Seite - 157 - in Maximilian Hell (1720–92) - And the Ends of Jesuit Science in Enlightenment Europe

157The 1761 Transit of Venus and Hell’s Transition to Fame totally unexpected problem occurred. Instead of entering and leaving the Sun in the form of a well-defined round spot, Venus was seen to take the form of a black drop around the moments of second and third contact. To some observ- ers, this phenomenon seemed to last for almost a whole minute. There may have been several causes of the “black-drop effect”: disturbances in the Earth’s atmosphere or that of Venus; some diffraction of light in the astronomical tubes of that time; astigmatism in the eye of the observer; or merely the stan- dard blurring of an image when two objects are very close to each other and the light is too dim for the human eye to distinguish between them. In any case, “a combination of solar limb darkening and telescopic point-spread func- tions” has been a matter of dispute right up to the present time,58 and whatever the cause, the phenomenon contributed to making the results of 1761 ambigu- ous. For the 1769 transit, the astronomical community was better prepared, and several reports include illustrations detailing the optical difficulties in- volved (see fig. 6). This did not eradicate the ambiguity of the data, but it was helpful when the observations of various observers were compared. The third problem was that the path of Venus in front of the Sun as seen from widely separated sites turned out to shift far less than anticipated by Hal- ley. There was no way that the difference in latitude between stations could suffice: knowledge of each station’s longitude was required as well. In theory, the difference in longitude between two places could be measured simply by transporting a running clock between them. The difference in local time, as revealed by simple observations of the Sun or stars, would then reveal the dif- ference in longitude between the two places. However, as mentioned earlier, this is an impossible procedure when using a pendulum clock. Hence, astrono- mers on expeditions did their best by adjusting their clocks to local meantime and then used celestial phenomena such as occultations of the moons of Jupi- ter or eclipses of the moon or the Sun, compared to observations of the same event communicated by other astronomers in faraway places, to compute the longitude: a very delicate and time-consuming process indeed.59 For the 1769 transit of Venus, however, a solar eclipse was predicted to take place on the 58 See, e.g., Maor, Venus in Transit, 95–97; Bradley Schaefer Jr., “The Transit of Venus and the Notorious Black Drop Effect,” Journal for the History of Astronomy 32, no. 4 (2001): 325–36; J.M. [Jay Myron] Pasachoff, Glenn Schneider, and Leon Golub, “The Black Drop Effect Explained,” in Kurtz, Proceedings, 242–53; quotation from J.M. Pasachoff and Naomi Pasa- choff, “Helge Kragh, The Moon that Wasn’t” (review), Physics in Perspective 12 (2010): 105–8, here 107. 59 Even at a foremost center of astronomical research like Paris, astronomers spent almost the entire eighteenth century defining the exact position of the observatory. See Moutch- nik, Forschung und Lehre, 101. Bearing this in mind, it should come as no surprise that the