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Astronomical Images : Demonstration of Blagrave's drawing method for construction lines

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 first book of <i>The Mathematical Jewel</i>, from which this page is taken, Blagrave dealt with basic drawing techniques that would have been needed to inscribe the lines of the astrolabe. Each technique was called a 'conclusion', and the conclusions often had the same results as the proofs by construction which featured in Euclid's <i>Elements</i>. For example, in his fourth conclusion, accompanied by the third image from the top on the page shown, Blagrave showed how to erect a perpendicular to a given line from a particular point. It is interesting to note that Blagrave saw these conclusions as presentations of drawing techniques, rather than instructions in the arrangement of the geometrical figures required to achieve a given end. Thus, while most editors of Euclid would have mentioned the inscription of two overlapping circles, Blagrave instead wrote only of arcs of those circles. Moreover, only these arcs are shown in the accompanying figures, where they resemble construction lines rather than geometric figures in their own right. This approach is indicative of the practical concerns that were at play in this first book of <i>The Mathematical Jewel</i>.</p>


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