Hi,
There are a number of threads which have come up in the past few
months to which I've intended to reply, but work, the holidays and a
research trip to India have all intervened. So, here with a bit of bottle
age are a few threads which bear on my primary subject: the Dodwell Dial.
i) the search for the beautiful, accurate dial.
The one bit of this thread which I have came, I think, from Tony Moss:
>What the world needs are some beautiful, accurate and durable sundial
>designs that can be >adjusted for any location, that tell Standard Time,
>and that can be mass-produced and >marketed like the current garden
>variety.
Actually, there is a second thread nested here, which I'd like to
revive and underscore:
ii) designs which are suitable for placing in public spaces.
Some time ago we touched on this problem. I seem to recollect a
comment from one of us about the vandalisation of even huge dials. What ARE
the most suitable designs for public dials? The Piet Hein dial on top of a
20 metre pole is pretty safe: but can you read it? I don't think we've
really had a sustained examination of this problem.
iii) unfolded EoT lines
While I was in India, Art Carlson raised this when he introduced
his 'Dali' dial and it prompted me--when I finally found it among the
400-odd messages which came in my absence-- to dig out my photos of
Dodwell's dial, get them scanned, etc.
>It seems reasonable to suppose that everybody has a pretty good idea of
>the date already, >so we are making the sundial do unnecessary work. If we
>make the user do this work >instead of the nodus, the figure-eight can be
>unfolded and made unambiguous. (I am sure I >have seem such a dial design
>somewhere, but I can't remember where.)
The Dodwell Dial.
The Dodwell Dial is germaine to all three strands of our
conversation. As it has probably not been described before, it may in any
case be of general interest to you. The dial is located in Adelaide, in the
Pioneer Womens's Memorial, just to the north of Government House. According
to the brass plaque beneath it, the dial was designed by George F. Dodwell,
the South Australian Government Astronomer in 1941 (a position which was I
fear abolished many a moon ago). [I'm appending two JPEGs to this section
of the message, and two to the next, which should make its structure and
operation transparent.]
The dial is a form of polar dial, made to appearances of bronze,
engraved or cast. It is about 40 cm (15.5 in) wide and 42.5 cm (16.75 in)
long and about 16 cm (6.25 in) from base to central ridge.
Kinship to Ferguson Dial...
Although the dial has a distinct kinship with Ferguson's Solar
Chronometer which is pictured on p. 192 of Cousins _Sundials_ (why isn't
THAT reprinted by someone??), Dodwell has introduced several innovations
which I think make his design superior.
As can be seen from the image I've appended, Ferguson's instrument
is essentially a segment (perhaps 10 cm? wide) of a semi-cylindrical
surface (that is: of a cylindrical tube, cut in half down its axis). At
each end of the segment there is a semi-circular metal frame whose ends are
joined at the diameter. (The shape is rather that of the protractors we
used when I studied geometry in school...). The dial has an adjustment
(something Dodwell's doesn't) to permit the plane of the dial to be aligned
parallel to the Earth's polar axis. Instead of the usual solid gnomon of an
ordinary plane polar dial, Ferguson has placed a rod in the axis of the
cylinder which is attached to the end frames. The rod is notched in the
centre to form a nodus.
Finally, Ferguson's dial has two printed cards (or probably two
sides of a single card) which have curved lines for the hour, half and
quarter hours which adjust for the equation of time and successive
'straight' lines for the changing declination of the Sun. Each card covers
half a year from equinox to equinox. The card can be 'shifted' to the left
or the right to compensate for local latitude.
[continued in part 2]
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