> The longer the shadows become relative to the length of the
> stick the less distinct the shadow is. I viewed the shadow
> from a thin wire rod on the snow and very quickly there is no
> distinct shadow at all that could be used ....
If the gnomon is a pole (elongated cylinder), the axis of the gnomon is always
centered in the shadow. Gnomons with a few other cross sections share this
property: square, rectangle, hexagon, and so on.
However (as may have been mentioned on this list a few years ago), if the
gnomon's cross section is a triangle, trapezoid, pentagon, etc., the axis will
be OFF CENTER in the shadow at certain times of day, leading to errors in
reading the sundial.
It's neat that, in a horizontal dial, the rectangular cross section (or a
"sword blade" gnomon) can compensate for the varying projection distance of the
shadow:
(1) Around noon, when the projection distance is least, the silhouette of
the gnomon is thinnest. We want this because the hour and minute lines are then
more bunched together.
(2) In early morning or late afternoon, when the projection distance is much
longer, the silhouette of the blade is fatter. This keeps the umbra visible.
-- Roger
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Roger W. Sinnott, Senior Editor
Sky & Telescope magazine
Sky Publishing, a New Track Media Company
90 Sherman St.
Cambridge, MA 02140-3264
Tel: 617-864-7360 x2146; Fax: 617-864-6117
E-mail: [EMAIL PROTECTED]
Website: http://SkyandTelescope.com/
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