Dear Frank, Chris and the List,
   
  An experiment to determine the position of the shadow edge was reported by 
Allan Mills in his article "Sunlight and Shadows - or, what's the point of big 
sundials?", British Sundial Society Bulletin 96(1) pp.22-27 (February 1996).  
Allan's key result for the question now under consideration is: 
   
  "the perceived edge of the 'optimum shadow' corresponded to a point where 
just a little of the sun remained uncovered. ...... but correspond to about 12% 
of the Sun's diameter being uncovered."
   
  Allan's paper considers the circular shape of the light source, the effects 
of limb darkening and background illumination, and the non-linear response of 
the eye.
   
  I haven't tried the calculation to convert the 12% into a time offset.
   
  Regards,
   
  John
  --------------------------------------------------

Chris Lusby Taylor <[EMAIL PROTECTED]> wrote:
  Dear Frank,
Try as I might, I can find no published information on where people judge
the edge of a shadow to be. It would certainly be an interesting experiment.
As John Davis observed, the human eye is very non-linear. Experience with
photography seems to suggest that, like the ear, it is logarithmic. I
imagine one's judgement of a shadow would to some extent depend on how clear
the sky was and might well vary from person to person. For instance, someone
well experienced with the phenomenon might have trained themselves to judge
the 50% point more accurately. My own informal experiments with a shadow
sharpener suggest that I tend to judge the edge at about 10% of full
illumination, which corresponds to about 70% of the 64 seconds passed, or 45
seconds, but it may depend on a lot of factors including the colour and
optical qualities of the surface - white matt paper v polished brass, for
instance.

One point you make that I don't understand is that you expect different
results going from dark-to-light than light-to-dark. Why should that be?
Shadows move so slowly that I'd imagine one would err towards the darker
side consistently regardless of whether the dark area is shrinking or
growing.

Regards
Chris

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