Thursday, January 15, 2015

Sun Dogs

Sun dogs are a natural phenomenon which can occur any time of the year but are more prevalent in winter.  They have been visible in the skies above Fredericton at least twice recently.  

Sun Dog Fredericton 8 Jan 2015 
Sun Dog Fredericton 14 Jan 2015

Sun dogs are more prevalent in winter for two reasons.  Because it is so cold, there are often ice crystals in the atmosphere.  The other reason is the low angle of the sun making them more visible from the surface of the earth.

Light passing through ice crystals is refracted and splits up into rainbow colours making the sun dog.  The dog itself is curved because the visible spectacle is usually in an arc around the sun.  That makes another dog on the other side of the arc which sometimes can be seen.  In the two cases above, we could only see a single dog.  In both pictures above the curvature is apparent.  The reddish part of the refracted light is on the inside of the arc, i.e., on the side closest to the sun.  

Why do we call them 'sun dogs'?  Etymologists are not certain but it may be from an old English term for mist, dag.  Others believe it is from Norse mythology where a constellation of two wolves chased the sun and the moon.  The proper name for them is parhelia (parhelion singular).  Other names include mock suns or phantom suns.

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