In reply to John Davis: > >I have a question/challenge to all you sundial designers: what is the most >accurate design for a Standard Time dial? >
My vote is of course for a dial with the EOT built into the hour lines to give the annalema shapes such as used in the Swensen Sun dial at : http://www.uwrf.edu/sundial/welcome.html >The criteria for the dial are, in my opinion (John Davis's): > >a) it should tell Standard Time, (or possibly Daylight Saving Time - BST in >the UK) It does. >b) it should be in a fixed location It is:-) >c) it must have no moving parts (which rules out adjustable equatorials and >changeable gnomons etc) and > should be as physically robust as possible. It is. >d) it must not require reference to a separate table or computer program eg >to get an exact declination for the sun. It does not. > All data must be built into the dial plate. >e) the "accuracy" should be interpreted as the mean error for the hour >lines 3 hours either side of noon (or 12:00) for the years 2000 to 2050. > Its less than a minute except for the 2 weeks about the Winter Solstice at which I would put the reading uncertainty to about 3 minutes. >As a starter, the "Singleton" dial recently discussed here would seem to be >a reasonable candidate. It's main limitation, common to all dials which >incorporate an EoT correction, is that it is drawn for a some MEAN EoT >curve, and no allowance is made for the leap year cycle and the other minor >variations. This is not so. If the annalemmas are used and the sun's declination incorporated so that the time is read by a fixed point on the gnomon, (the end in the case of the Swensen Dial), no problem occurs at the leap year. It is only when dates are used to determine the correction that there is a jump at the leap year. Cheers, John Professor John P.G.Shepherd Physics Department University of Wisconsin-River Falls 410 S. 3rd. St. River Falls,WI 54022 Phone (715)-425-3196, eve. (715)-425-6203 Fax (715)-425-0652 44.88 degrees N, 92.71 degrees W.
