DECLINATION LINES
The free booklet Declining Shadows (and its DeltaCAD
macro (calendarDeclination.bas) here, and you can set the
declinatio values) are very well worth downloading. This is
only an extract from the section in Illustrating More
Shadows which deals with calendar curves in depth, and
discusses several trigonometric approaches, and provides
complete tables for a wide range of style heights
(latitudes). Also
see here.

The DeltaCAD
programs display calendar curves, as does
one of the JustBASIC
programs, and of course the main
spreadsheet  
illustratingShadows.xls which has many ways
of calculating declination line data.

The DeltaCAD macros provide for animation showing how
declination curves vary. They also show how they vary by
nodus to dial plate height, or, by style length. The results
appear inconsistent, but they are not, and as with all the
DeltaCAD animations, they are highly instructive.
For other dial furniture, look at the Italian line page page
The main page here has online Visual Basic programs for declination curves
The analemma page has excellent DeltaCAD macros discussing not just the analemma, but also the calendar
curves.

However, this
calendarDeclination.bas macro allows you to specify the declination values to use for all the dials,
whereas the
analemma.bas macro has fixed values except for a few cases.  

The analemma uses the EOT (equation of time) together with the sun's declination to derive the analemma, and that
same declination is the basis for calendar curves. Actually, that is true for armillary, equatorial, polar and meridian
dials. For horizontal, vertical, and vertical decliners, the sun's azimuth and altitude are used as well. And those
analemma notes are helpful in understanding what is going on. Chapter 24 of Illustrating Time's Shadow is by far
the best description available. I think.
For calendar curves on any hour
angle dial with
analemmas,
check this page here. However,
this
calendar macro allows more
flexibility on specifying declination
values.
DeltaCAD V7 changed how "dcMirror" worked, I have
a workaround that works for most, but not all, of the
text that the V7 function altered.
 April 24, 2010Also,
DeltaCAD v6 and v7 differed on handling a -ve single
as a paremater, fixed.  
 June 13, 2010
GOOD DECLINATIONS TO USE FOR REASONABLE             
SYMMETRY FOR THE AMBIGUOUS
MONTHS                        
23.50                      20                       10                     0
                    or 19                   or 11
                    or 18                   or 12        
Jun, Dec        Jan, May             Feb, Apr         Mar, Sep
              Jly, Nov              Aug, Oct        
For other dial furniture, look at the analemma page
for example... CALENDAR CURVES