May 18, 2012 2:40:08 PM
Next month the world will enjoy a rare astronomical treat. Venus will cross the face of the Sun; it did this in 2004, because these transits tend to come in pairs separated by eight years, but the paired events occur only about every 120 years or so. Of course astronomers all over the world will be looking carefully, and recording, and timing, and there will be an international effort to gather all possible data from the event. We take such scientific cooperation for granted now, but it did not always exist, and it was transits of Venus that started the tradition of worldwide scientific cooperation. In Chasing Venus: The Race to Measure the Heavens (Knopf), Andrea Wulf has told the amazing stories of the transits of 1761 and 1769, and the expeditions made by intrepid astronomers all over the world to get some idea of the vastness of our solar system. Looking at the sky every night, documenting points of light and their changes might be considered dull; indeed, at the time Britain's Royal Observatory was looking for assistant astronomers and suggested that they be "obedient drudges." The picture here, however, is of adventurous and hardy men (which is not to say they never complained about the hardships) who ventured into the wilds, literally risking their lives for the sake of getting astronomical data. Wulf's entertaining book is a fine tribute to that admirable human trait of scientific curiosity.
The expeditions had been set in motion in 1716, when Edmond Halley suggested the worldwide scientific project. Halley, more famous for computing the orbit of the comet that bears his name, knew that he himself would not be around for the transits. He instructed those who would live after him, though, to measure the timing of the transits from observation points all over the globe. Precisely knowing when, from different observation points, Venus started to cross the face of the Sun, and when it exited, would yield data that could be entered into the trigonometric tables to give precise distances. In Halley's time, astronomers knew, by the times planets took to go around the Sun, only proportionate distances; the distance from Earth to Jupiter, for instance, was known to be five times the distance from Earth to the Sun. But no one had a specific distance to the Sun, and Halley knew that close observations of the transit would make an accurate calculation of this essential figure possible. The observations had to come from different parts of the Earth to show different tracks of the planet, but they did not have to be measured except for the exact time of the entrance and the exit, and the exact location of the observer; only a telescope and a clock would be needed, and whatever it took to mark the observer's longitude and latitude.
Halley's call to action was well heeded. The French astronomer Joseph-Nicolas Delisle had met the aging Halley in 1724 and discussed the transit, and in 1760 the aging Delisle had presented his own proposal to the Académie des Sciences for global measurements of the transit the next year. Britain and France were then undergoing the Seven Years War, but such vulgar enmities were forgotten among the scientists who were to join together and share data in the efforts for the 1761 and 1769 transits. Delisle was a one-man clearinghouse of information, and knew astronomers all over the world. He was a skilled surveyor, and one of the documents he used to recruit astronomers to the cause was a "mappemonde," a map showing where upon Earth a transit could best be seen, and showing the observation points, as widely separated as possible, where astronomers could record Venus's entry and exit times.
And so a small army of astronomers, many of whom may have been drudges devoted to scanning the sky every night and going to their homes to sleep during the day, enlisted for the sort of work they could not have predicted when they started their careers. They may have been a scientific fraternity, but as they spread out over the oceans, some of them could not avoid being part of the national conflicts of the time, besides participating in all the other risks of sea travel. A couple of Britons named Mason and Dixon were eventually to be associated with the famous line in America, but in this effort they were trying to get to Sumatra and were nearly blown out of the water by a French warship. The Frenchman Jean-Baptiste Chappe d'Auteroche went from Paris to the depths of Siberia for the 1761 transit, going by sled for much of the frozen and forsaken way. When he got to his post, the villagers not only distrusted him as a stranger, but seeing his telescope, quadrant, and clock, thought he was a magician, and blamed him for the recent floods. He had to be put under guard by the local governor because peasants threatened to murder him. Eight years later, he was measuring the second transit after a grueling travel to Baja California, and he caught the typhus that was killing off others in a Jesuit mission. He was delirious with fever, but kept himself alive long enough to make all his observations and record them for others to use. Tahitian natives stole and dismantled equipment brought by Captain James Cook for viewing the transit in 1769, one of the first voyages that was to make his name as an explorer and cartographer. Among the saddest stories here is that of the Frenchman Guillaume Le Gentil de la Galaisière, who was sailing to French possessions in India for the 1761 transit, was taken off his itinerary by British war moves, and wound up going to Mauritius instead. On the day of the transit he was aboard ship and although he could see the transit, the ship's movements prevented accurate measurements. Then he got to India for the 1769 transit, only to have clouds make observation impossible. He thought himself doomed, and then his return to France took so long that his heirs had declared him dead and divided up his property. His was not the only sighting obscured by clouds. The long-planned observations by Anders Planman in Finland looked like they were going to go perfectly, in a cloudless sky, but then his view was obscured by smoke; local farmers were clearing woodland by burning it.
The viewings of the first transit were generally unsatisfactory, but they pointed the way to doing better ones for the second transit. For instance, it was realized that positions that saw the transit with the Sun near the horizon produced less reliable data, because of atmospheric interference, than those seeing the Sun high in the sky, so that observation points for 1769 could be planned with this in mind. There were still inaccuracies; the astronomers were mystified by the display that showed that Venus did not show up as a simple black disk entering the large bright disk of the Sun, but seemed to distort and flow into the Sun, making the measurement of crossing times inexact. The newly calculated distance to the Sun was still not exactly agreed upon, but it was far closer than any that had come before.
It wasn't just that we got a better picture of our place in the universe from these expeditions. The hundreds of voyagers produced better charts of their observation points, as well as descriptions and specimens of plants, animals, geography, weather, and human activity from the regions they traversed. The great inspirational lesson, though, is from the heroic astronomers and crews who endured daunting hardships to work together on a shared goal. "Never before," writes Wulf, "had scientists and thinkers banded together on such a global scale - not even war, national interests or adverse conditions could stop them." This is an inspiring story. May the trust that nature could best be understood by reason, a cornerstone of the Enlightenment, continue to unite and embolden us.
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