Page - 162 - in Maximilian Hell (1720–92) - And the Ends of Jesuit Science in Enlightenment Europe

Chapter 3162 One conspicuous element of the transits of Venus, which had already been noted by the two amateur astronomers who observed the transit of 1639, was that the planet’s size appeared to be considerably larger in the night sky than it did in front of the Sun during a transit. In fact, observers of the transit reported that Venus appeared to have a diameter of less than one arc minute; most mi- crometer determinations gave about fifty-eight seconds, or just short of a hun- dredth part of the Sun’s disc. In his 1761 report, Hell explains how, during the nights before and after the transit, Venus appeared in the sky as a bright star of approximately one arc minute and seventeen seconds,78 or almost thirty-three percent larger than when viewed against the disc of the Sun. What could be the cause of this sudden diminution? Hell discusses various hypotheses that might explain the phenomenon and concludes that it was most likely to be caused by a certain optic tendency, causing dark objects to appear smaller when viewed against a light background and light objects to appear larger in front of a dark background. This tendency, combined with other optical factors caused by the lenses of the astronomical tubes and the smoked glasses that were used for observations of the Sun, seemed to Hell to be the most likely reason for the change of size of Venus. Further research was needed, however, and his word- ing in this context is very cautious.79 As to the adjustment of the nodes of the planet’s orbit, Hell used various observations of the path of Venus across the Sun’s disc to determine the coming transits of Venus in the years 1769 and 1874 as seen from the center of the Earth. He also calculated the visibility of the so- lar eclipse that was to take place on June 4, 1769.80 A final inference that could be drawn from the transit was that Venus prob- ably had no moon. From time to time, various astronomers had argued its ex- istence, among them the conseiller au Grand Conseil Armand-Henri Baudouin de Guémadeuc (1734/37–1817). In a lecture held at the Académie Royale des Sciences in Paris on May 20, 1761 and printed immediately afterward, Baudouin reported observations made by his friend Jacques Montaigne (1716–85?) in Li- moges, who had seen a gleaming object beside Venus on four occasions earlier the same month. Montaigne—and his patron Baudouin—interpreted this as the much-sought moon of Venus.81 The most likely appearance of such an 78 Hell, “Observatio transitus […] 1761,” 24–25, 99–100. 79 Hell, “Observatio transitus […] 1761,” 24–28. 80 Hell, “Observatio transitus […] 1761,” 110–15. 81 The full title was Mémoire sur la découverte du satellite de Venus, & sur les nouvelles ob- servations qui viennent d’être faites à ce sujet; Lu à l’Academie Royale des Sciences le 20 Mai 1761. On this memoir and its reception, see Kragh, Moon That Wasn’t, esp. 44–56. To his references, we may add J.G. [Johann Georg?] Krünitz, “Verzeichniß der vornehmsten Schriften von der Venus und dem Merkur, und dem Durchgange dieser Planeten durch