Dodwell's dial is novel in several respects. His most radical
innovation is to cut Ferguson's dial in half (that's how I imagine it, any
way!) and to transpose the two halves. This permits his dial to operate
from earliest sunrise to full sunset. The shadows are cast by two vertical
gnomons 7.5 cm (3 in) high on the outer edges of the dial. The dial itself
is not a quarter of a cylindrical surface. After MUCH fruitless curve
fitting, I've concluded that Dodwell probably used what Piet Hein later
christened a 'superellipse', that is a curve which uses the formula for a
circle but with exponents higher than 2, in this case, 2.5 (i.e. x ^ 2.5 +
y ^ 2.5 = 1). There is a gain and a loss in this. The dial is much easier
to read. But the distances between hours are unequal and thus this most
difficult aspect of construction is made additionally complicated. I've
made a rough-and-ready model of the dial using large diameter PVC pipe and
find it generally satisfactory. Dodwell has also displaced the hour lines
to adjust for the 4 degrees that Adelaide is to the west of
its--non-standard--Standard Time longitude of 142 degrees 30 minutes.
Dodwell's other innovation is to dispense with the nodus and unfold
the EoT curve in much the fashion that Art Carlson suggested is possible.
The months march in succession down the dial and only the constructor has
to worry about which particular point on the edge of the gnomon is casting
the shadow on any particular day.
Finally Dodwell's design seems an especially robust one for public
display. Being in a quiet and obscure corner of the parklands helps, no
doubt.
The area in which the dial most signally fails the Tony Moss Test
is in the difficulty of construction. Perhaps a Dodwell (that is: full-year
EoT, without nodus) version of the Ferguson dial would be suitable, needing
only to be made in northern and southern hemisphere versions to be usable
at--am I right?--any latitude.
cheers,
Peter
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Peter Mayer | [EMAIL PROTECTED]
Politics Department |
University of Adelaide | 'phone:+61.8 8303 5606/5610
Adelaide, SA 5005 | FAX: (+61.8) 8303 3446
AUSTRALIA |
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