Hi Angelo,
I am not familiar with the sundial 'meridiana dei Cesari', but I do
know of the two types of ring dials that are available from
http://www.cris.com/~stircraz that Roderick Wall mentioned. If you
look at that page they have examples of both a universal ring dial
(they call it an "explorer dial") and an elevation ring dial (which
they call an "aquataine dial" I think). Which one are you interested
in?
The construction of the elevation ring dial is described in Waugh's
book "Sundials their theory and construction", and also in Mayall's
book "Sundials How to Know, Use, and Make Them". Mayall's book just
came out with a third edition that I haven't seen yet, and Waugh's
book is still available as well. I haven't tried to make one from
either of these but their descriptions seem good. Rohr's book is also
a good source for information about portable dials and how to
construct them. It has been recently reprinted by Dover.
The universal ring dial is described somewhat in Waugh's book, but not
in detail. You can get most of the idea from understanding equatorial
dials and armilary spheres. It should be almost enough to build one.
I built a prototype from this, but there is one aspect of the
universal ring dials that I still don't understand:
All of the old universal ring dials that I've seen pictures of had a
sliding hole for a gnomon rather than a wire. This hole would be
adjusted for the day of the year. The new dials that you can buy (for
example from the web page above) don't have the sliding holes, and
neither do the armilary spheres that I've seen. So, I haven't been
able to figure out what the sliding hole is for. Does anyone know?
Does it adjust for the equation of time? Does it help to orient the
dial to face north? Or does it simply make the reading of the dial
more accurate because it is a dot of light instead of a shadow?
- Bryan Loyall
p.s. Does anyone know of other good sources of information about
portable dials?
