Astronomical Images : Theory of the motion of the eighth sphere

Peter Apian

Astronomical Images

This Venetian edition of Peuerbach's Theoricae novae was copied from Apian's 1528 edition, printed in Ingolstadt. Subsequently, the work went through several further editions. Compared to the early editions of Peuerbach's Theoricae novae, Apian's edition included more woodcuts or woodcuts with additional notations. Some errors in the woodcuts in the 1528 edition were repeated in this Venetian edition of 1537. In this diagram, Apian has combined two illustrations from Peuerbach's Theoricae novae planetarum [Aiton, figs. 25-26] in such a way as to clarify Peuerbach's geometrical construction of trepidation and minimise any confusion that might have arisen from Peuerbach's original images. In the diagram, the thick black circle CPQARO is the celestial equator and CGNALI is the (fixed) ecliptic of the ninth sphere, while HPMR and FQKO represent the oscillating ecliptic of the eighth sphere. The two ecliptics intersect at the solstices; here Apian may have intended to mitigate the potential for misunderstanding in Peuerbach's image [Aiton, fig. 26] that does not make this intersection explicit. The accession and recession of the eighth sphere is the result of the rotation of the small circles that are fixed to the Aries and Libra points of the ninth sphere (C and A). As the circles rotate, taking the Aries points of the eighth sphere (FGHI and KLMN) with them, the intersections of the ecliptic of the eighth sphere with the equator, i.e. the equinoxes, move between O and P, and between Q and R. The arcs drawn on the small circle at Libra highlight a difference between Peuerbach's theory of trepidation, and that of ThÄ?bit ibn Qurra, the ninth-century astronomer who first developed the idea. ThÄ?bit measured the equation of the eighth sphere, which governs the oscillation of the eighth sphere by up to nine degrees in each direction, as an arc of the ecliptic of the eighth sphere. However, Peuerbach made it an arc of the ecliptic of the ninth sphere; on Apian's diagram this arc would be between A and a point found by drawing a line parallel to AK, from one of the many small black circles (indicating the moving Libra point), to the ecliptic of the ninth sphere.