>When I did this with the big dial I first set out a parallel noon gap near the plate edge expecting the shadow to fill this at noon!! It was only when I came to observe this in sunlight that I found the apparent shadow was less than 10" wide because of the umbra/penumbra effect. The shadow was *tapered* of course and so should the noon gap lines be too. <
Hi John, This is not really a comment on Tony's very important observation about large dials with solid triangular gnomons but just a note to say that if the gnomon is instead made tubular then, because time assessment is then made from the centre of the resulting shadow and not from an edge, the observed umbra-penumbra does not affect time measurement in the same way - unless of course the dial is too large for the gnomon's diameter - by which point there is no umbra at all at the point of the chapter ring. Large dials designed with tubular gnomons can in fact be remarkably accurate and their timescales can be predictably calculated too. Regards Patrick --------------------------------------------------- https://lists.uni-koeln.de/mailman/listinfo/sundial
