Regular readers will recall that we are in the midst of a series of columns on the 18th-century transits of Venus. A transit of Venus is the occasion of that planet coming directly between the earth and the sun, so that we see it as a black blob moving slowly across the face of the sun. As explained in part I, the timing of this event leads eventually to a knowledge of the scale of the solar system, a quantity essential to astronomy but poorly known in the mid-18th century. It was also the case that although there would be transits of Venus in 1761 and 1769, there would be none thereafter for more than 100 years, so it was important to make the most of the opportunity. Thus strenuous efforts were planned by the major scientific bodies of that era to make the necessary observations. The difficulty, though, was twofold. First, adequate precision required that observers doing the timing be as widespread across the earth as possible, even though exploration of distant lands was still decidedly limited. Second, the major powers, particularly the two strongest naval powers, Britain and France, were at war with one another in 1761, making sea travel extremely hazardous.

In my second column on the topic, I described two of the British expeditions to observe the 1761 transit—that of Mason and Dixon to South Africa, and Winthrop's Harvard expedition to Newfoundland. In addition, we looked at the misfortunes of a French expedition, that of Pingré to the island of Rodrigues in the Indian Ocean. Here we deal with the two other French expeditions of 1761, that of Jean Chappe d'Auteroche to Siberia, and of Guillaume-Joseph-Hyacinthe-Jean-Baptiste Le Gentil de la Galaisière to India.

Chappe came from a family of the lower French nobility, but since he was only 31 when he was admitted to the Académie Royale des Sciences in 1759, and since the 1769 transit cost him his life, his career was short-lived and we know little about him. Certainly his early entrée to the Académie, and the work he did on his transit expeditions, bespeak a person of talent and determination. Who knows what he might have achieved in a longer life?

Through an invitation from the Russian Imperial Academy of Science, the Académie Royale des Sciences appointed Chappe to observe the transit of June 6, 1761 from Tobolsk, a city in central Siberia some 5,000 kilometers from Paris. This site was chosen because both the start and finish of the transit would be visible from it, granted clear weather. Protocol dictated that Chappe pay his respects to the Russian Academy in St. Petersburg en route, and since travel would be slow he would necessarily face a crossing of the Ural mountains and Siberian travel in a Russian winter.