Giovanni Francesco Zarbula

Laying out the dial

• With a plumb bob he drew a vertical line on the wall, the "Noon Line".
• He hammered a rod into the wall at right angles to the vertical, the "centre of the dial" C
• He observed and marked the shadow cast by the tip of that rod throughout a day, the hyperbolic declination line.
• Using a set of compasses, he determined the line of symmetry on the wall., the "substyle line".

• He drew a horizontal line at a point of choice on the dial plate, the "horizon".
• The square was placed on the substyle facing outwards, sliding into any convenient position where it cuts the horizon. He drew a perpendicular line drawn to that point, the "equatorial". A line was drawn from that point to the centre of the dial, the 18h line. The points where the equatorial crosses the noon line and the substyle line were important.
• He placed the apex of the square on the substyle line, so the sides passed through the equatorial points. The apex point is called the "auxiliary equatorial centre".
• A protractor or squares drew off 15° intervals on the equatorial. These were connected to the centre of the dial: the hour lines. The distance from the auxiliary equatorial centre to the equatorial formed the substyle height at that point.

The colours

Existing Dials (cadrans)