Moon Ring, Moon Halo, and Winter Halo.
The halo appears as a whitish ring over 80 times the diameter of the Moon, and centered on the lunar orb. It only occurs when a thin veil of high cirrus clouds covers the sky, which often precedes a change in the weather as a cold front approaches. The cirrus clouds are composed of hexagonal ice crystals suspended in the upper atmosphere. Each ice crystal acts like tiny 6-sided prism with 60° sides.
As bright moonlight shines through the ice crystals, some of the light rays are refracted or bent through an angle of approximately 22°. The actual angle of refraction depends on the color of the light. For instance, red light is refracted about 21.54° while blue light is refracted 22.37°. Since the Moon’s light is simply reflected sunlight, it consists of all the colors of the rainbow. This gives the inner edge of the halo a reddish tinge while the outer edge is bluish.
The resulting Moon halo has an apparent radius of 22° (or a diameter of 44°). It is several degrees thick because the Moon itself is 1/2 degree in diameter, and the ice crystals are randomly oriented in the cirrus clouds. These factors tend to spread the ring out a few degrees.
Moon halos are best seen when the Moon is within several days of Full Moon. Because the Moon is then at its brightest, it is easier to see a faint Winter Halo if the weather conditions are right. This 22° halo phenomenon is also visible around the Sun but is frequently overlooked because of the Sun’s intense glare. By covering the Sun with your outstretched hand it’s easier to spot a Sun Halo.