Page - 156 - in Maximilian Hell (1720–92) - And the Ends of Jesuit Science in Enlightenment Europe

Chapter 3156 aid available, both in 1761 and in 1769. Cook did not even take a chronometer on his first circumnavigation of the globe: the moments of ingress and egress were determined by means of standard pendulum clocks.55 A pendulum clock, however, cannot be transported while it is running and needs to be corrected astronomically over several days in order to be held reliable. Besides, its retar- dation or acceleration compared to the Sun would vary from day to day, de- pending on the temperature. For most purposes, George Graham’s (1675–1751) temperature-compensated mercury pendulum solved this problem, but a re- tardation or acceleration of a few seconds every twenty-four hours was still common. For the delicate observations of a transit, where each moment need- ed to be determined to the exact second, this uncertainty was unacceptable, which is why so many of the Venus transit reports include tables of time- keeping stating the retardation or acceleration of the clock over many days. To  pick another example from 1769, Hell used two pendulum clocks in Vardø: one from Vienna, the other from Copenhagen. Both were constructed with temperature-compensated pendulums, so as not to be too severely affected by climatic factors. Nevertheless, they had to be tested against astronomical ob- servations over several weeks leading up to the transit.56 The problem regard- ing clocks was particularly acute in the case of temporary observation sites set up during expeditions. However, as shown by the case of Caspar Müller above, lack of proper time-keeping also rendered the data of some amateur observers questionable. As a second difficulty, the moments to be observed were particularly the second, third, and fourth contacts of Venus with the Sun’s limb. The very first contact, that of Venus’s exterior contact at ingress, was generally held to be too difficult to observe. (Venus being invisible on a daytime sky, the observer would simply not know where to look for it until the contact had taken place and the ingress had in fact started.57) However, during the transit of Venus in 1761, a 55 See, e.g., Peter Aughton, Endeavour: The Story of Captain Cook’s First Great Epic Voyage (Moreton-in-Marsh: Windrush, 1999), 11. 56 Only a small extract of these tests found inclusion in the Venus transit report (Observatio transitus Veneris […] Wardoehusii, 61–69). Hell’s manuscript “Observationes astronomicæ et Cæteræ in itinere litterario Viennā Wardoëhusium usque factæ” (from 1768 to 1769, preserved at the wus) contains a longer series of tests, starting April 26 and ending June 4, 1769. Another description containing extracts from these tests is extant in an untitled manuscript of Hell, starting with the words “NB De Horologijs” (1769, wus). 57 Halley in fact insisted that only the interior contacts were to be used, i.e., the time span between the occurrence of the second and third contacts was the focus of his attention (cf. Halley, “Methodus singularis”). Later astronomers extended their attention to the ex- terior contacts as well, particularly the fourth and last contact of Venus with the limb of the Sun.