Hello dial makers: There is an important point that I should make concerning the difference in sundial precision achieveable with stone and metal.
Generally speaking, we all know that large sundials can be made more precise because the space between hour lines can be divided and marked with smaller increments of time. It is easier to put five minute lines on a big sundial than a small one. You also know that the time lines are closest together near apparent noon. (except for equatorials). Just how close can time lines be to each other? What are the limiting factors? First is the minimum separation easily readable by the unaided eye. According to Tony Moss, this distance is about 2 mm. The other limiting factor is mechanical and structural. Photoengraving on metal can easily be made with a 2 mm line separation, but stone sundials, however, cannot be engraved with time lines so closely spaced because this would severely weaken the stone. The closest line separation that I can do on stone is 6 mm. This means that for a stone sundial to have the same degree of precision as metal, it must be three times larger. Tony can photoengrave 5 minute lines on an 8 inch diameter horizontal sundial, whereas I need a 24 inch diameter dial to do the same on stone. Also, the thickness of a photoengraved line (as small as .01 pt or .05 mm) can be much smaller than that of a line engraved on stone ( about 1 mm). For these reasons, I only make large stone sundials. These are important considerations to make when designing your sundials. Remember that your decision to use 1,5,10,15, or 30 minute time lines should be based on these minimum line separation distances near apparant noon when the greatest time line compression occurs. John Carmichael Tucson Arizona 520-696-1709 http://www.azstarnet.com/~pappas
