Astronomical Images : Orbs and motion of Venus in relation to the motion of the Sun

Erasmus Reinhold

Astronomical Images

<p style='text-align: justify;'>This Parisian edition was copied from the first edition of the commentary of Peuerbach by Erasmus Reinhold, printed in Wittenberg by Hans Lufft in 1542. Subsequently, in 1556, Charles Perier published a new edition, copied from the revised edition printed by Lufft in 1553, which contained additions to the theory of the Sun (<i>[Theoricae] auctae novis scholiis in theoria Solis ab ipso autore</i>). In the first editions of the <i>Theoricae novae</i> there is no diagram specifically devoted to Venus. Oronce Fine, first in his edition of Franciscus Capuanus' commentary on Peuerbach (Paris, 1515), then in his edition of the <i>Theoricae novae</i> (Paris, 1525), was the first to illustrate the short chapter <i>De Venere</i>. The next step was to illustrate what constitutes the main topic of Peuerbach's chapter: the relationship between the motion of Venus and that of the Sun. In his 1528 edition Apian has copied (and simplified) the Fine diagram of the orbs and motion of Venus and inserted it inside the representation of the solar orbs. This diagram is an improved version of the Apian diagram (fol. 18v). It is a close copy (or a re-use) of a woodcut first printed in the Wittenberg edition of the <i>Theoricae planetarum</i> (1535). We see the three orbs of Venus, similar to the orbs of the superior planets: the two blackened deferent orbs of the apogee with their common axis, the vertical line, also called the line of the apogee (<i>linea augis</i>), and the white eccentric orb sandwiched between them. The centre of the epicycle is attached to the circle in the middle of the latter, also called the eccentric deferent. The body of the planet is represented by a star at the circumference of the epicycle. The three orbs of Venus are marked 'B'. The eccentric equant, intersecting with the eccentric deferent, has been drawn and the three centres (of the equant, the eccentric deferent, and the World) are labelled 'H', 'C', 'D' respectively. Around the orbs of Venus, and separated from them by a white circle, the three orbs of the Sun are marked 'A'. The body of the Sun is visible in its eccentric deferent, above the epicycle of Venus. All six orbs are surrounded by a circle that represents the zodiac. We see that the line of the apogee (or axis of the deferent orbs of the apogee), passing through the three centres, H, C, D, is common to Venus and to the Sun. According to Peuerbach the deferent orbs of the apogee of Venus follow the movement of the eighth sphere in such a way that the apogee of the eccentric of Venus 'is always under that place of the zodiac under which the apogee of the Sun lies' (<i>ita tamen ut aux eccentrici eius sub eo loco zodiaci sit semper, sub quo aux eccentrici Solis</i>). Some of the lines that measure the motion of the planets show the relationship between the motion of Venus and that of the Sun. The line of the mean apogee of the epicycle of Venus is drawn from H, the centre of the equant, passing through the centre of the epicycle. Line DE, drawn from the centre of the World to the zodiac, is parallel both to the line of the mean apogee of the epicycle of Venus, mentioned above, and to the line drawn from C, the centre of the eccentric deferent, to the centre of the body of the Sun. Thus, it is the line of the mean motion both of Venus and of the Sun. According to Peuerbach, the centre of the epicycle of Venus, on its eccentric deferent, 'makes one revolution precisely in the time in which the orb carrying the Sun makes one' (<i>in eo tempore revolutionem unam centrum epicycli faciat, quo precise orbis Solem deferens unam</i>). For the relationship between Venus and the Sun is such that the line of the mean motion of the former 'terminates in the same point of the zodiac in longitude as the line of the mean motion of the Sun â?¦ Therefore, they are always in their mean conjunction' (<i>linea medii motus [Veneris] in eo loco zodiaci secundum longitudinem, in quo linea medii motus Solis terminetur â?¦ Semper igitutur est media eorum conjunctio</i>). Line DF, drawn from the centre of the World to the zodiac, and passing through the body of Venus, is the line of the true motion of Venus: <i>Linea DB ostendit verum motum Veneris</i>. Peuerbach also mentions the period of revolution of the epicycle: nineteen solar months, and notes that this period is not equal to that of the revolution of the Sun, as in the case of the superior planets. Reinhold notes that although this diagram is also found in earlier copies of the <i>Theoricae novae</i>, he has reproduced it only because it represents the orbs (<i>hanc figuram tantum propter picturam orbium adjeci, cum et in prioribus exemplaribus haberetur</i>). To explain the relationship between the movements of Venus and the revolution of the Sun, he introduces a new diagram, <i>Theorica centrorum linearum et aequationum Solis et Veneris</i>, accompanied by scholia. Translated quotations of Peuerbach's <i>Theoricae</i> are from Aiton (1987). Quotations from Reinhold's commentary are translated or paraphrased by Isabelle Pantin.</p>