I think there is another option.
You can built a bifilar sundial with a negative thread.
The negative thread is behind the dial so that it is inside and may be
drawen on the wall or on the floor. The outside thread casts a shadow
through the stained glass and the shadow will cross the negative thread in a
point : the knot. The stained glass will be casted too and the hour will be
shown by the knot on this projected dial.
The negative thread may be hanging too, in this case the knot will be the
cross of the shadows of the 2 threads.
With these bifilar dials the stained glass may correspond to a dial with a
different declination regards to the wall or may be like an equinoctial
dial.
The bifilar dial may also have both the threads negative.
I have drawen some of these bifilar dials last year on the italian
mail-list. If someone is interested on them I can put the pictures on my
website or send them by e-mail.
I have no news about bifilar realized with negative thread.
ciao Fabio Savian
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Please, Fabio, place them on your (excellent) web! I am sure I am not
the only one interested in them!
Anselmo
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