Thanks for the various replies to my recent postings on self-southing dials and also on the finding south when setting up a vertical dial. Apologies for the delay in responding.
Regarding the posting about an itinerant C17th dialmaker, the consensus seems to be that he would find south by the method of equal shadow lengths. The task would be set up on the ground below where the dial was to placed. Then he would use a plumbline to position of the gnomon over the newly established meridian. On the question of what dip angle (i.e. latitude) to use, the solution offered is to measure the sun's altitude at noon, and then adjust for solar declination. This is easy to measure and a simple calculation, but it does mean a sundial maker would need knowledge of and access to tables of seasonal declination. Although this is an entirely reasonable requirement and likely method, but I was hoping that someone would offer a purely "craft" solution. I mean a method which just involves adjusting something until a shadow has a certain alignment. I also asked whether the iterant would do a new latitude calculation for each location where he worked, as opposed to using a regional rule-of-thumb figure. Nobody commented on that topic. I guess that if you have to wait for the sun so that you can find NS, then you can use its presence to do a latitude check too. Now, back to a craft method of adjusting for latitude without knowing solar declination. I think there may be at least one way, although I've not tried it out and I'm certainly not suggesting it was used in practice: You take a disc with a perpendicular rod through its centre like an equatorial dial - it could be double as your hourline marking tool in which case you simply slide this over the gnomon rod of the dial you are constructing. Adjust the gnomon until it is true NS using the plumbline method mentioned above, and set it so that the dip angle is about right for your guess of the current latitude. Now make a mark on the gnomon rod at the edge of the shadow of the equatorial disc. Wait a while. If the shadow has travelled upwards towards the wall, your gnomon rod is dipping at too slight an angle. If it has travelled down towards the disc, the rod is dipping too steeply. Or possibly vice versa. Do you think this would work? How accurate would it be? Steve
