John Hoy, I'm not a Postscript user, but if you are programing to design Capuchin dials, you presumably have available provisions for the scale of hours, solar declination, local latitude as independent variable, etc.. You can get the line you want by ploting time vs declination via:
cos HA = - tan L tan D
where HA is the hour angle in degrees of the sun at its rising or setting,
L is the latitude of the place, and D is the declination of the sun.
Bill Maddux
