Hi John, Your observation is correct that the hour points and date line scale with the ellipse size. For a dial with semi-diameter "a", the (x,y) coordinates of the hour points are (a Sin t, a Cos t Sin L) and the date line points are (0, a Cos L, Tan D).
There is however a clever way to use two sets of hour points ellipses to adjust for the daylight savings time change. Just change the scale of the date line when the time changes. Use one scale (a1) for the dates from October to April (or whenever the time changes) and the other (a2) for the rest of the year. The scale change would be subtle as the it occurs pretty close to the equinox line, the east west axis. The upper part of the date line for spring and summer would be scaled to one ellipse, the lower part for fall and winter would be scaled with the other ellipse. I do not know if Douglas Hunt uses such an asymmetric date line for his Sunclocks, but it would work quite well. Roger Bailey Walking Shadow Designs N 51 W 115 where we don't worry about winter use as the dials are snow covered -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Behalf Of John Carmichael Sent: September 3, 2002 9:29 AM To: [email protected] Subject: Re: Seeking any sundial enthusiasts in Turkey, (Istanbul area) Hi Doug A comment you made in your letter about Daylight Time scales on analemmatic dials was intriguing because just last week I was looking at the beautiful analemmatic dial in Louisiana on the Sunclock website picture gallery. http://www.argonet.co.uk/education/sunclocks/info/select.htm I struck me as very odd that the Standard Time ellipse has much larger radii than the smaller Daylight Saving Time ellipse, yet both ellipses use the same dateline. (I was going to ask Roger Bailey and Fer about it). From my limited knowledge of analemmatics, I think this design will not function correctly because if you change the size of the ellipse, you must also change the size of the dateline. This could explain why you've had complaints about Daylight Savings markings. The only way you could show both Standard and Daylight Saving would be to have removable numerals that use the same ellipse that you change twice a year. (But even then it won't show Standard or Daylight Saving unless corrected for EOT). It would be a shame to limit your sales market to those areas that only have Standard Time because you be abandoning 95% of your potential customers. I might be wrong though. (Roger, Fer any comments?) John John L. Carmichael Jr. Sundial Sculptures 925 E. Foothills Dr. Tucson Arizona 85718 USA -
