Hi All, (See attached picture) Concerning the backwards moving of the sun draw an analemmatic sundial for a latitude between the tropics, let's say 18 degrees. The result is an ellipse with a dateline which lies partly outside the ellipse Assume it is 21 june with the gnomon at the end of the dateline and the sun just rises. The shadow lies as the line A and the time is about 5:30. The shadow then moves to the right until it is tangent to the ellipse as line B. Then the shadow moves to the right, back to line A and so on. In the afternoon the reverse proces may be seen. Also it is clear that the line A intersects the ellipse in 2 points which means that we have 2 sulutions fr the time reading.
Best wishes, Fer. Fer J. de Vries [EMAIL PROTECTED] http://www.iae.nl/users/ferdv/ Eindhoven, Netherlands lat. 51:30 N long. 5:30 E ----- Original Message ----- From: John Carmichael <[EMAIL PROTECTED]> To: <[EMAIL PROTECTED]> Cc: <[email protected]> Sent: Wednesday, December 13, 2000 4:14 PM Subject: Re: I think there can be 2 hour angle solutions for a single azimuth > In the October 2000 BSS Bulletin, there is an interesting article by Allan > Mills called "Backwards Motion of The Shadow of a Sundial". > > While I don't understand all the math Mr. Mills presented, the article shows > that at lower tropical latitudes on or near the summer solstice, the sun can > have two different hour angles at the same azimuth! This seemed impossible > at first, but the diagram in figure 5 makes this clear. > > Maybe this article will help > > best > > John Carmichael > > >Steve L, > > > >I think there can be 2 hour angle solutions for a single azimuth. I think > >one h.a. will be less than 90 degrees, and the other greater than 90. See > >attached DeltaCad file. I wish I could comment on the derivation of your > >math, but I am unfamiliar with your arctan, arc cos solution for ha. I hate > >to say it, but this is where spherical trig will yeild the answers. Any > >takers? > > > >Bill G. > > > >Attachment Converted: C:\STARNET\EUDORA\STEVEAZI.DC > > > > Attachment converted: Macintosh HD:ellipse.gif (TIFF/JVWR) (0001FD48)
