Astronomical Images : Projection of the polar circle and a tropic (2)

Gemma Frisius

Astronomical Images

<p style='text-align: justify;'>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 <i>De Astrolabo Catholico libellus,</i> 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. <i>De Astrolabo Catholico libellus </i>also includes, in its later parts, several contributions from Frisius' son, Cornelius Gemma (1535-1578). This figure shows the second stage in the projection of a polar circle and a tropic from the point of view of the centre of the World. In the first stage, Frisius projected lines from the centre of the World through features of interest, here a polar circle and a tropic, to the plane of projection. The points where the projected lines intersected with the plane were then used to set the diameters of the circles in the projection. Thus, the three-dimensional sphere was represented on a two-dimensional surface. <i>De Astrolabo Catholico libellus</i> was one of the sources invoked by John Blagrave in <i>The Mathematical Jewel</i>. 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.</p>