Bill Maddux gives a very logical reasoning to the subject of how accurately one can read a sundial, assuming the reading is of the centre of a symmetrical shadow line with fuzzy edges.
I am inclined to agree that judging the centre of such a shadow must be easier than judging the position of the shadow cast by an edge. I believe the optimum width of gnomon is such that the umbra is of zero width. Even then, the figure of 1/60 width seems surprisingly good to me. But not all sundials cast shadows. I offer three alternatives: 1. A spot dial casts a spot of light which again has fuzzy edges but which might be easier to judge the centre or edge of. 2. I have seen described, but not in the flesh, a sundial consisting of a crystal which is rotated until two objects (spots of light? I forget.) appear to have the same brightness. It is said to be highly accurate. 3. When a sextant is used to find the sun's altitude in a known place, it is a sundial. You adjust a sextant to make the lower 'limb' of the sun appear to touch the horizon. This can be done with great accuracy. Even my plastic sextant is calibrated to 0.2' of arc, so I am sure the best brass sextants are better than that. You add on the radius of the sun, and correct for refraction in the atmosphere, to get the true altitude of the centre. This is going to beat any spot or shadow at times of day when the sun's altitude is changing fast. Chris Lusby Taylor
