Warren Thom wrote: > > I have been thinking about a dial described by Ing. Gianni Ferrari in > the March issue of the NASS Compendium. The article is based on > Ptolemaic coordinates. A background was laid by Fred Sawyer in the > September 1998 issue. Starting simple with the dial on page 19 and > labeled figure 12. Questions: > > (a) Can (should) the time line be extended upward to the south to give > hours after 6pm during the summer? (I think so.) > > (b) With corrections for the equation of time and longitude difference > from the standard time zone meridian on the declination scale, by moving > the E-W rod up or down (like an analemma or figure 8 fashion) -- can the > time show clock time fairly accurately? > > (c) How might I start, if I wished to look at Italian hours (hours to > sunset) on any of the dials described in the article on Ptolemaic > coordinates? I know Italian hours can be considered as great circles on > the globe. > snip....
> Thanks to all -- Warren Thom (Lat=41.649 Long=88.096) Hello Warren, Still I need to study the Ptolemaic coordinate sndials but I will try to answer your questions. (a) For the late afternoon hours and early morning hours the time scale may be extended upwards. In the article is pointed to the Parent dial ( 1701 ), but also Samuel Foster has described the principle of this type of dial. ( 1654 ) ( Rectilineal or Diametral Horologiography ) (b) To correct for longitude easily can be done. That correction is a constant value all the year, so add or subtract the longitude correction to each hourangle t. To my opinion a corretion for EoT by moving the E-W rod isn't possible. In other types, where you have a series of date lines, it is possible to correct for the EoT. (c) In the types where you have a series of date lines in principle it is possible to draw Italian hour lines. Proceed as follows : For a certain Italian hour calculate for a given sun's declination the hourangle t of the sun and calculate one point for these sun's declination and hourangle. Proceed with another sun's declination for a second point and so on. Connecting all these points with a line give the Italian hourline. Many other types of lines can be calculated with such a procedure. Best wishes and happy dialling, Fer. -- Fer J. de Vries [EMAIL PROTECTED] http://www.iaehv.nl/users/ferdv/ lat. 51:30 N long. 5:30 E
