Seite - 138 - in Maximilian Hell (1720–92) - And the Ends of Jesuit Science in Enlightenment Europe

Chapter 3138 A planetary transit can only occur with either of the two planets Mercury and Venus, since the other planets in our solar system have orbits farther out, thus never passing between the Sun and the Earth. However, although transits of Mercury occur fairly frequently (between twelve and fourteen times a cen- tury), they are of little use in calculating the solar distance. Mercury is simply too close to the background (the Sun) to offer any substantial parallax, no mat- ter how far apart the terrestrial observers spread themselves.8 The planet Ve- nus, on the other hand, orbits the Sun much closer to the Earth and should therefore be of far better use, according to the ideas of influential eighteenth- century astronomers. A transit may last for several hours, depending on how close to the center of the Sun’s disc the planet makes its passage. As a result of parallax, the time spent by the planet crossing the disc of the Sun will also vary according to where on the surface of the Earth an observer is situated. The transit of Venus in 1769, for example, as observed by Hell in Vardø, lasted 6h 29′ 34.5″ (six hours, twenty-nine minutes, thirty-four-and-a-half seconds). At the same time, as- tronomer Charles Green (1735–71) of Cook’s crew on Tahiti saw Venus spend 6h 5′ 37″ crossing the Sun (i.e., nearly twenty-four minutes less).9 This difference in time was a key figure in the calculation of the Sun’s parallax. By measuring the exact time spent by Venus in crossing the Sun, astronomers were able to determine how close to the center of the Sun’s disc the transit took place as seen from each station. Theoretically, the position of Venus on the Sun’s disc could be measured. In practice, such observations turned out to be difficult, and the displacements of Venus insufficiently large to yield a satisfactory re- sult. Exact time-keeping, combined with the determination of each observer’s geographical position, therefore came to constitute essential data for the cal- culation of the solar parallax (see fig. 4). The crucial stages of the transit were the moments of contact between Ve- nus and the limb of the Sun, commonly designated as the exterior and interior contact of ingress, and the interior and exterior contact of egress, sometimes referred to in order of appearance as the first exterior, first interior, second in- terior, and second exterior contacts, or sometimes just first, second, third, and fourth contacts (see fig. 5).10 It was the two interior contacts, that is, the second and third contacts, that were of primary concern to Halley. But what if cloudy 8 See, e.g., Woolf, Transits of Venus, 35–51; Marlot, Les passages de Vénus, 92, 99. 9 Duration of the entire transit as reported by Hell, Observatio transitus Veneris […] Wardoëhusii […] facta […] 1770, 78–80, and by Charles Green in James Cook, “Observations Made […] at King George’s Island in the South Sea […],” ptrsl 61, no. 1 (1771; published 1772): 397–421, here 410. 10 The terms immersion and emersion are also used as synonyms for ingress and egress re- spectively (immersio and emersio in Latin literally mean “diving in” and “diving out,”