Published: 2008-01-28 09:03:20
Updated: 2008-01-28 09:03:20
Posted January 28, 2008 9:03 a.m. EST
By Joyce B
MIKE MOSS SAYS: Joyce, You likely saw a 22-degree halo, an optical effect produced by a shallow layer of cirrus cloud composed of well-formed but poorly aligned hexagonal ice crystals. The crystals act as prisms that refract light from the moon as it passes through the cloud. There is an angle of minimum deviation near which many of the light rays become concentrated, and that shows up as the bright ring you saw - it often has some rainbow-like coloring (reddish at the inside and bluish toward the outside) since different wavelengths are refracted at slightly different angles in a process called "dispersion."
Because there is an angle of minimum deviation of about 22 degrees, light from the moon that passes through the clouds inside the halo is directed away from your location and the thin clouds inside the halo may appear dark or invisible from your perspective, while clouds outside the halo may be more visible because the light can be refracted at angles greater than 22 degrees and passed down to your location. For example, if you look about halfway between the moon and the halo, you are looking toward an area where light from the moon would have to be refracted at an angle of 11 degrees to reach your eye - since the minimum angle is 22 degrees, no light is visible to you from this direction. Looking outside the ring at an angle of 35 degrees from the moon, however, some light can be refracted toward you. Note that this discussion applies only to the cirrus cloud composed of well-formed crystals. If there are other clouds close to the moon that are composed of different kinds of ice crystals or water droplets, those may scatter light in a different manner and they would be visible.
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