It seems to me that the indented dial-surface could serve a useful purpose, by ensuring that the nodus's shadow falls on that dial-surface whenever the Dyptich's flat horizontal surface has sunlight. . Maybe it's desired to make the indented dial-surface much larger than the hemispherical indentation that could be accomodated by the thickness of the horizontal plate of the Dyptich. . Of course not having the nodus at the center of the sphere would complicate the marking of the lines on the indented spherical surface, explaining why that isn't encountered more often. . It's related to stereographic lines, by having a projection-point not at the center of the sphere.
It sounds like a great idea for a birdbath or bowl, but it might be too distracting for a skateboard-park. . Michael Ossipoff Aprilis 7th, 2020 16 Tu Aries 19th On Tue, Apr 7, 2020 at 7:07 AM Peter Mayer <[email protected]> wrote: > Hi, > Dan-George Uza's recent post reminded me of a question I have. I was > looking at Mark Lennox-Boyd's lovely *Sundials* the other day, especially > photos of some of the beautiful Renaissance ivory diptychs, and realised > that I didn't understand the principles behind the small scaphes on many > dials. (Here's a Wikicommons photo of a Leonhard Miller dial). > The Greek and Roman scaphes, like the one in Dan-George's photo, were > hollow sections of either spheres or cones, with a gnomon at their centre. > Their mode of operation seems quite straight-forward. > But the scaphes in diptyches weren't like that. They appear to be tiny > slices of much larger spheres. And the gnomons are, of course, far from the > centre. The 'furniture' on several of them seems similar to stereographic > projections. But, since instrument-makers then were well able to make > astrolabes with stereographs, perhaps not. > So: can someone point me to an article which discusses these small > scaphes? Or, in the meantime, help me understand how and why they were > used? Why don't we make them any more? Or: are there any contemporary > examples, using, say, bowls, or bird-baths or skateboard parks? > best wishes, > > Peter > > ----------- > Peter Mayer > Department of Politics & International Relations (POLIR) > School of Social Scienceshttp://www.arts.adelaide.edu.au/polis/ > The University of Adelaide, AUSTRALIA 5005 > Ph : +61 8 8313 5609 > Fax : +61 8 8313 3443 > e-mail: [email protected] > CRICOS Provider Number 00123M > ----------------------------------------------------------- > > This email message is intended only for the addressee(s) > and contains information that may be confidential > and/or copyright. If you are not the intended recipient > please notify the sender by reply email > and immediately delete this email. > Use, disclosure or reproduction of this email by anyone > other than the intended recipient(s) is strictly prohibited. > No representation is made that this email or any attachment > are free of viruses. Virus scanning is recommended and is the > responsibility of the recipient. > -- > > --------------------------------------------------- > https://lists.uni-koeln.de/mailman/listinfo/sundial > >
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