Angelo Brazzi asks about how best to determine the actual centre point on a dial with a cylindrical gnomon. I think his calculations are correct, but it occurs to me that, regardless of declination, the centre is mid-way between any parallel tangents, such as the horizontal or vertical tangents. Therefore I would be inclined to mark the tangents, measure the distance between them and halve it, to determine the centre of the ellipse. No trigonometry needed. A more difficult case is a semi-cylindrical style on top of, or underneath, a solid sub-style (or gnomon, assuming we're agreed on these terms now ;-) Here, the hour lines need to be tangents to the ellipse, so you cannot avoid calculating the ellipse, I think.
As for the true length of the style, it seems to me to depend on how the end is formed. If it is merely cut off at right angles, its shadow will be a little too long except at the equinoxes. One solution is to have it pointed, like a pencil, in a cone with an apex angle of not more than 2*(90-23.5) degrees. Chris Lusby Taylor Email: [EMAIL PROTECTED] (Formerly [EMAIL PROTECTED])
