Folsom Street Shadows
Down, back, right, forty-five degrees was the mantra.  That was the angle of the sun we used to cast shadows in architecture school.  At that angle, the length of a object's shadow would be the same length as the object itself.  The shadow would be at a forty-five degree angle on the adjacent surface (if the surface is flat and perpendicular to the object).  If the surfaces receiving the shadow were on varying planes, then the shadow shifts and modulates defining its features.  Light and shadow define how we see form and therefore how we see architecture.  See an example below of how we cast shadows by hand did it by hand.

Beaux Arts Shades and shadow

I was among the very last generation of students who received formal training in the Beaux Arts technique of casting and rendering shades and shadows.  Considered quaint and outdated, it was soon cast aside in favor of more relevant coursework.    This illustration is from a 1896 translation by Julian Millard from a Beaux Arts text, "Shades and Shadows".  See an on-line version here.    

Quaint yes, but at Mock/Wallace, when I use free software like Google Sketchup to cast shadows, I still use the classic down, back, right forty-five degrees. 
San Francisco Civic Center Lantern
Postscript:  I'm glad I don't have to draw this shadow. 

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