Dee, Mercator, and Louvain Instrument Making: An Undescribed Astrological Disc by Gerard Mercator (1551)

The present paper complements the publications of Gerard L'E. Turner on Mercator's astrolabes by presenting an account of an astrological disc which Mercator published at Louvain in May 1551. This instrument, of which only one copy is known, is described, and a transcription of its instruction sheet, with commentary and English translation, is provided. My preliminary study of the astrological content and context of the instrument indicates that it is connected with John Dee's astrological studies at Louvain from 1548 to 1550. The didactic functions of the instrument are discussed, as well as the relationship between astrological theory and practice in Mercator's work.