Saturday, December 7, 2013

Sun Dog Prisms

Sun Dogs in Fargo, North Dakota.
February 18th, 2009.
The luminous patches that occasionally appear on each side of the sun during freezing atmospheric temperatures are called sun dogs.

According to folklore, sun dogs follow the sun like a dog follows its master.

Parhelia is the scientific term for sun dogs. Parhelion derives from the Greek parēlion which means beside the sun.

Sun dogs are an atmospheric optical phenomena. The luminous patch occurs when sunlight passes through and is refracted by hexagonal ice crystals.

The hexagonal ice crystals, falling with their long axis perpendicular to the earth, act as rotating prisms bending the sunlight. Because of the prism-like configuration of the falling crystals, sun dogs happen 22° to the left and 22° to the right of the sun viewed above the horizon.
Top View of Falling Crystal: Sunlight passing through two
surfaces of a rotating hexagonal ice crystal which is a 60° prism.

Sun dogs appear in art and literature. Shakespeare wrote about them in  King Henry VI:
Dazzle mine eyes, or do I see three suns?

King Henry VI, Part 3
Three glorious suns, each one a perfect sun;
Not separated with the racking clouds,
But sever'd in a pale clear-shining sky.
See, see! they join, embrace, and seem to kiss,
As if they vow'd some league inviolable:
Now are they but one lamp, one light, one sun.
In this the heaven figures some event.

'Tis wondrous strange, the like yet never heard of.
I think it cites us, brother, to the field,
That we, the sons of brave Plantagenet,
Each one already blazing by our meeds,
Should notwithstanding join our lights together
And over-shine the earth as this the world.
Whate'er it bodes, henceforward will I bear
Upon my target three fair-shining suns.

— King Henry VI, Part 3, Act II, Scene 1

Sunrise or sunset is the best time to observe sun dogs.