Dear All, An intriguing question has been raised on the Italian Sundial list by Fabio Savian. Slightly re-expressed he is asking...
When specifying the orientation of a plane dial, is your order: A) Declination then Inclination B) Inclination then Declination ? This is important because rotations are not commutative: R1.R2 does not equal R2.R1. My own answer is firmly (A) and (slightly simplified and avoiding using either word) I think as follows: 1. Start with a vertical plane facing due north (sic). 2. Rotate about a vertical axis until it has the correct azimuth. 3. Rotate about a horizontal axis until the tilt is right. I use this order because both rotations are about axes IN THE PLANE being used. This keeps rotation matrices simple and so on. An alternative is: 1. Start with a vertical plane facing due north. 2. Rotate about a horizontal axis until the tilt is right. 3. Rotate about a vertical axis until the azimuth is right. This is fine but the second rotation is not about an axis in the plane being used. Here I quote a delightful observation by John Davis in the BSS Glossary referring to a definition of inclination: Beware: this convention is not followed by all authors. He could be referring to a host of other definitions :-) OK. Which way do you all do things: Dec then Inc or Inc then Dec ? There is no right answer but it would be interesting to hear who does what! Frank King Cambridge, U.K. --------------------------------------------------- https://lists.uni-koeln.de/mailman/listinfo/sundial