Astronomical Images : Projection of several celestial circles and a particular horizon

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 several celestial circles and a particular horizon, an extension of a simpler example that Frisius had treated on the previous pages. In the first stage of this example, Frisius projected lines from the pole of the World through the features of interest to two alternative planes. In this figure, he shows the projection resulting on just one of these planes, the points where the projected lines intersect the plane being used to set the diameters of the circles in the projection. Note that the horizon as represented in the projection is not concentric with the principal parallels since it does not share its pole with them. The other 'off-centre' circle in the projection represents the ecliptic, which Frisius has presumably constructed by drawing a circle which touches both tropics at a tangent, although he does not mention the ecliptic or its construction in the text here. <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>