Page - 165 - in Maximilian Hell (1720–92) - And the Ends of Jesuit Science in Enlightenment Europe

165The 1761 Transit of Venus and Hell’s Transition to Fame The debate on the notorious “moon of Venus” was soon followed by another one in which Hell was involved. As mentioned, Hell added as a sequel to his 1761 memoir some remarks about the visibility of Venus during the coming transits of 1769 and 1874. One of his conclusions was that the 1769 transit was not going to be visible in Vienna because the Sun would then be below the horizon. This conclusion was not obvious to every contemporary specialist, in- cluding the French astronomer Claude-Étienne Trébuchet (1722–84). When in 1764 Trébuchet published a work in which he argued against the conclusions of Hell, this provoked a brief review by the Viennese astronomer in the appendix of his Ephemerides.88 Trébuchet, for his part, defended his position in a lengthy letter published in the Journal des Sçavans for October 1766, concurring with the editors of the journal in their praise for the 1765 issue of the Ephemerides, but maintaining his disagreement. A summary of Hell’s reply was then pub- lished in the Journal des Sçavans in August 1767; Trébuchet’s rejoinder to this was also printed separately in 1770 and even personally sent by the author to Hell—though by this time Hell had been proven right, as the 1769 transit of Venus was indeed not visible in Vienna.89 However, although the core of the debate was a disagreement about the exact orbit of Venus, it soon also involved the existence (or non-existence) of the “moon of Venus,” the feasibility of solar eclipses for longitude determination, the methods of Halley and Delisle for computing the solar parallax, the correct interpretation of a mappemonde of the transit of 1769 that had been published by Lalande, and so on.90 Trébu- chet’s doggedness in the affair may have been fueled by more than purely sci- entific concerns. Introduced in the “Lettre à Messieurs les auteurs de Journal des Sçavans” (Letter to the gentlemen authors of the Journal des Sçavans ) as an “old servant of the queen” from the town of Auxerre, he was in fact a calcula- teur employed at the Connoissance des temps by Lalande.91 While Connoissance 88 Hell, “Observatio litteraria,” in Ephemerides 1765 (1764), 364–68. 89 Trebuchet, “Lettre à Messieurs les auteurs du Journal des Sçavans sur les passages du Vé- nus, & sur l’eclipse de Soleil arrivée en 1764,” JS (October 1766): 644–57; Hell, “Extrait d’une lettre du R.P. Hell […] sur le passage de Vénus observé en 1761,” JS (August 1767): 624–26; Trebuchet, Lettre à Messieurs les auteurs du Journal des Sçavans sur le passage de Vénus [separate reprint, with additions] (Bouillon: Société Typographique, 1770). The second Lettre of Trebuchet was—according to a handwritten message in the hand of Trebuchet on the back page of Hell’s copy (wus), dated Auxerre, September 16, 1770—written in 1768 and published in a Recueil philosophique. Only a brief resumé appeared in the Journal des Sçavans, as late as February 1771, 118–19. 90 Trebuchet, “Lettre à Messieurs les auteurs”; Hell, “Extrait d’une lettre.” See also Alexandre Guy Pingré, “Mémoire sur la parallaxe du Soleil, déduite des meilleurs observations de la durée du passage de Vénus sur son disque le 3 juin 1769,” hars (1775): 398–420. 91 Guy Boistel, “Nicole-Reine Lepaute et l’Hortensia,” Cahiers Clairaut 108 (2004): 13–17.