Giovanni Francesco Zarbula

Laying out the dial

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

• He drew a horizontal line at a point of choice on the dial plate. This was the Horizon.
• The square was placed on the substyle facing outwards, it is slid into any convenient position where it cuts the horizon, and he drew a perpendicular line drawn to that point. This line is called the equatorial. A line was drawn from that point to the centre of the dial; it was the 18h line. The points where the equatorial crosses the noon line and where the equatorial crosses the substyle line were important.
• He placed apex of the square on the substyle line, so the sides passed through the equatorial points mentioned.. 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)