Seite - 263 - in Maximilian Hell (1720–92) - And the Ends of Jesuit Science in Enlightenment Europe

263Observing Venus and Debating the Parallax A more straightforward method of calculating the latitude was to observe the apparent distance of the Sun’s upper limb from the horizon when it reached its highest point at noon. This method yielded data of sufficient accuracy for the needs of ordinary navigation, but not for the delicate calculations of the solar parallax, where each observatory had to be determined as exactly as pos- sible. The method presupposed, for example, that the refraction of the site was exactly known. On his trip back and forth, Hell used this less exact method to determine the latitude of thirty-seven sites between Copenhagen in the south and Vardø in the north. He estimated the degree of uncertainty involved in these measurements to be around ±15″, or for some around ±30″.17 Fifteen arc seconds in the latitude would equal only 0.05 mm on the circle of the quadrant during observation,18 making it hard to believe that Hell’s claim to an uncer- tainty of only ±15″ is a reliable figure. It may be added that for surveys in central parts of Sweden around the mid-eighteenth century, an uncertainty of ±30″ was deemed acceptable, whereas Hellant in his surveys of Lapland argued that ±1′ must suffice.19 It would have been interesting to learn whether Hell tested the two methods comprehensively against each other in Vardø. However, neither his Venus tran- sit report nor his subsequent treatises on the solar parallax give any evidence of this.20 In order to answer this question, we need to look into the letters of 17 Maximilian Hell, “Nogle Steders Geographiske Breder,” 622; repeated in Hell, “Observatio- nes astronomicae latitudinum,” 309–10. 18 Personal communication from Truls Lynne Hansen, based on the study of Hell’s descrip- tions of Niebuhr’s quadrant. The radius of the quadrant is stated to be two feet in Hell, “Nogle Steders Geographiske Breder,” 621–22; Hell “Latitudines geographicae,” fol. 4, and in Hell, “Observationes meteorologicae,” 308–9. The same size is given in a letter from Niebuhr to Franz Xaver von Zach in Gotha, dated Meldorf, July 9, 1801 (originally pub- lished in von Zach’s Monatliche Correspondenz zur Beförderung der Erd- und Himmels- kunde 4 [September 1801]: 240–53, here 244: “Mayer had made for me a quadrant of two-foot radius for observations on land.” In a more detailed description in Hell’s MS “ Observationes astronomicæ et Cæteræ Jn Jtinere litterario Viennâ Wardoëhusium usque factæ” [1768–69], [1], the radius of the quadrant is said to be one foot and two Viennese inches, whereas its tube was two feet and two inches. Probably, “i ped. ii dig.” is a slip of the pen for “ii ped. ii dig.” In that case, the exact radius of Niebuhr’s quadrant was two feet and two inches, or twenty-six inches). 19 Cf. Sven Widmalm, Mellan kartan och verkligheten: Geodesi och kartläggning, 1695–1860, Institutionen för idé- och lärdomshistoria, Uppsala universitet, Skrifter 10 (Uppsala: Insti- tutionen för idé- och lärdomshistoria, Uppsala universitet, 1990), 79. 20 Admittedly, Hell mentions an initial result of 70° 20′ for the pole height in a more elabo- rate treatise on his method of calculating the latitude, but gives no details as to whether he cross-checked this result with other solar observations later in his stay in Vardø. Hell, “Methodus astronomica Sine usu Quadrantis,” 31.