Astronomical Images : Orbs, axes and poles of the Moon's motion

Erasmus Reinhold

Astronomical Images

This Parisian edition was copied from the first edition of the commentary of Peuerbach by Erasmus Reinhold, printed in Wittenberg by Hans Lufft in 1542. Subsequently, in 1556, Charles Perier published a new edition, copied from the revised edition printed by Lufft in 1553, which contained additions to the theory of the Sun ([Theoricae] auctae novis scholiis in theoria Solis ab ipso autore). This woodcut derives from the figure of the axes and poles of the motion of the Moon in Peuerbach's original (c. 1474) edition of the Theoricae novae. But, whereas the original diagram represented only the circles, axes and poles of the motion of the Moon, this one also represents the orbs, as did the diagram devised by Peter Apian in 1528 (see Apian, Theoricae novae planetarum Georgii Peurbachii (1537), fol. 7v). However, this is not obvious, for the two deferent orbs of the apogee (the two 'deformed' orbs) are not printed black as they are in Apian's diagram. This figure had been first used in the edition of the treatise printed in Wittenberg by Johann Klug in 1535. The 1535 diagram was not lettered; the lettering was added in the first edition of Reinhold's commentary (Wittenberg, 1542). The axis of the deferent orbs of the apogee of the eccentric of the Moon (axis orbium au[gem] defe[rentium], or line ITF in the diagram) intersects the axis of the ecliptic (axis eclip[ticae] 8 sphae[rae], the line passing through GTK in the diagram) at the centre of the World (T), and, according to Peuerbach's text, its poles (pol[us] au[gem] defer[entium]) decline from the poles of the ecliptic (pol[us] eclip[ticae]) always by five degrees. As the orbs rotate, the poles of the axis of the deferent orbs of the apogee describe circles around the poles of the axis of the ecliptic. This rotation is represented by arcs of circles (IKL and HGF) roughly drawn around the pole of the ecliptic. Parallel above the axis of the deferent orbs of the apogee is the axis of the eccentric deferent of the epicycle of the Moon (axis eccen[trici]). One of its poles is marked 'pol[us] eccen[trici]'. The diagram shows, as did its models, that the axis of the eccentric rotates around the axis of the deferent orbs of the apogee. Three arcs of circles indicate this rotation: one around the centre of the World, T (SNV), and two on the circle in the middle of the eccentric orb of the Moon (DE and ABC). Peuerbach's text only specifies that the axis of the eccentric, as its orb moves, remains parallel to the axis of the deferent orbs of the apogee, so that 'the distance between the poles of both axes will be equal to the value of the eccentricity' (etiam poli motus istius a polis orbium augem deferentium distabunt secundum eccentricitatis quantitatem). Also marked on the diagram are the lines indicating the plane of the ecliptic (superf[icies] pla[na] eclip[ticae]) and the plane of the deferent orbs of the apogee (superf[icies] plana defere[ntis]). These two lines, each perpendicular to its own axis, intersect at the centre of the World, T. For a more detailed description of these motions and axes, see the 1482 edition of Peuerbach (sig. e5r). The spatial relations of these axes and their motion are more clearly shown in the three-dimensional diagram provided in Erasmus Oswald Schreckenfuchs' Commentaria in novas theoricas planetarum Georgii Purbachii (Basel: Henricus Petri, 1556), plate after p. 38. Translated quotations of Peuerbach's Theoricae are from Aiton (1987).