Page - 140 - in Maximilian Hell (1720–92) - And the Ends of Jesuit Science in Enlightenment Europe

Chapter 3140 weather deprived an observer of one of these crucial contacts, or a station’s geographical position made only the ingress or egress of a transit observable? Following Halley’s method, precious data from many stations were at risk of having to be discarded. This concern was raised by Delisle, the “grand old man” of European astronomy and a leading figure in the planning of the Venus tran- sit project of 1761, since Halley himself had passed away in 1742. In various memoirs, articles, letters, and unpublished lectures, Delisle presented an alter- native method for the computation of the solar parallax. If a single contact of Venus with the Sun’s limb was observed from two stations ranging far apart, he argued that the difference in latitude and longitude between the two stations would provide the necessary basis for the computation of the parallax.11 This suggestion no doubt came as a relief to many astronomers on the European continent. The transit of 1761 was pre-calculated to take place during the night and early morning hours as seen from the heartland of Europe, leaving hope only for the egress (i.e., the end stages of the transit) to be observed. By follow- ing Delisle’s method, however, data from these observatories would be just as valuable as those from faraway places on other continents. This is where Hell saw his golden opportunity. Although no expeditions overseas were planned by the Habsburg monarchy, which lacked territories in which the entire duration of the coming transit would be visible, it was still possible—thanks to Delisle’s modification of Halley’s idea—for the Imperial and Royal Observatory of Vienna to provide the global community of astrono- mers with crucial datasets for the calculation of the solar parallax. And not only that, Hell could employ his prestigious title of imperial and royal astrono- mer in combination with his budding fame as editor of the only official astro- nomical yearbook apart from France’s Connoissance des temps to organize observations all over the Habsburg lands and beyond. This is exactly what he did. A concrete step to facilitate this was the publication of a twenty-page print- ed instruction, written by Hell, explaining how the transit should be observed. Written in a language accessible even to those with relatively little previous ingressus and egressus “going in” and “going out”). Furthermore, internal and external are common synonyms for interior and exterior. 11 In one of his manuscripts, read as a memoir to the Royal Academy of Sciences in Paris, April 30, 1760, Delisle explains: “This method consists of using observations of the entry or exit [of Venus] at places where one of these two stages will take place at points of time differing as much as possible between them.” Jean-Eudes Arlot, ed., Les rendez-vous de Vénus/Venus’s rendez-vous, cd-rom (Les Ulis: edp Sciences, 2004), caption Delisle, manu- scrits 1753 et 1760, 10. See also Woolf, Transits of Venus, 33–35.