We have had many discussions on this list on how to determine the declination of a vertical wall. Let me suggest a new technique for not only wall declination but also location (latitude and longitude). Use the satellite images on "Google Earth". An example is posted in three PowerPoint slides at: http://www3.telus.net/public/rtbailey/WalkingShadow/WallDecG.ppt
For my "Timelines" presentation, I was interested in analysing sundials on mosques in Istanbul. When I took pictures of the three sundials on the New Mosque "Yeni Camii", I did not take field measurements to determine the wall declination. No problem. Start up Google Earth and fly to Istanbul. Without rotating, just using the X/Y controls, zero in on the New Mosque and save the image. You can insert the saved image into a PowerPoint presentation, add the North South vertical axis, draw a line along the wall and determine the wall declination, perpendicular to the wall. Then start up Zon 2000 and enter the latitude and wall declination from the Google image into Fer de Vries program. Calculate for the example sundial: local hours, Italian hours and Islamic Prayer times. Clip the Zon drawing, save it as a dxf file and load the dxf file into Delta Cad. There you can trim the lines to recreate the dial on the wall. Select all and copy the lines as an overlay on the photograph (corrected for perspective) in your presentation. The Google Earth technique is sufficiently accurate with large buildings and high resolution images. I have used it successfully for three mosques in Istanbul and the Klementinum in Prague. It even works for my home! The full story will be shown in Vancouver. A conclusion from this analysis is the sundials on the mosques in Istanbul show the mid afternoon prayer time "Asr" as in the standard horizontal shadow length definition. But a different convention was used in Istanbul for "Zuhr", the prayer just after noon. The Zuhr lines are not based on the altitude giving the horizontal shadow length as the noon shadow length + 25% of the vertical gnomon height. Regards, Roger Bailey Walking Shadow Designs N 48:38:22.90, W 123:24:10.80, declining east 90 degrees by Google Earth --------------------------------------------------- https://lists.uni-koeln.de/mailman/listinfo/sundial
