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

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. Apian's edition added new woodcuts as well as notations to some of those from earlier editions. Some errors in the woodcuts in the 1528 edition were repeated in this Venetian edition of 1537. The left-hand woodcut shows the four orbs of the Moon that define its motion: the three interior orbs similar to those of the Sun and an outermost orb, which, according to Peuerbach's text, is called the deferent of the Head of the Dragon (deferens Caput Draconis); Apian marks it in this way in the preceding diagram depicting the lunar orbs. In this diagram the outermost circle is simply marked 'Orbis Signorum', as if it represented the sphere of the fixed stars (the distance between this sphere and the outermost orb of the Moon being thus ignored). There are two eccentric orbs printed black, which are called the deferent orbs of the apogee of the Moon's eccentric. In the middle of these two orbs is a third (white) orb, called the deferent orb of the epicycle, or the eccentric orb. Its axis is marked 'axis eccen[trici]', and its poles 'polus eccen[trici]'. Inside this third orb is a circle that describes the path of the centre of the epicycle to which the body of the Moon is attached. The axis of the deferent orbs of the apogee of the eccentric of the Moon (axis [or]bium au[gem] deferen[tium]) intersects the axis of the ecliptic (axis eclip[ticae]) at the centre of the World and, according to Peuerbach's text, its poles (polus aug[em] def[erentium]) decline from the poles of the ecliptic (polus eclip[ticae]) always by five degrees. As the orbs rotate, the poles of the axis of the deferent orbs of the apogee (polus aug[em] def[erentium]) describe circles around the poles of the axis of the ecliptic. This rotation is correctly depicted, on the left-hand part of the diagram, by a portion of circle (quite roughly drawn) around the pole of the ecliptic; but on the right-hand part there has been some confusion between the poles, as the portion of circle is incorrectly described around the pole of the axis of the deferent orbs of the apogee. Parallel above the axis of the deferent orbs of the apogee is the axis of the deferent orb of the epicycle of the Moon (axis eccen[trici]). One of its poles is marked 'polus eccen[trici]'. This axis rotates around the axis of the deferent orbs of the apogee: three arcs of circles - one around the centre of the World, two on the circle in the middle of the eccentric orb of the Moon - indicate this rotation. Also marked on the diagram are the lines indicating the plane of the ecliptic (superficies plana eclip[ticae]), and the plane of the deferent orbs of the apogee (superficies plana deferentis). These two lines, each perpendicular to its own axis, intersect at the centre of the World. For a more detailed description of these orbs, motions and axes, see the 1482 edition of Peuerbach (sig. e5r). The spatial relations of these orbs 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. The right-hand woodcut demonstrates how the line of the mean motion or longitude of the Sun is always in the middle, between the centre of the epicycle of the Moon and the apogee of its eccentric.