Hi ChiLian,
That's very interesting - I haven't heard before of a dial delineation being
"pre-distorted" in this way to allow for the penumbra effect.
Perhaps the reason that the 1 minute correction that you adopted was too much
is that your latitude (24.79 N) is so much less than the Shetlands at nearly 60
degrees N. Hence the shadow is much closer to the gnomon edge and the penumbra
effect is less marked. It is a complicated problem as human eyesight does not
respond linearly to varying intensity levels.
Regards,
John
--------------------------
Chiu ªô,Chi lian <[EMAIL PROTECTED]> wrote:
Hi!
>An article by Vicki de Kleer (BSS Bulletin 19(iii), pp.116-117, September
2007) describes an 1890 brass horizontal dial made by the London maker Charles
Baker for Sumburgh in the Shetland Islands at latitude 59 degrees 52 minutes N.
The dial has a comprehensive Equation of Time table engraved around its
periphery and also the two intriguing instructions:
"AM Sub.[tract] 1 min"
and
"PM Add 1 min". <
I designed an 8m sundial with solid gnomon for Nat'l Tsing Hua University
(Hsinchu, Taiwan, 24.79N, 120.99E) in April, 2006. I noticed the problem of
shadow. No one can tell where the shadow starts or where it is perfect. That
means no one can really deside by his bare eyes where the middle of the
penumbra is. Most people read the dial by the shadow edge and never care about
the middle of the penumbra. So I decided to use perfect shadow as time
indicater. I took the "one minute off" policy since the full sun angle is about
2 minutes. The policy was "mark the hour lines one minute ahead in the morning
hours and one minute behind in the afternoon hours". That is to draw 9:01 line
and mark it as 9:00, to draw 2:59 line and mark it as 3:00. So the sundial was
done.
However, it turns out that "one minute" off is too much. I overlooked the
fact that the sun, as a light source, is not square but circular.
Regards,
ChiLian
2008/2/10, JOHN DAVIS <[EMAIL PROTECTED]>: Hi Patrick, John C and the
List,
The problem of the shadow not properly filling the noon gap is the same
umbra/penumbra related one which affects the reading accuracy of dials with
'solid' gnomons at all times of the day. This is usually ignored by dial
makers but I know of one historical example where the maker seems to have
offered a possible solution.
An article by Vicki de Kleer (BSS Bulletin 19(iii), pp.116-117, September
2007) describes an 1890 brass horizontal dial made by the London maker Charles
Baker for Sumburgh in the Shetland Islands at latitude 59 degrees 52 minutes N.
The dial has a comprehensive Equation of Time table engraved around its
periphery and also the two intriguing instructions:
"AM Sub.[tract] 1 min"
and
"PM Add 1 min".
It seems to me that these instructions are implying that the true time is
being indicated slightly further into the penumbra area of the shadow than the
observer might normally expect. With a dial so far north, the effect of 'shadow
tapering' will be more prononounced than on most other dials.
Does anyone have a different interpretation of these instructions, or know of
other dials wich have similar ones?
Regards,
John Davis
--------------------------------
Dr J Davis
Flowton Dials---------------------------------------------------
https://lists.uni-koeln.de/mailman/listinfo/sundial
