My request is that you will take the time to
"pull together" the results of your investigations, and publish, at
least via email and the Sundial List, a discussion of this
fascinating topic, including the implications for sundial designers.
I have tried some of your experiments myself, but confess that I'm
not sure how to interpret what I see.
Mac and Others,
Since the last description of an experiment on the difference in time between the passing of the center of the penumbra and the passing of the perceived edge of the shadow of a style, I too have been thinking about the implications for sundial designers. A 30 to 40 second difference will be of little consequence on most dials of moderate size. As Roger Bailey pointed out, EoT error and other errors may be of comparable size. However, the article in the June NASS Compendium "Subtleties Of Shadows: The Penumbra At Noon" set me to thinking about this phenomenon in more detail. In reading the time on a very large sundial, the "pinhole" shadow sharpener effect can reduce the random errors in reading a fuzzy shadow and thus lead to more precision (say down to +/- 10 sec). At the same time, knowledge of the penumbra effect can account for some of the systematic error (difference between the geometric shad! ow and the perceived shadow of about 30 sec) and thus lead to a more accurate reading of time.
As I said, I have been thinking about these things, as a different way of looking at the "Penumbra at Noon" effect and as to their implications on the design of large sundials and heliochronometers. I hope to do a few further experiments and some calculations, and to write these up in the form of a letter to the editor of the NASS Compendium.
Thanks for your encouragement, Mac.
Bill Walton
Plymouth, MA, USA
42 N 71 W
Please note my new e-mail address: [EMAIL PROTECTED]
