The booklet Declining Shadows is 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). The DeltaCAD programs display calendar curves, as does one of
the JustBASIC programs, and of course the main spreadsheet reference-spreadsheets.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.