I am also a fan of Italian and Babylonian hours as they are based on the significant solar events in the day. They cannot be legislated to be something different. OK, the definition of sunrise and sunset need to be arbitrarily defined. I use the mathematical zero altitude that ignores refraction and semi-diameter.
I agree with Gianni that establishing hour points along the equatorial line is a good practice. This shows quickly what the dial will look like and as a check on the calculation of the Italian and Babylonian lines as they cross on the equinox. Once while vacationing without access to files and computers, I set myself the challenge of designing a sunset sundial from scratch. This used the journeyman's procedure with the following steps. Calculate polar sundial using equatorial disc Solve for sunset times at solstices: Cos t = Tan Lat x Tan Dec Tabulate t = sunset time – N hours (15º/hour) Calculate Altitude for t knowing Lat & Dec Sin Alt = Sin Lat x Sin Dec + Cos Lat x Cos Dec x Cos t Calculate Azimuth for t knowing Altitude Sin Az = Cos Dec x Sin t / Cos Alt Project point onto wall knowing declination of wall, altitude and azimuth Plot the results and join the points for hours to sunset and solstice declination lines This exercise was successful and I left a mock up of the design for the owner of our rental suite. The results presented at the NASS Conference in St Louis. The presentation "Designing a Sunset Sundial from Scratch" is available on my website"www.walkingshadow.info" . Click on Walking Shadows (Sundials) for the index page of publications. The presentation contains drawings showing the geometry both spherical and plane. Also included are the program steps for a programmable scientific calculator like the ones distributed at the NASS Conference in Chicago. The conversion of the presentation to an article fro the compendium is stalled at 80% complete. This follows the classic 80 20 rule defining work progress. Regards, Roger Bailey Walking Shadow Designs -------------------------------------------------- From: "Frank Evans" <[email protected]> Sent: Monday, March 29, 2010 3:47 AM To: "Sundial" <[email protected]> Subject: how italian hours > Greetings, fellow dialists, > Following the pictures of the fine dial of Frank King in Selwyn College, > Cambridge (congratulations) I began to wonder how it was laid out. Most > of the commonly consulted books on dial construction (in English), > Waugh, Mayall & Mayall, Cousins, etc. do no more than glance at Italian > and Babylonian hours. Only Rohr has some account. His practical method > appears to be to find the time and nodus point of sunrise and sunset at > the solstices, count the hours back from them and join the winter and > summer nodus points for each hour. This seems a pretty journeyman’s > procedure (nothing wrong with that) but I wonder if there is some more > sophisticated method. > > Also, the assumption seems to be made that sunrise and sunset occurs > when the altitude of the sun’s centre is zero. This is far from sunset > in any practical sense. Any comments, please? > Frank 55N 1W > > > --------------------------------------------------- > https://lists.uni-koeln.de/mailman/listinfo/sundial > --------------------------------------------------- https://lists.uni-koeln.de/mailman/listinfo/sundial
