I would like to say something about the oldest two sundials of the dome of Braunschweig. The drawing of the first one has done following the Erfurt rule. This rule is opinion was written by the famous Johannes Dank. This rule is for temporal hours and gives only the angles between the lines. These angles represents the shadow of the gnomon at every hour in an equinoctial time for the latitude of Erfurt without any other explanation. My opinion is that the second dial, that is an improvement of the first one, has not been made with calculation but following a second Erfurt rule, tat is the same rule but with inverted angles. For a certain property, this works almost correctly for that time and for equal hours sundials between a lot of latitudes (the best one is Florence). I'm writing an article about it. But nevertheless you can read about the first Erfurt rule in K. Schaldach, Di due manoscritti di Rostock e la regola di Erfurt, in «Gnomonica Italiana», n. 2, giugno 2002, pp. 28-31, with the edition of the rule latin text.
Mario
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