Quite a while ago I asked the following question on sundial trigonometry. > >> My challenge to all for today is to simply explain >the basic formula for horizontal dials: Tan HA = Sin L x Tan t.
Mac Oglesby responded with the following answer which is much better than my proposed proof. > >Fred gave a demonstration of why this is true in an early issue of >Compendium (1-4, I think). I find the attached drawing (Tan_A.GIF) >easier for me to follow. > >Best, Mac Luke Coletti responded by pointing to the followin graphic that demonstrates the phenomenon very well. > A very nice derivation can be found in "A Choice of Sundials", by >Winthrop W. Dolan, pg. 20-21. For those seeking a more intuitive >approach, the graphic, inside the book's dust jacket, of a horizontal >plane "embedded" within the celestial sphere gives a marvelous >description of why the hour lines are drawn at the angles they are. >Simply put, the hour lines of the dial must reach out to intersect the >projected meridian lines. An example of the graphic (made for lat=36.5d) >can be viewed at the URL below. >ftp://ftp.gcstudio.com/pub/misc/gcdial.jpg Thanks Mac, Fred and Luke for SENDING ANSWERS. Roger Bailey Walking Shadow Designs N 51 W 115 Attachment converted: Macintosh HD:Tan_A.GIF 1 (GIFf/JVWR) (0000810C)
