PIETRA Star Sundial - pure

Baroque star sundial with 50 dials

In the Baroque period of the 16th to 18th centuries, which was focused on symmetry, this star sundial is a typical mathematical gimmick of the era. You will look in vain for embedded shadows on this sundial, only edges parallel to the earth axis, above all those of the star that gives the clock its name, cast their shadows on the 50 precisely calculated dials. There are all variations of gnomonics: horizontal, vertical, equatorial and polar dials on flat and curved surfaces show the solar time (officially called local apparent time) in unison as soon as they are illuminated by the sun. Only princely houses or wealthy landowners could afford such a complex sculpture carved in stone.

Dipl.-Math. Willy Bachmann from Richrath, who has been a member of the "Fachkreis Sonnenuhren der Deutschen Gesellschaft für Chronometrie e. V." for more than 30 years, had the great fortune in 2020 to acquire the original star sundial from 1735 in very good condition. In his blog post (in German)

Die barocke Sternsonnenuhr mit dem Initial „Aˮ, Teil 1

he tells the exciting story of the acquisition and installation of this extraordinary sundial in his garden.

In the soon to be published part 2 of the blog post, the collaboration with Dr.-Ing. Carlo Heller, owner of HELIOS sundials, to create a digital copy of the original will be documented. In painstaking detail work, the star sundial was completely measured and built up as an exact 3D volume model in the CAD system.  The hour lines and the alignment of the faces and edges were calculated and constructed for the latitude of the original (53°N). Care was taken to come as close as possible to the original, but deviations of the original from the ideal geometry due to manual work were straightened.

For the production of the PIETRA sundial, the 3D volume model created in the computer is produced layer by layer from fine quartz sand on a 3D printer for sand moulds and then permanently consolidated by infiltration with epoxy resin. Such a replica of the historical sundial is ideally suited for permanent outdoor use. In order to ensure that the sundial also works correctly at your location, the base of the sundial is already provided in the model with a seamlessly constructed wedge, which causes the sundial to tilt in a north-south direction corresponding to the difference in latitude. This ensures that the sundial also shows the exact solar time at your location. Example: For a location in Mainz (50° N), the sundial calculated for 53° N is tilted by 3° to the north.

The sundial is available in three versions:

• PIETRA sundial - pure: in the 3D printing manufacturing process, the natural sand is given an anthracite colouring by the binder. Exposed to the weather, the sundial gradually takes on a greyish colour and thus becomes more and more similar to the original. Without the model losing any of its substance, the surface will look like weathered sandstone after a few months.
• PIETRA sidereal clock - sealed: the anthracite colouring is permanently preserved by coating with transparent matt lacquer.
• PIETRA sundial - sand-coloured: the 3D printing manufacturing process is carried out here with a more expensive, lighter-coloured binder. The sundial is then coated with a weather-resistant, dull matt PU lacquer sand-coloured (RAL colour 1001). This gives the PIETRA sundial the appearance of being carved out of light-coloured sandstone.
• An inscription according to the customer's wishes is possible on the base edge of the south side.

The following video shows the PIETRA star sundial in sunny operation on 27 May from sunrise to sunset. The solar time can be read simultaneously on up to seven dials. The edges marked with an arrow are currently casting their shadows on the corresponding dials.