Transits in the Seventeenth Century and the Credentialling of Keplerian Astronomy

Consider for a moment the problem that the inferior planets posed to early cos- mologists such as Ptolemy or Copernicus. In principle, observations of Mercury or Venus at elongation could establish with fair accuracy the size and eccentricity of their orbits, although in reality the difficulty of observing Mercury at certain critical elongations made this determination somewhat problematic, and the fairly close commensurability of the period of Venus to that of the Earth (5/8) made it almost impossible over extended periods to observe satisfactory configurations. Finding the heliocentric longitude of Mercury or Venus (equivalent to the actual position in the epicycle) was even more difficult, however, because as the planet rounded the curve near elongation it was moving almost directly toward or away from the observer. The angle of elongation gave an excellent fix on the size of the orbit (or epicycle), but the distance from the observer that could have determined the longitude of the planet in its orbit was practically unobtainable. In contrast, an observation of a transit could provide strong leverage for establishing the heliocentric longitude and could furnish a check on the other parameters as well.Transits in the seventeenth century could therefore provide the astronomer with a significant test of planetary theories, and in particular could have revealed that Kepler's astronomy, as expressed in his Tabulae Rudolphinae, was substantially superior to the alternatives.' In principle the Keplerian physics, melded with the large base of Tychonic observations, provide a sophisticated interpolating scheme for the previously unobserved parts of the orbits. One historical fact especially sup- ports the notion that transits played a critical role here. Noel Duret (Natalis Durret), a little-known French astronomer who published a set of ephemerides for 1637 to 1651, started out using the 1632 tables of the Dutch astronomer Philips Lansbergen.2 He was not quite halfway through computing his ephemerides when he noticed that the Lansbergenian-based retro-calculations for the transit of Mercury observed by Gassendi in 1631 were not nearly as satisfactory as those reckoned from Kepler's tables. So, in midstream, Duret switched the basis of his calculations, and when his ephemerides were published in 1641, he prominently explained why Kepler's methods were to be preferred.3Two more publications, from the end of the century, seem to confirm the role of the transits by using and comparing multiple tables for specific transits. Ulrich Junius in Ulm (Figure 1) calculated the circumstances of the Mercury transit of 1697 from eight different tables (including Kepler and Lansbergen), and he also computed the circumstances for earlier transits including the one in 1631 predicted by Kepler and observed by Gassendi.4 Matthew Honold followed suit for the transit of 1721, calculating in advance from six tables, but conspicuously omitting Lansbergen and the Prutenics.5 He dedicated his thesis to Junius.The archives of the Paris Observatory contain the greatest depository of seventeenth-century transit material. These records were systematically collected and arranged by Joseph-Nicolas Delisle in the 1740s during his two decades in St Petersburg. Unlike the majority of his French colleagues, Delisle was a convinced Newtonian, and a fan of Edmond Halley, who had proposed that planetary transits could help determine the solar parallax.6 Delisle hoped that by gathering the data from the Mercury transits of the previous century, he might single-handedly be able to improve the numerical value of the solar parallax. Not only that, but as a Halley disciple he had the advance printed pages of Halley's tables, which would not be finally published until 1752, well after Halley's death. Thus Delisle was not particularly interested in validating Kepler's century-old tables, but in checking out the Halley tables. Nevertheless, his well-organized files showed who had observed the various transits, even if they rarely demonstrated that those observers had any interest in checking up on Kepler. …