Astronomical Images : Frisius' visual account of projection

Gemma Frisius

Astronomical Images

Gemma Frisius (1508-1555), born Jemme Reinerszoon, was born in Friesland in the present-day Netherlands, and we know him by the name conferred on him by his birthplace. However, Frisius soon moved to Louvain where he attended university and became a lifelong member of the faculty of medicine. While Frisius was involved in many fields, including medicine, philosophy and mathematics, he is remembered primarily for his practical skill. He founded a workshop which produced globes and mathematical instruments and was noted for his talent as an instrument-maker by Tycho Brahe, among others. His practical interests were passed on to several of his students, notably Gerard Mercator and John Dee. In De Astrolabo Catholico libellus, Frisius outlined the construction and use of an astrolabe of his own devising. This excerpt is taken from the first book of the work that was printed as part of a compilation of books on practical matters, many by Frisius himself. De Astrolabo Catholico libellus also includes, in its later parts, several contributions from Frisius' son, Cornelius Gemma (1535-1578). This figure extends a basic example featured on the previous pages of the work, and shows the first stage in the projection of several celestial circles. In the previous example, Frisius had dealt with the projection of just two circles. Here, he moves on to include several more features, including the horizon of a particular locality, represented by the slanting line within the circle. He also changed the viewpoint from the centre of the World to a pole. The two upright lines represent two possible planes of projection, although Frisius only shows the one running through the centre of the circle in the second stage of this more advanced example. As on the previous pages, the Latin can be hard to follow since the Greek characters used in the text do not always closely resemble the characters on the figure and again the entire figure has been printed upside down. De Astrolabo Catholico libellus was one of the sources invoked by John Blagrave in The Mathematical Jewel. Notably, though, Blagrave's account of the projection of the celestial sphere onto the surface of the astrolabe was more intuitive than Frisius' rather abstract account.