Re: Shadow Sharpener

[John Carmichael]

Bill,   I think you found the solution!!!  Your idea is much better than mine. I just tested it. I was going to mark the hour points at the place under the style's penumbra where the sharpener's point of light disappears and as Roger pointed out, this point doesn't represent the center of the solar disk. I had been holding my pinhole too close to the dial face (about 20cm).  At this short distance the image of the sun is too small and round to see it when it is half covered. It never occurred to me to raise the sharpener higher so that the half-disc effect is noticeable. I feel stupid for not having thought of it. Good one Bill.  However, I think waist high is too high. I was still able to discern the half disk when holding the sharpener at 18 inches (45cm).  At this lower height, my wobbly hand movements aren't amplified so much and it will be easier to mark the ground while crouching down.   p.s. I hear the tour guide at Jaipur uses his finger tip as a shadow sharpener. The tour guide on Kitt Peak could carry a pinhole sharpener in his pocket. And then there's the "Noon Table" polar dial setup I mentioned before which would have a permanent pinhole sharpener over the apparent noon mark under the east style. (But now that I think about it, I don't think a little analemma on the time scale underneath would work.  I'll let you ponder about that!)   Good job Bill!   John   John L. Carmichael Jr. Sundial Sculptures 925 E. Foothills Dr. Tucson Arizona 85718 USA   Tel: 520-696-1709 Email: [EMAIL PROTECTED] Website: <http://www.sundialsculptures.com> ----- Original Message ----- From: [EMAIL PROTECTED] To: sundial@rrz.uni-koeln.de Sent: Tuesday, May 21, 2002 10:15 AM Subject: Re: Shadow Sharpener John, Roger and Friends,       The pinhole shadow sharpener is indeed a way out of the problem of finding the center of the Sun's penumbra.  By my experiments the center of the penumbra is well outside the dark shadow of a style (say 1/4 the width of the penumbra, a distance the sun travels in 30 seconds).         Try this experiment on the shadow of the roof of a building (I used the roof of my home, about 20 feet high).  Make a small hole near the center of a 3x5 inch card with a pencil point.  My hole was about 1/8 inch (2 mm) in diameter.  Place a dull white surface on the ground across the edge of the shadow and hold the card waist high above the surface so that the hole intercepts the light near the edge of the shadow.  Move the hole in and out of the shadow.  You will see a pinhole image of the Sun cut off by the image of the edge of the roof or gnomon.  The dark edge of the image of the gnomon will be away from the dark edge of the shadow.  Note that when the pinhole image shows half the image of the Sun (center of the penumbra) the image made by the hole in the card is well outside the dark edge of the shadow of the gnomon.         By marking the center of the penumbra in this way, one can accurately mark the  geometric shadow of a large style, or tell time from a sundial with a very large style.  You could use this technique on the Kitt Peak Sundial Project to lay out your lines.  Observers could also use the technique to read the dial accurately, but since few people would take the trouble to use a "pinhole," even if furnished, it is probably not a practical addition to your dial.        Keep up the good work!                     Bill Walton        Plymouth,  MA, USA        42 N   71 W Please note my new e-mail address:  [EMAIL PROTECTED]