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

Gregor Reisch

Astronomical Images

<p style='text-align: justify;'>The <i>Margarita philosophica</i> 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 <i>Margarita</i> includes a summary of Peuerbach's <i>Theoricae novae planetarum</i> and reproduces some of its original diagrams. This woodcut, which is a copy of the corresponding diagram in the 1503 edition, shows the three orbs of the Sun and the axes and poles of their motion. The outermost circle represents the ecliptic, while the distance between this circle and the exterior orb printed black is meant to contain (without regard for proportion) the orbs of the superior planets (Mars, Jupiter and Saturn) and the space between them and the fixed stars. The three orbs of the Sun are comprised of the two orbs printed black and the white orb sandwiched between them and divided in two by a circle. The black orbs are said to be 'deformed' because their two outer surfaces are not concentric: one of them is concentric with the World, while the other has the same centre as the eccentric or deferent of the body of the Sun. Both centres, that of the World and that of the eccentric deferent, are rather confusingly marked 'C'. In fact, 'C' is only an abbreviation for 'centre'; this was also the case in the original (c. 1474) diagram. The white orb is eccentric to the centre of the World on both its inner and outer surfaces. It is called the deferent orb of the body of the Sun (or the deferent orb of the Sun). The Sun is attached to it and moves according to its motions. This deferent has its own motion (an annual revolution around its centre) and is also subject to the motion of the other (black) orbs. These orbs are called the deferent orbs of the aux (or apogee) of the Sun (<i>orbes augem Solis deferentes</i>), 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 aux and <i>oppositum augis</i> (perigee) of the Sun (or of its deferent) are marked on the diagram. The poles of the motion of the deferent orbs of the aux are the poles of the ecliptic of the eighth sphere (marked on one side <i>polus eclipticae</i>), '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 (<i>polus deferentis</i>) is at the extremity of the axis of this deferent orb (<i>axis deferentis</i>), which is 'parallel to the axis of the deferent orbs of the apogee' (<i>axi orbium augem deferentium aequidistantis</i>), and passes through the centre of the solar deferent orb and of the eccentric circle in the middle of this orb. Through the different motions of the orbs (the motion of the deferent of the Sun, which causes the annual revolution of the Sun, and the combined motions of the 'black' deferent orbs of the aux), 'the axis of the solar deferent orb with the centre of the eccentric circle, 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 (<i>axis mundi</i>), and is, more precisely, the axis of the zodiac, or of the ecliptic of the eighth sphere, passing through the centre of the World, C. 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. The diagram shows a line perpendicular to the <i>axis deferentis</i> and <i>axis mundi</i> and passing through the centre of the deferent, the centre of the World, and the points of the apogee and perigee (<i>aux</i> and <i>oppositum augis</i>). It is marked 'the plane of the ecliptic and the deferent' (<i>superficies plana eclipticae et deferentis</i>) and 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' <i>Commentaria in novas theoricas planetarum Georgii Purbachii</i> (Basel: Henricus Petri, 1556), plate before p. 1. Translated quotations of Peuerbach's <i>Theoricae</i> are from Aiton (1987).</p>


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