Re: Shadow Sharpener

[Wuwalton]

      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

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