Sun dogs and Moon dogs explained

Sun dog effect seen inside solar halo from Wikipedia

When I was in one of my college physics courses dedicated to the study of optics, one of my most dreaded and beloved professors Dr. Joe Ferguson explained a strange optical solar phenomenon called a sun dog which to this day is something I look for every time I’m outside and the conditions are right.

Dr. Ferguson retired from the Mississippi State Department of Physics and Astronomy in 2005 but is still a professor I strive to emulate in many ways. His lessons were often very hard but very memorable.

Sun dogs (and moon dogs) are caused by the prism-like refraction of sunlight through hexagonal ice crystals in a specific configuration in the Earth’s atmosphere. Halos around the Sun and Moon are often seen 22 degrees out from the celestial body and the sun or moon dog (or parhelia and paraselenae) will be in a horizontal line coinciding with this ring.

There doesn’t have to be a halo or ring to see strange prismatic patterns 22 degrees from the sun or moon and they happen fairly often here in Houston Texas. So next time you are out and sun or moon are not too high in the sky and there are some clouds out and about try covering the sun or moon with your hand and looking a bit to the left or right of the sun or moon and you just might get lucky and see a sun dog or moon dog.

Check out the wonderful explanation of sun dog formation over at the Atmospheric Optics website.


Sun dog geometry explained - Atmospheric Optics

The Astronomy Picture of the Day for June 11th features a beautiful moonlit sky and one of these paraselene effects or a moon dog.

APOD for 2009 June 11

APOD for 2009 June 11

One Response to “Sun dogs and Moon dogs explained”

  1. kate roxburgh Says:

    Thanks for this clear explanation. I studied astrophysics a while ago and had forgotten how this phenomoenon worked! Now 20 years later I am a comic who is performing my own show and I decided to call it Moon Dog!

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