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

Gregor Reisch

Astronomical Images

The Margarita philosophica was a compendium, or 'Epitome' of university learning in the sixteenth century. It was written by the prior of the house of Carthusians at Freiburg, Gregor Reisch (d. 1525), and was first published in 1503 in Freiburg by Johannes Schott, a printer from Strasbourg. The work was illustrated amply with somewhat crude woodcuts, and was divided into twelve books, with one book each dealing with the trivium (grammar, dialectic and rhetoric) and quadrivium (arithmetic, music, geometry, astronomy), four books devoted to natural philosophy, and one book on moral philosophy. It was a popular work, reprinted numerous times during the sixteenth century, including the unauthorized, augmented editions by another printer at Strasbourg, Johann Grueninger. Oronce Fine edited and added to the Latin text of the 1535 edition. In the 1512 edition, Grueninger attached an appendix consisting of material not discussed extensively in the original Schott edition. This appendix included Greek and Hebrew alphabets, musical notation, perspective and architecture, and explanation of such instruments as the quadrant, astrolabe, and torquetum. In book 7, the Margarita includes a summary of Peuerbach's Theoricae novae planetarum and reproduces some of its original diagrams. This woodcut is a close copy of the corresponding diagram in the original 1503 authorised edition (sig. n6v). It 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). In this diagram it is simply labelled 'the fourth orb' (orbis quartus). There are two eccentric orbs printed black, which are called deferent orbs of the apogee of the Moon's eccentric: the interior one is marked (in white) 'deferens augem inferior' and the exterior one, 'deferens augem superior'. Between these two orbs is a third (white) orb, called the eccentric deferent of the epicycle. In the middle of 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 (deferens epicyclum); 'epicyclus' and 'corpus Lunae' are also marked on the diagram. The axis of the deferent orbs of the apogee of the eccentric of the Moon intersects the axis of the ecliptic at the centre of the World (c. mundi). The poles of these two axes are marked on the diagram: 'polus augem deferentis', 'polus eclipticae'. According to the text, the poles of the axis of the deferent orbs of the apogee decline from the poles of the ecliptic by five degrees ' but the diagram is not accurate. We see also that the three points that determine the axis of the deferent orbs of the apogee, the two poles (polus augem deferentis) and the centre of the World (c. mundi), are not perfectly aligned, as they ought to be. Here, Grueninger has simply copied the erroneous diagram from the original edition. As the orbs rotate, the poles of the axis of the deferent orbs of the apogee (polus augem deferentis) describe circles around the poles of the axis of the ecliptic (polus eclipticae). This rotation is represented by two rough portions of circle, one at each pole. Parallel above the axis of the deferent orbs of the apogee is the axis of the eccentric deferent orb of the epicycle of the Moon, passing through the centre of this axis (marked 'c. deferentis'). This axis of the eccentric deferent 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, two at the interface between deferens augem inferior and the eccentric orb of the Moon. Also marked on the diagram is the line indicating the plane of the ecliptic (superficies plana eclipticae, barely legible), perpendicular to the axis of the ecliptic. The corresponding line indicating the plane of the deferent orbs of the apogee (superficies plana deferentis), perpendicular to the axis of the deferent orbs of the apogee, is not marked, but it is partly drawn: it leads to the lunar epicycle, pointing to the direction of the word 'aux' (apogee) inscribed in the fourth orb (while 'oppositum augis', that is perigee, is inscribed in the opposite direction). 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.