Page - 261 - in Maximilian Hell (1720–92) - And the Ends of Jesuit Science in Enlightenment Europe

261Observing Venus and Debating the Parallax because of the high southern declination of Jupiter, partly because of overcast weather. The only feasible data Hell obtained were those of the solar eclipse that took place the day after the transit, as well as the observation of the transit itself.7 However, the use of the Venus transit data for the purpose of determin- ing the longitude would only be of indirect value, as a crosscheck after the so- lar parallax had been calculated. At the time Hell wrote his Venus transit report from Vardø, this was way too early, since only European observations had reached him by then.8 As for the solar eclipse, this was obviously followed closely not only by Hell and other astronomers on Venus transit expeditions across the world but also by staff at all the high-standard observatories of Eu- rope. For observing the eclipse, Hell used the eight-and-a-half-feet long tele- scope, and Sajnovics the ten-and-a-half-feet. According to Hell’s report, the two astronomers determined the end of the eclipse as identically as could be, only a single second differing between them.9 Having returned to Copenhagen in the autumn of 1769, Hell was able to cal- culate the longitude of Vardø by means of corresponding observations of the solar eclipse of June 1769 provided by Maskelyne in Greenwich, Messier in Paris, Christian Horrebow and assistants in Copenhagen, Wargentin and Bengt Ferrner in Stockholm, Christian Mayer in St. Petersburg, Pilgram and the ama- teur Sambach in Vienna, and Cäsar Aman (Amman, 1727–92) in Ingolstadt.10 In this way, he found a longitude of 3h 14m 41.8s east of the island Ferro, or 1h 55m 6s east of Paris, corresponding to 2h 4m 27s east of Greenwich.11 As Hell saw it, however, this was only a preliminary result, for he was still waiting to check his figure on the basis of Venus transit reports from “America” (which in Hell’s par- lance included the Pacific). The site of Hell’s observatory is nowadays determined as 31° 6′ 27″, or 2h 4m 25.9s east of Greenwich. This means Hell’s initial determination was only longitude—that is, the moons of Jupiter, eclipses of the Sun and Moon, occultations of fixed stars by the moon, or transits of the moon through the meridian compared with positions of fixed stars—and that they show me the kindness of sharing these observa- tions with me.” It is likely that Hell asked colleagues at other sites for similar observations, but no other letters have been available for the present study. 7 Hell, Observatio transitus Veneris […] 1769, 30–31. 8 Hell, Observatio transitus Veneris […] 1769, 31. 9 That is, within a margin of error of ±½ second (cf. Hell, Observatio transitus Veneris […] 1769, 31). Other observations of the same moment made at identical sites by multiple ob- servers varied up to ±5 seconds (cf. Hell, Observatio transitus Veneris […] 1769, 33–42, esp. 38). 10 Hell, Observatio transitus Veneris […] 1769, 33–45. 11 Hell, Observatio transitus Veneris […] 1769, 50 (mentioning only Ferro and Paris). We are indebted to astronomer Truls Lynne Hansen, former head of Tromsø Geophysical Obser- vatory, for calculating the Greenwich value.