Moon Halo
Perhaps you are someone who has noticed an ominous halo around a full moon on hazy nights of the year. Perhaps you wondered where it came from, that there must be some logical explaination. Or, perhaps, you thought it signaled the coming of the apocalypse. Well, the moon halo is in fact no such doomsday omen, but a perfectly explainable atmospheric phenomenon, and it doesn't just happen with the moon, but with the sun as well.
The moon halo, if you are lucky enough to see it, is a fantastical sight composed of a bright circle in the sky wrapped around the moon. When the conditions are just right, there may even be two halos. The cause of this halo is surprisingly simple. Sunlight, reflected off of the moons surface, is refraced by ice crystals in the atmosphere. On humid nights, when there's some moisture in the air, that moisture forms ice crystals at the higher altitudes where the temperature is cooler. This cooling with rising altitude is another atmostpheric phenomenon. When the sunlight reflected off of the moon travels though hexagonal ice crystals, the resulting rays emerge at 22 degrees from the plane which they entered in. The result is that light is focused at a regular distance all around the moon. A second halo may result from a further angle of refraction. The halo is most spectacular on nights when the moon is full.
posted by Thomas @ 12:09 PM
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