Astronomical Images : Blagrave's theoricke of the orbs, lines and motions of Venus

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. This figure recalls the iconography of the <i>Theorica planetarum</i>, which explained the rules for each planet's motion - either as a theoretical construct, or possibly, as a representation of the supposed physical mechanism. However, Blagrave's purpose here is rather simple: he does not present any of the abstract mathematics necessary in order to make predictions about the motion of Venus, for example. Instead, the purpose of the figure is just to introduce terms that the reader would encounter in the following books of <i>The Mathematical Jewel</i>. In the accompanying text, therefore, Blagrave relates the name of each of the circles shown in the figure, referring to them according to their position (inner or outer) and their colour (black or white). He also alludes to the way that the circles relate to each other in motion, noting for example that the epicycle is carried in the eccentric. He remarks that all planets have a 'theorike', and that similar names pertain to the circles in those cases.</p>