Dear Gianni, I enjoyed your explanation and (I liked the deliberate mistake which you included to make sure we were paying attention)...
> If we have a horizontal sundial we > cannot use the method that I have > described yesterday. Of course, we CAN use your yesterday's method provided we accept that the horizon line is at infinity! Today is just a special case of yesterday! In general: B + I = 2.F (1) B = Babylonian I = Italian F = French = Local Sun Time = Modern = ... This means two things: 1. Local Sun Time, F, is the average of Babylonian and Italian Time 2. At ANY crossing point (when B and I are both integers) then Local Time is an integer or half-integer. The horizon line (even when at infinity) indicates sunrise or sunset when: EITHER B = 0 OR I = 24 B or I is an integer so all intersections on the horizon line are for integer B and I. At sunrise or sunset we have from (1): EITHER I = 2.F OR B = 2.F - 24 [I do not agree with HBAB=24-2HMOD this is the deliberate mistake you included to test us all :-) ] Worked examples for a horizontal dial: 1. To set out I = 18 Mark P on the equinoctial line at F=12 [I-6] Noting that I=2.F, draw a line through P parallel to the F = 9 line. 2. To set out B = 6 Mark P on the equinoctial line at F=12 [B+6] Noting that B=2.F-24, draw a line through P parallel to the F = 15 line. The world needs educating. Let's have more Italian-hours sundials outside Italy!! Frank --------------------------------------------------- https://lists.uni-koeln.de/mailman/listinfo/sundial