LAUSR

This article offers the first comprehensive study of a newly discovered type of medieval sundial made of ivory which might well be the precursor of the well-known diptych dial form made from ivory and wood. These sundials are unique for the combination with a wax writing tablet (tabula cerata) on the reverse side, such as has been deployed as a reusable and portable writing surface in Antiquity and throughout the Middle Ages. Three previously unpublished examples of this type of sundial have been located in Germany, Italy and England. This article gives a detailed analysis of the sundials and the underlying construction principles, including considerations from the history of science, chronology and cultural history in order to answer the questions of where, when and by whom these sundials were made.