Warren: here's a dial that shows the current position of the Sun. It's an Oughtred Dial on a gnomonic projection (instead of the stereographic that Oughtred used). I've also added the current sunset/sunrise terminator and the area of the globe that is currently illuminated (ignoring refraction, the terminator is a straight line, as are all great circles on such a projection). The terminator is perpendicular to the shadow line. The distance between the terminator and the center of the dial is proportional to the height of the dial node and to the tangent of the Sun's altitude. Also, the point of closest approach of the terminator to the center goes through the altitude line that is 90 - the altitude of the Sun (note that on the dial, the Sun is at 30 degrees altitude and the terminator just touches the 60-degree line). I suppose one could devise a nomogram to make the calculation. Oh, I've also added Babylonian hours (this is supposed to be a sundial, after all). Brad
On Sat, Apr 3, 2010 at 6:30 AM, Warren Thom <thom...@gmail.com> wrote: > All, > > I have enjoyed the discussion about Italian hours. My first dial was an > hours to sunset dial on my garage door done in the 1970s. Mac Oglesby's > models and dials are inspiring to me. I had a globe parallel to the earth > in the 1980s that I liked to view on a sunny day. I could observe where > on earth the sun was overhead and where the sun was setting at that very > moment. About six years ago I worked on a design that would cast a shadow > from a nodus onto a map that would show where on earth the sun was > directly overhead. It was an ugly confusing map. My question is: > > Hour and prayer lines are fine. Is there other information, that uses > Italian hour calculations (or solar declinations) , that would be of > interest to a broad audience? Fred Sawyer created a universal dial of > modern hours based on a world map. Is it possible to create a "map" dial > that would show where the sun is setting at that moment? What could it > look like? Would it require a line shadow casting instead of a point? > Any thoughts? > > Thank you -- Warren Thom > > > On Sat, 03 Apr 2010 06:33:11 -0400, Gianni Ferrari <gfme...@gmail.com> > wrote: > > > In Ottoman Sundials we find often the Italic hours because these hours > > give > > the time to the sunset, when the Muslims must say the Prayer Maghrib. > > > > These hours are given not from 0 (sunset) to 24 (sunset of the next day), > > but, as Roger has written, in two cycles of 12 hours and are called > > Ezanic > > hours. > > > > So at sunset we have the end of the 12th hour of a cycle and the > > beginning > > of the 1st of the next. > > > > In such a way only at the equinoxes at noon the Ezanic, Modern (French), > > Babylonian and Temporary hours have all the value 6. > > > > In different days this is not true as you can see in the table attached. > > > > > > > > In Ottoman Sundials generally we don’t find Babylonian hours, while we > > have > > often the lines of the Solar Time that give to the Muslims the time to > > noon, > > that is the time to the Zuhr prayer. > > > > These hours are not numbered as in our clocks or sundials, but, > > generally, > > with the number of hour to noon (in the morning) or after it (in the > > afternoon). So : 5, 4, 3, 2, 1, noon, 1, 2, 3, 4, 5. > > > > > > > > They use “Arabic numbers” or, in oldest sundials, numbers written with > > “letters” ( Abjad method) > > > > > > > > Gianni Ferrari > > > > > > > > > -- > Using Opera's revolutionary e-mail client: http://www.opera.com/mail/ > > --------------------------------------------------- > https://lists.uni-koeln.de/mailman/listinfo/sundial > >
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