Astronomical Images : Projection 1: showing the passage of a glass plate across the circles to be projected

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 Johann 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. In the first stage, the reader was asked to imagine a sheet of glass, represented here by the lines AB, cutting a meridian circle with the viewer's eye positioned at the North Pole. To project the various circles onto AB, one had to mark the points where 'prospective lineaments', that is, lines of sight to the various circles of latitude would intersect with AB (this was equivalent to Duerer's advice to draw what one saw directly onto the glass). For example, when AB was at the tropic of Cancer, the points of interest would be Q (the horizon), M (the tropic of Cancer), K (the equinoctial), H (the tropic of Capricorn), F (the Antarctic circle), E (the Antarctic circle), P (the horizon), G (the tropic of Capricorn), I (the equinoctial) and L (the tropic of Cancer). These points would then be fed into the second stage of the process, which produced projections of the celestial circles mentioned. The fact that Blagrave's instructions were not particularly precise, coupled with the fact that he wrote that he would set aside the inscription of other lines on the astrolabe for another occasion, suggest that this account was intended only to aid the reader's understanding of projection, rather than to enable to reader to project the lines of an astrolabe for himself.</p>