A ring around the Moon

Have you seen a ring around the Full Moon? It's called a halo and it's an optical effect caused by ice crystals high in the Earth's atmosphere.

The Astronomy Picture of the Day web site has a photograph of a Moon halo.
The University of Illinois at Urbana-Champaign provides a good explanation.
Keith Cooley's site discusses Moon dogs, Moon pillars, Moon coronas and Moonbows as well as Moon halos.
Go to the atmospheric optics web site to find a program which simulates all kinds of ice crystal effects.
Robert Greenler's book Rainbow, Halos and Glories has lots of colour photographs and good explanations of a wide range of optical effects. It is published by Cambridge University Press.