Moon Halo

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.

Moon Halo

A halo around the Moon can form when the ice crystals in high altitude clouds refract the Moon’s light into a ring. For complete details about this image, see Moon Halo. Photo copyright 2012 by Fred Espenak

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.

Fred Espenak

4 thoughts on “Moon Halo

    • Distortion caused by the 20mm wide angle lens makes the Moon appear offset from the center of the Halo. If I shot this image with the Moon near the center of the field, then the halo would be centered on the Moon.

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