Hi Frank, In general you are correct. I assume you mean azimuth when you say amplitude. I use the "Double Dawn" criteria. The vertical declining sundial will see sunlight when two criteria are met: 1. It is daylight at the location of the dial. The sun has risen and has not yet set. 2. It is daylight at the location of the equivalent horizontal sundial.
In both cases theoretical sunrise and sunset occur when the altitude is zero at that location. This time is determined by the the formula: Cos t = +/- Tan Lat x Tan Dec. The latitude of the equivalent horizontal dial is the corresponding angle, Lat-90. You have to bring the times to the same longitude so you need to calculate the difference in longitude (DL) for the equivalent horizontal dial with the formula: Cot DL or 1/Tan DL = Sin Lat / Tan Dec. Of course many people have reduced this mat to practical forms. I have a messy but useful spreadsheet. Fer De Vries includes the optional in Zon 2000. Just click on the black and white square symbol. Robert Hough wrote a DeltaCAD macro called "Duration" that does a great job of plotting the results with DeltaCAD. There are many options. For background see the presentation on "Sunrise Phenomena" available on my website. Regards, Roger Bailey Walking Shadow Designs www.walkingshadow.info -------------------------------------------------- From: "Frank Evans" <[email protected]> Sent: Monday, April 27, 2009 7:18 AM To: "Sundial" <[email protected]> Subject: sunrise > > Greetings, fellow dialists, > Am I correct in thinking that the earliest that a declining dial can > begin to tell the time is when the sunrise amplitude lines up with the > amount by which the dial is declining, for instance, sunrise bearing 080 > deg., dial plate declining 10 deg east? It seems that with both greater > and lesser sunrise amplitudes the dial will start work at a later hour. > Am I right? > Frank 55N 1W > > > --------------------------------------------------- > https://lists.uni-koeln.de/mailman/listinfo/sundial > > > No virus found in this incoming message. > Checked by AVG - www.avg.com > Version: 8.5.287 / Virus Database: 270.12.4/2081 - Release Date: 04/26/09 > 09:44:00 > --------------------------------------------------- https://lists.uni-koeln.de/mailman/listinfo/sundial
