Astronomical Images : Cosmological spheres and orbs (the order of spheres in the Ptolemaic worldview)

Johannes Sacrobosco

Astronomical Images

<p style='text-align: justify;'>Very little is known about Johannes Sacrobosco except that he was probably British, taught astronomy at Paris University, and died there in the second quarter of the thirteenth century. <i>Sphaera mundi</i>, his major work, was an extraordinarily popular astronomical textbook for several generations. Manuscripts of it circulated through all the main European centres of learning. It was first published in 1472 in Ferrara, and went through dozens of editions up to the mid-seventeenth century. This edition of Sacrobosco's <i>Sphaera mundi</i> was printed with Georg Peuerbach's <i>Theoricae novae planetarum</i> and Johannes Regiomontanus's <i>Disputationes contra Cremonensia deliramenta.</i> It is illustrated throughout with woodcuts, some of which were coloured. These diagrams in chapter one of Sacrobosco's text accompany a discussion of the distribution or division of the Sphere in the Ptolemaic worldview. The image on the left-hand page is a typical cosmological diagram depicting the crystalline spheres. It divides the cosmos into terrestrial and celestial regions, with the elemental region in the centre, surrounded sequentially by the spheres of the Moon, Mercury, Venus, the Sun, Mars, Jupiter, Saturn, the firmament, and in the outermost sphere, the Prime Mover [<i>primum mobile</i>]. Due to different explanations of precession, the number of orbs utilised in such models varied; this example is comprised of nine spheres, though models with ten, eleven, and twelve layers had existed since the Middle Ages. The images on the right-hand page illustrate, firstly, the oblique and right horizons, and secondly, the spheres of the Elements.</p>

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