Dear Shadow Watchers, Some time ago an idea for a nodus on a Large Sundial appeared to me. It is the shadow of a thin-walled transparent sphere. This can be seen simply in the shadows of soap bubbles and transparent round helium balloons. This is not considering it a lens, just the shadow cast. As a lens, such a structure is a diverging lens, but the shadow has a bright spot when the distance is proper for it's size. The central light bundle is little refracted and outer bundles are reflected at Brewster's or other angles, leaving a surrounding shadow. It appears much like the shadow of a disk with a hole in it, but works the same with light from all directions. At a proper, and quite large distance, the central aperture acts much like a pinhole. I've not tried large spheres, but possibly those of glass or plastic, especially coloured transparent materials would give a distinctive small bright spot surrounded by shadow. I've no idea how to calculate the proper dimensions, size, wall thickness, surface shinyness, etc. But, it could work very well at hundreds of feet away. If someone figures out how to calculate this effect, I'd be pleased to hear about it.
Thanks! Edley McKnight PS. John, I decided to let the list know. --------------------------------------------------- https://lists.uni-koeln.de/mailman/listinfo/sundial
