Another tool of the master builder was the gnomon, a straight pole to find the four cardinal directions by measuring its shadow. It had a little sphere on top for a more distinctive shadow. The length of the shadow was marked on the ground during one day, from sunrise to sunset (the red curve in the drawing above).Then a circle was drawn with the pole as its center (blue in the drawing). This circle crosses the shadow line in points 1 and 2. Tracing two equal circles centered in 1 and 2 produces crossing 3. The line pole-point 3 indicates true North, coinciding with the shortest shadow during the day.
This method, known since Antiquity, one can even suppose since the Neolithic era, was used everywhere in the construction of temples. This simple pole planted in the ground on flat ground is effective to study the solar course. It also allowed by repeated measurements during the two solstices to determine the latitude of a place with remarkable precision.
The gnomon represents the “axis mundi”, the world axis, the symbolic connection between sky and earth. One could say that the gnomon is an astronomical instrument activated by the sun to regulate time and space of man and to construct a geometric model of the universe at a human scale.
Gnomon in the Saint-Sulpice church, Paris