Regiomontanus and the Armillary Sphere
In his letter of 1469 to Archbishop Vitez of Gran, introducing a tract he had written on the torquetum, Regiomontanus listed the armillary sphere among the various kinds of observational instrument. A description he wrote of the armillary sphere appeared on his tradelist of intended publications. It seems likely that he owned and used one or more of these instruments in the course of his life, and his Nuremberg pupil Bernhard Walther (d.1504) relied on an armillary sphere for his observations of stars and comets.
Regiomontanus wrote a treatise on another instrument which he called a Meteoroscope. This tract, dedicated to Bessarion, described a version of the armillary sphere used for determining the co-ordinates of terrestrial locations. Regiomontanus did not claim to have invented this device, but merely proposed to explain its use. It is possible, however, that work on the Meteoroscope prompted him to design and construct various types of annular sundial.
The description of the Meteoroscope also appeared on Regiomontanus' tradelist of intended publications; it was printed as an appendix to Johann Werner's edition of Ptolemy's Geographia (Nuremberg, 1514) and again in a 1537 treatise on ring-dials by Johann Dryander. The tract on the armillary sphere appeared, along with accounts of the torquetum, Ptolemaic rulers, and astronomical staff, in the Scripta clarissimi mathematici Ioannis Regiomontani (Nuremberg, 1544) compiled by Johann Schūner (1477-1547) from the astronomer's manuscripts. The woodcut of the armillary sphere or Armillae Ptolemaei shown here comes from this printed version.