is there a picture available? It might make the text a lot clearer. Thibaud Chabot
At 22:18 03-02-2005, Frank Evans wrote:
Greetings fellow dialists, The other day I bought a copy of Weir's Azimuth Diagram. This was the first time I had seen it although I had read about it in earlier days. It is a remarkable document, devised by Captain Weir in 1890. Its purpose is to enable a navigator to find the azimuth of a heavenly body by graphical means. In appearance it is a circle of 50 cm diameter on paper, containing a central vertical ordinate, a series of concentric ellipses and mirrored hyperbolic curves spreading from the central ordinate. It makes a very pretty picture and is in colour. The diagram is entered with the elements latitude (expressed as an ellipse), declination of the heavenly body (expressed linearly on the vertical ordinate) and the hour angle (crudely, sun time) of the heavenly body (expressed as a hyperbolic curve). Marking the declination on the vertical ordinate (which is pricked out in degrees from 65N to 65S) and joining this point to a second point where the appropriate latitude ellipse crosses the hour angle hyperbola gives a line which plots the azimuth of the heavenly body in question. It occurred to me that this graphical method of solving azimuth problems could be modified to make a sundial. If the Weir diagram were to be levelled and oriented N and S and a vertical shadow pin were placed on the central ordinate at the sun's declination then the shadow would represent its azimuth. Follow this until it reaches the appropriate latitude curve and read off the time from the conjoined hour angle curve. Two points (1) This diagram is a fascinating survival, last printed in 1950 and still obtainable from UK Admiralty chart agents. It is catalogued as chart no. 5000 and is printed on good quality chart paper. It costs 5 pounds 50. There is a second diagram available for the hardy ones, extending from 65 deg to 80 deg. (2) Have I got all this right? Is anyone else familiar with Weir's diagram and has it been mentioned before in this sundial group? Frank 55N 1W -
--------------------------------------------------------------------- Thibaud Taudin-Chabot 52° 18' 19.85" North, 04° 51' 09.45" East, alt. -4.50 m home email: [EMAIL PROTECTED] -
