Astronomical Images : Orbs of the Sun

Georg von Peuerbach

Astronomical Images

<p style='text-align: justify;'>Georg Peuerbach (Georgius Aunpekh) was born in Peuerbach, near Linz. He studied at the University of Vienna, obtaining his BA in 1448 and MA in 1453. He held positions as court astrologer to the king of Hungary, and then to Emperor Frederick III. At the request of Cardinal Johannes Bessarion, Peuerbach began an abridgement of Ptolemy's <i>Almagest</i>, which was incomplete when he died in 1461. Peuerbach had also compiled <i>Theoricae novae planetarum</i>, a revision of the <i>Theorica planetarum </i>attributed to Gerard of Cremona. This originated as lectures given in Vienna in 1454, which were attended by Johannes Regiomontanus, who published the first edition in Nuremberg around 1474. This is the 1482 edition by Erhard Ratdolt, which contains copies of the original diagrams. As in the original edition, some woodcuts were coloured. Peuerbach's text was printed in a compilation that also included Johannes Sacrobosco's <i>Sphaericum opusculum</i> and Johannes Regiomontanus' <i>Contra Cremonensia in planetarum theoricas delyramenta disputationes</i>. This collection of astronomical treatises, and other similar ones, together comprised the main elementary texts available in the late fifteenth century. This volume's printer, Erhard Ratdolt, was active in Venice and Augsburg, and was particularly interested in astronomical subjects. He also produced editions of Johannes Engel's <i>Astrolabium planum in tabulis ascendens</i> and G. Julius Hyginus' <i>Poetica astronomica</i>. This figure shows the three orbs of the Sun with two points: the centre of the World and above it, the centre of the eccentric (or deferent). The outermost sphere (the coloured outer ring in the woodcut) has the characteristic that its outer (convex) surface has the centre of the World as its centre, but its inner (concave) surface takes the eccentric centre. The innermost orb (the coloured smaller ring) has the eccentric centre as the centre of its outer (convex) surface and the centre of the World as the centre of its inner (concave) surface. These two orbs are called the deferent orbs of the aux (or apogee) of the Sun. They move together in that the narrower part of the larger orb is always above the wider part of the smaller orb; their motion defines the aux of the Sun. The orb sandwiched between these two orbs is eccentric to the centre of the World in both its inner and outer surfaces. 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 this orb's motion.</p>