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
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