Dear Gianni, As always, you are quite right, but there is more to say...
> In all the methods to calculate or > draw sundials, geometric or analytic, > the Sun is always considered punctiform, > and reduced to its center, Yes to all that. > ...and no account is taken of > refraction... Yes AND no!!! Read on... > Sunset and sunrise (for the calculation > of temporary or italic hours) are always > the instants when the center of the Sun > crosses the theoretical horizon. Consider a standard medieval dial: 1. Vertical, direct south-facing 2. Gnomon horizontal and south-pointing 3. Horizontal sunrise and sunset lines IF you have ideal conditions and a clear view of the sun at sunrise (in the winter months otherwise the sun is on the north side of the dial) THEN... The shadow of the gnomon falls on the sunrise horizon line when the sun is *observed* on the horizon. This moment is BEFORE the centre of the punctiform no-atmosphere sun is on the horizon. Therefore, I assert, the medieval design DOES take account of refraction (though quite by accident!!). Of course, at the (slightly) later time, when the punctiform no-atmosphere sun is on the horizon, its shadow will also be horizontal. So, you ARE right but I think a little extra comment is needed! Of course, the standard 15-degree interval design fails almost everywhere else! It is correct at sunrise, midday and sunset but not at other times (except on the equator). Saluti e Auguri Frank --------------------------------------------------- https://lists.uni-koeln.de/mailman/listinfo/sundial
