Astronomical Images : Projection of the polar circle and a tropic (1)

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 first stage in the projection of the polar circle and a tropic from the point of view of the centre of the World. The Latin is a little hard to follow since the Greek characters used in the text do not always closely resemble the characters on the figure. Note also that the entire figure has been printed upside down in this copy (look, for example, at the epsilon at the top of the circle, which appears back-to-front), making the labels on the figure even more difficult to read. <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>