Six sections of worked ivory are an extraordinary discovery on the “Tortugas” ship. These fragments derive from the upper vertical and lower horizontal leaves of a portable octagonal sundial diptych with sides interconnected with cuprous wire and originally closed with a brass hinge. The upper plane of the horizontal box leaf that once contained a compass is simply adorned with three bands of dual lines, on one fragment pierced with two holes. The octagonal base was originally secured to the overlying box sections with a three-part bronze hinge composed of two central linear pins on either side of a central hexagonal bolt. It is also pierced at center for securing in place a string gnomon and is incised on the outside edge with one set of dual lines.
The star-decorated ivory plaque was originally set in the center of the upper vertical leaf’s exterior surface. Also octagonal, it is covered with a star design subdivided into six rays with curved edges. Between the external edges of each is a single sun motif (six in total). Outside these symbols at the plaque edge are three incised lines. A similar dual incised line extends longitudinally through the center of the star symbol. The sundial’s overall decorative scheme is crude.
Two holes pierce the upper vertical leaf on one edge; on the opposite surface these are surrounded by cupreous stains from a three-part bronze hinge similar to that on the base plaque. This indicates that this point corresponds with the lower plaque edge joined to the underlying horizontal leaf. A single hole on the opposite edge of the vertical leaf accommodated the gnomon.
The “Tortugas” sundial seems to be the earliest octagonal example attested on a shipwreck. It was almost certainly manufactured in the great mathematical tool production center of Nuremberg in Germany under the control of six dominant ‘compass-maker’ families between c. 1550 and 1730: Troschel, Ducher, Karner, Lesel, Miller and Reinmann. The use of ivory in such great quantities was unusual for dials, but was the specialty of Nuremberg’s compassmakers.