Hello Art, In my programs suite zwvlak95 there is a program spin.exe to calculate such a dial for a horizontal plane with circular scales of date and a vertical style. You also may name this type of dial an azimuthal dial. All is for download at my URL below.
Happy dialling, Fer. Fer J. de Vries [EMAIL PROTECTED] http://www.iae.nl/users/ferdv/ Eindhoven, Netherlands lat. 51:30 N long. 5:30 E ----- Original Message ----- From: Arthur Carlson <[EMAIL PROTECTED]> To: <[email protected]> Sent: Monday, January 17, 2000 8:28 PM Subject: Dali dials > A "normal" sundial has the gnomon coaxial with the Earth. This is > done to keep the errors with respect to clock time to a minimum during > the course of the year. If we have the ambition to make our sundial > read clock time to better than +/- 15 minutes, then we have to correct > for the Equation of Time. There have been many public discussions > here and private ones in my head about the best way to do this. > Simply reading a table or graph is inelegant and subject to errors of > sign. Methods which use the declination of the sun, either by using a > specially shaped gnomon or by observing the shadow of a nodus, rather > than an edge, are perhaps more esthetic, but they are inherently > ambiguous at the solstices and double-valued the rest of the time. > > One way of thinking about the nodus methods (which has come up here in > discussions of the EoT with respect to leap years) is that the > declination tells you what the date is, and the figure-eight-analemma > allows you to find (with the restrictions mentioned above) the EoT for > that date. It seems reasonable to suppose that everybody has a pretty > good idea of the date already, so we are making the sundial do > unnecessary work. If we make the user do this work instead of the > nodus, the figure-eight can be unfolded and made unambiguous. (I am > sure I have seem such a dial design somewhere, but I can't remember > where.) For example, the date-lines can be made concentric circles, > from Jan 1 innermost to Dec 31 outermost, and the EoT for each hour > marked as a nearly radial wavy line. We could even trivially > accommodate the change between standard-time and daylight-saving-time. > As a practical matter, I think it would be easier to read clock time > (corrected for EoT) from such a dial than from any alternative. The > freedom opened up by this arrangement is astounding: The date-lines > can have (nearly) any shape, and they could all have different shapes > (as long as they don't cross). The gnomon need no longer be parallel > to the Earth's axis; it doesn't even have to be straight! > > I could envision a Salvador Dali sundial, but maybe I should start > with something for John Carmichael: Draw the outline of Arizona many > times at different scales and put them inside of one another, but so > that all the Tucsons overlap, and of course properly oriented with > respect to the compass. Put an obelisk at the location of the > Tucsons. Label each outline with a date and calculate (the hard > part!) where the shadow of the obelisk will fall across that outline > for that date and each hour of the day. For each hour, connect the > points for all the dates and label the resulting wavy line with the > hour. Voila! > > I'd love to do the design myself, but realistically I know I won't > find the time any time soon, so I'd rather through the idea out to the > world. Is the description clear enough? (The idea is probably > between 500 and 2000 years old anyway.) > > Regards, > > Art Carlson >
