The determination of the solar parallax from transits of Venus in the 18th century

The transits of Venus in 1761 and 1769 initiated the first global observation campaigns performed with international cooperation. The goal of these campaigns was the determination of the solar parallax with high precision. Enormous efforts were made to send expeditions to the most distant and then still unknown regions of the Earth to measure the instants of contact of the transits. The determination of the exact value of the solar parallax from these observations was not only of scientific importance, but it was expected to improve the astronomical tables which were used e.g., for navigation. Hundreds of single measurements were acquired. The astronomers, however were faced by a new problem: How is such a small quantity like the solar parallax to be derived from observations deteriorated by measuring errors? Is it possible to determine the solar parallax with an accuracy of 0.02" as asserted by Halley? Only a few scientists accepted this challenge, but without adequate processing methods this was a hopeless undertaking. Parameter estimation methods had to be developed at first. The procedures used by Leonhard Euler and Achille-Pierre Dionis Duse'Jour were similar to modem methods and therefore superior to all other traditional methods. Their results were confirmed by Simon Newcomb at the end of the 19(th) century thus proving the success of these campaigns.