----- Original Message -----
Sent: Thursday, May 30, 2002 5:12
AM
Subject: Re: Shadow Sharpener Again
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 -----
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.
