>Dear Dialers > >I question if a sundial with fixed features can ever be expected to read >time to within >a minute. Assuming there is an analemma to correct to mean time, any >analemma is going >to be in error from year to year with a rough period of about four years. >Ie., when choosing to place >an analemma on a sundial one has to choose an analemma for some point in >time. That analemma >will shift back and forth with respect to any longitude. > >Certainly a sundial can be made to read mean time to within a minute for >some period of time, but >not over a long time period. > >Do others agree? >
No you can see a sundial that is accurate to better than a minute on the hour at: http://www.uwrf.edu/sundial/ As Chris Lusby pointed out the use of an annalema avoids the date problem as the leap year is for our convenience as we insist on keeping time so that the sun is close to crossing the meridian at noon and to keep the Vernal equinox as close as possible to March 21st. An annalem plots the the EOT as a function of the Sunsdeclination or altitude which is continous and hence is not affected by the periodic leap year shifts. Cheers, 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
