Conical dial

Another very common type were conical dials, where the shadow-receiving surface was a portion of the inner surface of a cone. Typically, the conical depression was cut into the edge of a rectangular slab of stone. The stone-working involved in making a conical depression was simpler than that required for a spherical cavity. Conversely, the theory of the conical sundial was more complicated, as it involved projecting the celestial sphere onto a conical surface.

Commentary. Philipp Nothaft (May-June 2019)