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

Erasmus Oswald Schreckenfuchs

Astronomical Images

<p style='text-align: justify;'>This unique edition of Oswald Schreckenfuchs' massive commentary contained, for the first time in the <i>Theoricae novae</i> printed tradition, a series of three-dimensional diagrams intended to complement the ordinary diagrams of the orbs, circles, axes and poles of the planets and the eighth sphere. These diagrams are printed on one side of two <i>bifolia</i>. In some copies the <i>bifolia</i> form one gathering b (four folios), bound after the preface; in others the figures have been cut, and each is inserted in the appropriate place in the treatise. They are not lettered and have no legend, though some of their elements are labelled, as they must be used in association with the diagrams in the text. This diagram corresponds to the figure of the orbs, axes and poles of the motion of the Moon. The four orbs of the Moon are seen from above, in latitudinal section: the two black 'deformed' orbs (the deferent orbs of the apogee of the Moon's eccentric), the white 'deferent' orb sandwiched between them, which bears the epicycle of the Moon, and the outermost orb, concentric to the World, which encompasses the other three and revolves around them, called the 'deferent of the Head of the Dragon'. Each orb is labelled: <i>exterior defer[ens] augem eccentrici</i> (the exterior black orb), <i>interior deferens au[gem] eccen[trici]</i> (the interior black orb), <i>deferens epicyclum Lunae</i> (the sandwiched white orb, with a black 'hole' for the epicycle: <i>epicycl[us] Lunae</i>). The fourth orb has two labels: <i>deferens Caput Draconis</i> and, on the opposite side, <i>deferens Caudam Draconis</i>. The main axis is that of the ecliptic (<i>axis eclipticae octavae sphaerae</i>), which is also the axis of the deferent of the Head of the Dragon. It intersects the axis of the deferent orbs of the apogee of the eccentric of the Moon (<i>axis orbium augem defer[entium]</i>) at the centre of the World. According to Peuerbach, the angle at the intersection is always five degrees. The little circles described by the poles of the axis of the deferent orbs of the apogee of the eccentric around the poles of the axis of the ecliptic are drawn. One circle passing through the poles of the axis of the deferent orbs of the apogee of the eccentric determines the place of the Head and Tail of the Dragon (<i>Caput Draconis</i>, <i>Cauda Draconis</i>) as it intersects the deferent of the Head of the Dragon. The axis of the deferent orb of the epicycle of the Moon (<i>axis exce[n]t[rici]</i>) always remains parallel to the axis of the deferent orbs of the apogee. Their reciprocal movement is represented by three small circles drawn about the centre of the World and the poles of the axis. Whereas the 'flat' diagrams represent a perfect rotation of the axis of the deferent of the epicycle around the axis of the deferent of the apogee, this one shows that owing to the complex combination of different movements this is not so evident: though the two axes always remain parallel (as specified by Peuerbach), we cannot see clearly which of them revolves around the other.</p>


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