Fellow shadow watchers,
There's an interesting dial by John Davis on the cover of
the current NASS Compendium which illustrates a concept I encountered some
years ago when designing a horizontal dial with an 'overhung' gnomon viz. a
gnomon supported from above rather than from below. The dial was never built.
In the conventional dial, where the support is below the style, the shadow
plane rotates about the west edge or the siix hours approaching noon and the
eastern edge thereafter until 18.00 with an instantaneous interval where both
edges protect the well-understood 'noon gap'. The active style edges are
reversed in the 'overhung' gnomon so that the minutes approaching and leaving
noon overlap in the 'noon gap' area producing instead the 'noon overlap'
referred to by John in his accompanying article.
My recent adventure to install the dial on Spitbergen at 78° north produced an
interesting extension of the concepts surrounding 'thick gnomons'. The 24-hour
dial at Longyearbyen has both a noon gap and a 'midnight overlap' and a novel
way of dealing with the latter.
In recent weeks I've been working on three PowerPoint presentations* covering a
wide range of dialling concepts using animated diagrams where, among other
things, you can see the moving sun build up the solar noon analemma in the sky
in stages 'before your very eyes'! Part of this series contains three animated
slides illustrating the curious phenomenon of the midnight overlap in a unique
way that even a lay person can quickly grasp.
A short extract is available for anyone who would like to see it as an email
attachment of about 500k. The complete versions will eventually be available
on CD ROM.
* Mac Oglesby of NASS and Mike Shaw of BSS very kindly made many suggestions
and proof-read the results of this considerable labour.
Tony Moss
P.S. If there's some international XYZ code or somesuch for approving
PowerPoint presentations I'm pretty sure it doesn'y comply. ;-)
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