Astronomical Images : Projection 2: showing the lines resulting from connecting the associated points in Projection 1

John Blagrave

Astronomical Images

<p style='text-align: justify;'>This image comes from John Blagrave's <i>The Mathematical Jewel </i>(1585). In this work, Blagrave drew on a long tradition of guides to the construction and use of astrolabes, referencing works by Gemma Frisius and Johann Stoeffler amongst others. The new instrument that Blagrave presented as the mathematical jewel was an astrolabe of his own devising which had the benefit that it could be used anywhere in the World without the need to substitute different plates according to the latitude. In the second book of <i>The Mathematical Jewel</i>, Blagrave turned his attention to the making of the jewel. This part of the work opened with a discussion of the projection of the circles of the celestial sphere onto a flat surface. Blagrave acknowledged precedents for this sort of discussion in astrolabe books by both Gemma Frisius and Johannes Stoeffler, and also drew parallels with Albrecht Duerer's work on perspective. Blagrave noted Duerer's recommendation that one may produce a perspective drawing by placing a sheet of glass in front of the subject and, with the artist's eye fixed at one point, drawing whatever one saw onto the surface of the glass. Similarly, Blagrave explained, one could imagine a sheet of glass intersecting the celestial sphere in order to understand the projection of circles on an astrolabe. Blagrave's explanation worked in two stages. Once the points that set the diameters of the celestial circles had been established in the first stage, the line AB was to be redrawn, with the relevant points marked along it. These pairs of points were then to be connected with circles to give the projection of the circles required. Blagrave included a key to the left of the first figure on this page. The second figure then extended this simple example to give a cursory illustration of the projection of the lines on the mater of the jewel itself. As in the first stage, the description is not particularly full, suggesting that it was intended more as an aid to understanding than as a complete set of instructions.</p>