Message text written by Frank Evans >Douglas Hunt asked about an analemmatic dial on a slope. Yes, well, I've been planning an analemmatic dial on a vertical surface for quite a while. Trouble is, the plan has not left my armchair for the wall opposite my kitchen window, where I can watch it while I wash up. I'm in 55 north, nearly on the Greenwich meridian. The wall declines 34 degrees east of south, taking the morning sun. The solution seems to be to work out the layout for a dial in 35 degrees south latitude, as suggested by Tom McHugh and Tony Moss. The peg gnomon thus sticks out horizontally from the wall. This solution was proposed to me by Fred Sawyer at last year's meeting of the British Sundial Society at Dunchurch when I asked for his help (I hope I'm citing you correctly, Fred).
< Hi Frank, When you asked about this problem, you stressed that you wanted to work out the answer yourself - a laudable goal - but now that you've presented it here, perhaps I should reiterate what I said - which is not quite what you have indicated above - and then you can work towards coming up with the same result! I did say that the gnomon should come out horizontally from the wall. However, the dial should not have the same layout as a dial in 35 degrees south latitude. Actually it should be the same as a dial at 28.393 degrees south latitude. The minor axis should indeed be at a 21.383 degree angle from the vertical, but note that the minor axis does not intersect the noon point. What you would normally think of as noon on the ellipse is actually 9:22 am. This 2 hr. 38 min. shift carries through all along the ellipse. The earliest time that needs to be recorded on the dial is 4:04 am and the latest is 4:04 pm. local apparent time. This is all because the wall you described is exactly parallel to a horizontal plane placed at latitude 21.383 S at a longitude 39.469 degrees to the east of your location (whence the 2 hr. 38 min. adjustment). Software that solves for all of these values immediately is available to members of the North American Sundial Society in a back issue of The Compendium. Good luck, Fred Sawyer
