Once in a Blue 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.

Useful links

• The Astronomy Picture of the Day web site (http://antwrp.gsfc.nasa.gov/apod/ap000515.html) has numerous photographs of Moon halos.
• The University of Illinois at Urbana-Champaign (http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/opt/ice/halo/22.rxml) provides a good explanation.
• Keith Cooley's site (http://home.hiwaay.net/~krcool/Astro/moon/moonring/) discusses Moon dogs, Moon pillars, Moon coronas and Moonbows as well as Moon halos.
• Atmospheric Optics (http://www.atoptics.co.uk/) has software 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.

Once in a Blue Moon Who wrote this?

Copyright © 2002 by David Harper and L.M. Stockman
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