Transits, Travels and Tribulations, IV
In the last three of my Marginalia columns I described the adventures, successes and failures of astronomers who in the 18th century set out across vast expanses of the then relatively unexplored world to observe Venus on two of the rare occasions it comes between the earth and the sun and so is seen to transit the solar disk. The reason for such extreme endeavors was that those observations, if made from widely separated places on the earth, would lead to a greatly improved estimate of the scale of the solar system. This, we now know, would form the next step in an ongoing ladder of distance scales that would eventually reveal our place in our Milky Way galaxy, and the scale and age of the universe itself. Also, the immense efforts to observe these transits in 1761 and 1769 were in part inspired by the knowledge that no further such transits would take place until 1874 and 1882, and after those there would be none until 2004 and 2012.
Having looked at the adventures of the 1761 expeditions, as well as the horrors of Le Gentil's ill-fated travels for both transits, we here consider two of the other 1769 expeditions.
You may recall that Jean Chappe d'Autoroche succeeded in observing the 1761 transit from the town of Tobolsk in Siberia, having spent much of a Siberian winter getting there by sled, as well as narrowly escaping a lynch mob when an early thaw that flooded the town was said to be caused by his interfering with the sun. Nevertheless, he volunteered to observe the 1769 transit, asking only relief from snow and ice. The Académie Royale des Sciences in Paris obligingly sent him to Baja California.