> ... the shadow on a wall of any thin flat object parallel to the > wall is exactly the same shape and size as the flat object... > if you ignore fuzziness.
Indeed so but the crucial phrase is "if you ignore fuzziness" because if you DON'T ignore fuzziness a whole new world of interest opens up! The greatest examples which use an aperture nodus are the camera obscura dials in continental cathedrals. You will certainly find that a circular hole results in an elliptical spot of light even though the hole and floor are parallel. Moreover, the spot of light is an image of the sun so it is a bit unfair to use the term "fuzziness". Super pedants will note that the image isn't quite elliptical but is an ellipse with a fuzzy border whose width is the diameter of the hole. You can get a low-quality version of this effect with an ordinary circular nodus with a circular hole. If the nodus is parallel to the dial what you see is a roughly circular shadow with a roughly circular spot of light. Both circles are distorted by fuzz (I am happy with this term here!). The shadow is a false ellipse and the spot of light is too. The major axis of the shadow is aligned with the minor axis of the spot of light. The extent of these distortions very much DOES depend on the angle of incidence of the sun. Frank King Cambridge, U.K. --------------------------------------------------- https://lists.uni-koeln.de/mailman/listinfo/sundial
