Astronomical Images : Orbs, axes and poles of the Sun'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. These two diagrams are new versions of Peuerbach's original (c. 1474) diagrams. The left-hand woodcut shows the three contiguous orbs of the Sun. The outermost orb, marked A and printed black, is 'deformed' (its two surfaces are not concentric): its exterior convex surface is concentric with the World (the centre of which is point D), while the centre of its concave surface is point C, the centre of the eccentric or deferent of the body of the Sun. The innermost orb, marked E and printed black, is also 'deformed': the centre of its interior concave surface is point D, while the centre of its exterior convex surface is point C. Sandwiched between these orbs, the white orb B is eccentric to the centre of the World on both its inner and outer surfaces (its centre is C). It is called the deferent orb of the Sun or the deferent orb of the body of the Sun. The Sun is attached to it and moves according to its motion. This deferent has its own motion and is also subject to the motion of the other orbs. Orb A and orb E are called the deferent orbs of the aux (or apogee) of the Sun (orbes augem Solis deferentes), because the aux of the Sun varies according to their motion; they move together in that the narrower part of the larger orb is always above the wider part of the smaller orb. These two orbs follow the variations of the proper motion of the eighth sphere. The right-hand woodcut shows the same orbs of the Sun and, in addition, the axes and poles of their motion. The poles of the motion of the deferent orbs of the aux are the poles of the ecliptic of the eighth sphere (each marked polus eclipticae), 'for the apogee of the eccentric deferent of the Sun continually revolves in the plane of this ecliptic'. Each pole of the motion of the eccentric deferent orb of the body of the Sun (polus deferentis) is at the extremity of the axis of this deferent orb (axis deferentis), which is 'parallel to the axis of the deferent orbs of the apogee' (axi orbium augem deferentium aequidistantis), and passes through the centre C of the solar deferent orb and of the eccentric circle in the middle of this orb. Through the different motions of these orbs (the motion of the deferent of the Sun, which causes the annual revolution of the Sun, and the combined motions of the deferent orbs of the aux), 'the axis of the solar deferent orb with the centre of the eccentric circle [C], and also the poles of the solar deferent orb, describe the circumferences of small circles about the axis of the deferent orbs of the apogee, according to the size of their eccentricity', for the small circles are described around what is called the axis of the World (axis mundi), and is, more precisely, the axis of the zodiac, or of the ecliptic of the eighth sphere, passing through the centre of the World, D. This last motion is represented by small semicircles, which are to be considered to be in parallel planes perpendicular to the plane of the sheet. Oppositum on the diagram means oppositum augis (perigee), the point opposite to the apogee. The line perpendicular to the axis deferentis and axis mundi and passing through the centre of the deferent, the centre of the World, and the points of the apogee and perigee is shown on the diagram but bears no name. On the original diagram by Peuerbach, it is marked as 'the plane of the ecliptic and the deferent' (superficies plana eclipticae et deferentis). It signifies that the axis of the deferent and the axis of the ecliptic are always on the same plane, as the former revolves around the latter. 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 before p. 1. Translated quotations of Peuerbach's Theoricae are from Aiton (1987).