Bill Walton writes: > The much increased precision of the Shadow Sharpener is obtained > by casting a pin-hole image of the gnomon (wire, ball or building edge) in > the center of a pin-hole image of the Sun (as described by Charles). > ... > A wire or a bead that subtends 1/2 degree (1/8" at a distance of 14") when > viewed from the scale or surface on which the shadow falls, will just > obscure the Sun (as pointed out by Art Carlson). It will cast a tiny, dark > umbra that can be closely read, but not as closely as the centered image of > the Shadow Sharpener.
I believe the configuration that can be read with the most accuracy would be a wire or bead subtending just under 1/2 degree, viewed through a pinhole. Perfectly aligned, the image would be a uniformly illuminated ring. The slightest misalignment would result in one side being brighter than the other. This should be much easier to detect than the deviation of a thin shadow from the center of the sun's image disk. --Art Carlson
