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, knowing that he himself would not be around for the transits. Halley knew that observers at different points on the globe would only have to measure the exact times that the disk of Venus entered and exited that of the Sun, and that such times could calculate an exact distance to the Sun, something we did not know. The French astronomer Joseph-Nicolas Delisle took up the challenge in 1760, calling for global measurements of the transit the next year, and then for 1769. 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 were imperiled by the dangers of the sea, including the wartime activities of their respective nations. They endured disease (not every one successfully), hardships, foul weather, freezing temperatures, and the distrust of natives. 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.