Page - 266 - in Maximilian Hell (1720–92) - And the Ends of Jesuit Science in Enlightenment Europe

Chapter 6266 apparently never undertook a comprehensive, comparative study of the pole height yielded by observations of the Sun versus that yielded by the stars. Inci- dentally, Hell’s final conclusion concerning the latitude of his observatory—70° 22′ 36″—is today found to be somewhat more inaccurate than his determina- tion of the longitude: it should be 70° 22′ 15.5″ north, that is, 20.5 seconds, or 632 meters farther south than Hell’s figure.27 It is important to note, however, that this modern value depends on more accurate knowledge of the curvature of the Earth’s surface than that which existed in the eighteenth century. An overall examination of Hell’s activities as a surveyor during his expedition, with assessments of his results in the light of the history of Nordic geodesy, is a desideratum.28 In any case, as the third day of June 1769 was approaching, Hell felt that he had safely determined the latitude of his observatory. The running of the clocks had already been tested for weeks, and the frequency of these tests was intensi- fied in the last days before the transit. Ideally, such tests involved observations of the Sun as it passed the meridian in the south at noon and the meridian in the north at midnight. The transit was going to take place when the Sun was in the north, meaning that the northern room of his observatory (the observato- riolum septentrionale) would be used for this crucial observation. Having checked the time-keeping at twelve o’clock in the day, Hell had to move his instruments over to the northern chamber in the afternoon of June 3 in order to be prepared for the transit of Venus. By the next morning—June 4—at least two of his telescopes must have been moved back again, as these were used to 27 Personal communication from Bjørn Geirr Harsson. The astronomical latitude, observed by Hell, is related to the plumb line at the station, whereas the latitude obtained from a gps receiver is related to the normal at the ellipsoid. “The angle between the plumb line and the vertical of the ellipsoid is called the deflection of the vertical,” Harsson explains. “In Vardø the geoid is tilting to east northeast, which means that the deflection of the vertical has a component in direction north, even if the main component is in direction east. The north component of the deflection of the vertical is computed to be 2.5 second of an arc at today’s post office in Vardø. So if 2.5″ is added to the GPS-latitude, the two lati- tudes can be compared. Hell’s latitude was 70° 22′ 36″ and the GPS-latitude is 70° 22′ 13″. If we add the 2.5″ to the GPS-latitude we get 70° 22′ 15.5″. The difference of 20.5 seconds corresponds to a latitude for Hell to be 632 meters north of today’s gps position of the same place.” 28 An investigation of the latitude for a single location (in Christiania, now Oslo) by Harsson in 2003 gave a discrepancy of only nine arc seconds, or 270 meters between Hell’s determi- nation and the modern value. Cf. Per Pippin Aspaas and Nils Voje Johansen, “Astronomen Maximilian Hell: Fra Wien til Vardø for å se Venus,” Ottar: Populærvitenskapelig tidsskrift fra Tromsø Museum 249, no. 1 (2004): 3–11, here 5–6. It would be futile, however, to con- clude much from the examples of Oslo and Vardø alone.