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What is that ring (or rainbow) around the moon?

Tonight, the moon was full and the sky was clear. When I looked in the sky, the moon appeared to have a ring of light around it. The ring was not adjacent to the moon, but more like the outer ring of Saturn with blck sky between the ring and the moon. If you understand what I'm saying, can you e-mail me back as to why this condition exists?

The ring you describe is what atmospheric scientists call a "halo" or "glory." It probably looked something like this:

Moon halo
A halo around the Moon Credit: APOD

Halos are caused by the light of the sun or moon passing through a very thin layer of cirruform (ice-crystal) clouds in the upper atmosphere. The ice crystals refract the light of the moon, similar to the way water droplets in the lower atmosphere can refract sunlight to produce a rainbow. Just like a rainbow, strong halos can have bands of color in them, due to slightly different refractive properties of the ice crystals for different colors. Essentially, halos ARE rainbows caused by primary refraction in ice crystals.

Some interesting facts about halos: Halos always occur exactly 22 degrees away from the sun or moon. Occasionally, intense halos can be double halos, just as intense rainbows can be doubled. Intense halos can also produce "moondogs" or "sundogs," very bright regions on the halo evenly spaced at 90 degree intervals around the halo.

Have a look at this link. There are some very nice pictures of halos and moondogs there, as well as some informative diagrams showing how the light is bent by the ice crystals.

Update by Lynn Carter (April 2002): Also check out this image, from Astronomy Picture of the Day.

January 1999, Lynn Carter (more by Lynn Carter) (Like this Answer), Dave Kornreich (more by Dave Kornreich) (Like this Answer)

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