Astronomical Images : Orbs, lines and motions of Venus

Georg von Peuerbach

Astronomical Images

This 1515 Parisian edition, which contained the commentaries on Peuerbach by Franciscus Capuanus (first edition 1495) and Sylvester de Prierio (first edition 1514), and Jacques Lefevre d'Etaples' Astronomicon, was supervised by Oronce Fine, who redrew and improved all the diagrams of the preceding editions. Folio 31v belongs to the first part of the volume that contains the commentary of Franciscus Capuanus, professor in Padua, first printed in Venice by Simone Bevilaqua in 1495. Its abundant illustration exerted an influence on the subsequent printed tradition of Peuerbach's treatise. In the original (c. 1474) edition of the Theoricae novae, and in the first editions of the commentary by Capuanus (1495 and 1499), no diagram is specifically devoted to Venus. In the original treatise, the figure of the orbs of the superior planets is simply labelled Theorica trium superiorum et Veneris, as it applies also to Venus. Oronce Fine, in his edition of the Capuanus commentary, was the first to illustrate the short chapter, De Venere. This diagram of the orbs and motion of Venus was later reused in his edition of the Theoricae novae (Paris, 1525), not in the chapter on Venus but in the chapter on the three superior planets. It was probably influenced by a figure in Gregor Reisch's Margarita philosophica (1503, sig. o3r), devoted to the meaning of the terms concerning the superior planets, Venus and Mercury in the astronomical tables. Fine devised for the 1515 edition of Capuanus a much-simplified version of this Reisch diagram. There is something remarkable in the layout. On the next page (fol. 32r), there is another simpler diagram, which shows only the orbs of the planet. The reason why the simpler diagram has not been printed first is that, on this page, the diagram has two functions: it marks the opening of the chapter on Venus, and it illustrates the end of the preceding chapter, which explains the motion of the superior planets. In any case, Peuerbach concludes the chapter on Venus by asserting that the definitions of the terms concerning the motion of this planet are identical, in every way, to those for the superior planets (terminorum expositiones per omnia sunt hic, sicut in tribus superioribus). Venus has three orbs, similar to the orbs of the superior planets. The outermost orb, printed black, is said to be 'deformed' (its two surfaces are not concentric): its exterior convex surface is concentric with the World, while the centre of its concave surface is the centre of the eccentric deferent. The innermost orb, also printed black, is likewise 'deformed': the centre of its interior concave surface is the centre of the World, while the centre of its exterior convex surface is the centre of the eccentric deferent. These two deformed orbs are the deferent orbs of the apogee (orbes augem deferentes). Their common axis is the vertical line (bec), also called the line of the apogee (linea augis). The white orb sandwiched between these orbs is eccentric to the centre of the World on both its inner and outer surfaces. The centre of the epicycle (lhgf) is attached to the circle in the middle of the eccentric orb. The body of the planet is at l. Both orb and circle are called the eccentric deferent of Venus. The outermost circle (abmki) represents the ecliptic; the letter a, on the right, probably indicates the beginning of Aries. The line that crosses the linea augis at right angles at the centre of the World indicates the mean longitudes of the eccentric (longitudines mediae). As in the case of the superior planets, the movement in longitude of the centre of the epicycle of Venus is not regular in relation to the centre of the eccentric, but to the centre of another circle, the eccentric equant. This centre is situated on the line of the apogee, as the centre of the World and the centre of the eccentric; it is as distant from this centre of the eccentric as the centre of the eccentric is distant from the centre of the World. The eccentric equant, intersecting with the eccentric deferent, has been drawn and the three centres are marked, although the lettering is not easily legible: the centre of the equant, above the other two, is probably labelled 'e', and the centre of the World, 'c'. The main lines and points for measuring the motion of the planet are also drawn. The mean apogee of the epicycle (aux epicycli media), point g on the diagram, is the point of the epicycle marked by the line drawn from the centre of the equant and passing through the centre of the epicycle (v), while its true apogee (aux epicycli vera), point h on the diagram, is marked by the line drawn from the centre of the World and passing through the centre of the epicycle. The line of the mean motion of the epicycle and of the planet (linea medii motus) is line ci, drawn from the centre of the World to the zodiac, parallel to line evg, drawn from the centre of the equant and passing through the centre of the epicycle. The line of the true motion of the epicycle (linea veri motus epicycli) is line cvk, drawn from the centre of the World to the zodiac and passing through the centre of the epicycle. The line of the true motion of the planet (linea veri motus planetae) is line clm, drawn from the centre of the World to the zodiac and passing through the body of the planet. The apogee of the eccentric 'in second signification' (aux in secunda significatione) is arc ab. The mean motion of the epicycle and of the planet (medius motus) is arc ai, measured eastward from the beginning of Aries to the lines of the mean motion. The true motion of the epicycle (verus motus epicycli) is arc ak; the true motion of the planet (verus motus planetae) is arc am. The mean centre of the planet (centrum medium planetae) is arc bi, measured eastward on the ecliptic from the line of the apogee to the line of the mean motion. The true centre (centrum verum sive aequatum) is arc bk, measured from the line of the apogee to the line of the true motion of the epicycle. The equation of the centre in the zodiac (aequatio centri in zodiaco) is arc ki, measured on the ecliptic between the line of the true motion of the epicycle and the line of the mean motion. The equation of the centre in the epicycle (aequatio centri in epicyclo) is arc hg, measured on the epicycle between the true and mean apogees of this epicycle. The mean argument of the planet (argumentum medium planetae) is arc gfl, measured on the epicycle, according to the direction of the movement of this epicycle, from the mean apogee of the epicycle to the body of the planet. The true argument of the planet (argumentum verum) is arc hgfl, also measured on the epicycle in the same direction, from the true apogee of the epicycle to the body of the planet. The equation of the argument (aequatio argumenti) is arc mk, measured on the ecliptic between the line of true motion of the planet (linea veri motus planetae) and the line of the true motion of the epicycle (linea veri motus epicycli). On these terms and their definitions, see Reisch (1503), sig. o3r, and Reinhold (1553), fol. 56r.