Re: Shadow Sharpener Again

[Pete Swanstrom]

I apologize for my original explanation regarding the perceived edge of a shadow, it was wrong.  The corrected first paragraph should read as follows:     On a bright day with clear skies, the perceived edge of a shadow appears near the inner edge of the penumbra (near the umbra.)  With increasing haze or whenever the edge of a cloud passes by, the perceived edge of a shadow moves toward the outer edge of the penumbra (away from the umbra.)  This makes it impossible to design an accurate single-edged gnomon (without using a shadow sharpener.)  You eliminate this problem with a double-edged gnomon such as a rod, cable, pinhole, slit, etc. Thanks.  Pete S.   ----- Original Message ----- From: Pete Swanstrom To: sundial@rrz.uni-koeln.de Sent: Tuesday, May 28, 2002 10:14 AM Subject: Re: Shadow Sharpener Again     On a bright day with clear skies, the perceived edge of a shadow appears near the outer edge of the penumbra.  With increasing haze or whenever the edge of a cloud passes by, the perceived edge of a shadow moves inward toward the umbra.  This makes it impossible to design an accurate single-edged gnomon (without using a shadow sharpener.)  You eliminate this problem with a double-edged gnomon such as a rod, cable, pinhole, slit, etc.       For an analemmic/equatorial dial I made for a local park here in Boise, ID, I used a single-edged gnomon with an analemmic shape.  Prior to building the gnomon I carefully measured shadow widths under various haze conditions over a several month period.  For our local climate conditions, I found that I could best compensate for the average perceived edge of a shadow by adding 0.222° radius to the gnomon from solar centerline.  Trying to explain this another way, if you calculate a time mark's position on a sundial with a straight line drawn from the center of the sun across the top edge of the gnomon to the time mark, then when you build the sundial you should increase the height/width of the gnomon by 0.222° (measured from the time mark back to the gnomon) to compensate for the perceived position of the edge of the shadow.   Pete S.