Hello Charles and all, I tried your Shadow Sharpener test today and was amazed at the result. I now see that the small hole acts as the "lens" of a pinhole camera projecting the image of the sun onto a plane. I took a piece of cardboard and punctured a series of holes of different diameters. About 2 mm was best. Half a barley corn was at the upper limit to form a pinhole image. Smaller holes than ~2 mm resulted in images that were too small and dim. Standing by the shadow cast by a tall building, I brought the cardboard and backup screen from full sunlight into the penumbra. I was amazed to see the image of the sun's circular disk being eclipsed by the vertical shadow, but coming in from the other side as the image is reversed. The image did not dim as it did with larger holes but it was cut off. Moving farther into the shadow penumbra, the eclipsed portion grew until halfway through the image was a vertical hemisphere. Moving farther into the shadow the eclipsed portion grew and the image shrank until the narrow edge sector disappeared completely as the hole entered the umbra of the shadow.
By observing the shape of the image of the sun, the middle and the two edges of the penumbra were easily determined to a precision better than one inch over the width of the penumbra (about 2 ft). This gives a precision equivalent to about 5 seconds per day for this size of shadow. The bisected hemispherical image would be the appropriate position for the sun as a point source with no semi-diameter correction required. Fixing the hole and screen and timing the movement of the shadow would give even more precise results for these events (but if I had a watch, why would I need a sundial). I repeated the test using a magnifying lens (old timer's reading and nit picking glasses). The effect was the same. The glass gave a much brighter image but lacked the infinite depth of field of the pin hole. I obtained better results using the pin hole. Thanks for providing the tip. Now that the precision question has been resolved, we are back to the challenge of building an accurate, practical device that will not cause blindness among its users. Roger Bailey Walking Shadow Designs N 51 W 115 At 12:31 AM 5/3/99 -0400, [EMAIL PROTECTED] wrote: >As a test, you could make use of a device I read about called a "Shadow >Sharpener", supposedly used by Chinese astronomers centuries ago. > >The shadow sharpener is simply a stiff sheet of opaque material with a >clean edged round hole in the middle. I made one from the thin cardboard >backing of a pad of paper, with a hole about half a barleycorn in diameter. > >To use, hold the sharpener a short distance (1 to 3 feet) away from the >fuzzy zone, with one side facing the sun. An image of the sun will appear >on the surface the shadow is on. Move the sharpener around until the image >of the sun is bisected by the edge of the roof of the building, (or >whatever comprises the gnomon). I believe then that the image of the >gnomon will be in the exact place it would be if the sun was a point >source. Try this and see if this point falls in the same place that your >eye estimates it would. > >Charles >
