I agree with the recommendation on using the prime vertical to set sundials. John Schilke gave a presentation on this topic at the NASS conference in Tucson. My notes from the conference, as published in the Compendium and posted on the NASS website www.sundials.org are clipped in below.
"John Schilke's presentation 'Oh, East Is East and West is West' was a note on the Prime Vertical, that great circle which passes through the zenith and nadir and the east and west points of the horizon. The Prime Vertical is thus orthogonal to the meridian and to the horizon. It has a few properties that make it useful for the dialist; several terms of formulae in common use vanish, simplifying calculations. For example, the time angle when an object is on the prime vertical reduces to Cos t = Tan d/Tan f. John outlined two methods of establishing the Prime Vertical, the Indian Circle method of equal altitudes and the two string and candle method. He also gave examples of dials for which a readily determined local east-west line is even more convenient than the meridian. In the discussion Roger Bailey noted a couple of good reasons for using the Prime Vertical rather than the meridian to determine the axis of a sundial. The shadows are longer and the rate of change of azimuth angle with time is less. Unfortunately the Prime Vertical is only available for the sun for half the year, when the declination d is the same sign and smaller than the latitude f." There is real value in these NASS conferences. I hope to see you in Banff in August. There are still some spaces available. Registration information is on the NASS website. Cheers, Roger Bailey Walking Shadow Designs N 51 W115 -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Behalf Of Anselmo Perez Serrada Sent: June 17, 2003 3:42 PM To: [email protected] Subject: Re: Best Time for Setting Sundials John Carmichael wrote: >I've often wondered if there is a best time of day to set a sundial using >the "Time method" (as opposed to the compass, the Polaris, the GPS and >plumbob shadow methods). > >I'm thinking that the best time of day to precisely set and/or read a >horizontal sundial would be at mid-morning and mid-afternoon (those times >that are halfway between sunrise and apparent noon and apparent noon and >sunset). > >These setting & reading times would avoid the early morning and late >afternoon affects of maximum atmospheric refraction and would also avoid the >compressed hourline markings that are close together at noon which make time >estimation more difficult. > >Does anybody agree with my theory? Is there a best time for setting >sundials? > > You are right. Making a bit of mathematics it can be shown that the highest insensibility to errors is reached when the Sun crosses the prime vertical (ie, the one that contains the East and West points). You can ask a topographer or consult a good astronomy book for more information about this, but your reasoning is much crisper and essentially correct. Best regards, Anselmo Perez Serrada - -
