> When I first started using cables for my styles I thought a lot about what > would be the correct diameter cable. On one hand, I wanted the cable to be > as thin as possible, because a thin shadow gives a more precise time reading > than a thick one. On the other hand, if the shadow is too thin, it makes > for difficult time readings from a distance. Not knowing the resolution > formulas that some of you have mentioned, I determined the cable diameter by > experimentation.
I'm not sure how to solve this problem mathematically. Maybe that Compendium article does it. I just tried different diameter rods the design distance from a piece of sheet metal until I got a combination of umbra and penumbra that I felt was best for accurate reading. Exactly what you did. I ended up using 1/8" (3.175 mm) diameter rod to cast the shadow on the hour ring of my armillary sort of sundial (still at the machinist's, lo these many moons). The inside diameter of the ring is 6" (152.4 mm). The mathematical treatises presented of late are intended for line width and letter size rather than shadow size. I'm not sure how one would apply them to shadow size. John B
