The transit of Venus (seen centre right) across the sun in June 2004.  On June 5 and 6 (depending on where you live), we are likely to be the last people now living on Earth to witness a transit of Venus. If it is a clear day, we will be able to watch the brightest star of the night march for a few short hours as a small black dot across the fiery disc of the sun.  Not only is this one of the rarest astronomical events—the next one will not take place until December 2117—but in the 1760s, this heavenly rendezvous spurred the first international scientific collaboration, laying the foundation of modern science.  On June 5 and 6, 2012, one of the rarest astronomical events will occur: the Venus transit. This animation released by NASA shows an artist's conception of the event.  Transits of Venus always arrive in pairs, eight years apart. Then they don't recur for more than a century. As the transits of 1761 and 1769 approached, astronomers believed that by recording their exact time and duration they could answer one of the most pressing scientific questions of the age: the distance between Earth and the sun and, by extension, the size of the solar system. But this endeavor would only work if scientists combined the observations from viewing stations in the Northern and Southern hemispheres. The calculations would be valid only if the astronomers traveled to far-flung corners of the world and then shared their results.  Amid the global Seven Years War, hundreds of astronomers from the belligerent nations joined together to plan expeditions to see the transit from India, the Arctic Circle, Siberia, Tahiti, Newfoundland, Baja California and many other places—at a time when a letter posted in Philadelphia took two to three months to reach London.  The French astronomer Guillaume Le Gentil left first, in 1760, for the French port of Pondicherry in India. Beset by enemy attacks, hurricanes and dysentery, he finally saw the Indian coast at the end of May 1761. With only two weeks until the transit, Le Gentil learned that the British had taken Pondicherry and that he would have to see the transit from the sea, on a rolling boat, making precise observations impossible.  Having failed to find an appropriate perch for the first transit of his era, the stoical Le Gentil decided that nothing would prevent him from observing the second one in June 1769. To make sure, he waited for eight years in the region and eventually returned to Pondicherry—again in French hands after the war's end—with plenty of time to get ready and to build an observatory.  After weeks of clear skies, a howling wind woke Le Gentil on the night before the big event. Clouds, sand and dust hid the sun; he could see nothing of the transit. A few hours later, after Venus had passed, and as if the heavens were mocking him, the sun returned. Le Gentil declared himself "doomed." By the time he returned to Paris in 1771, his heirs had declared him dead and divided up his estate.  Another French astronomer, Jean-Baptiste Chappe d'Auteroche, faced freezing temperatures and deep snows when he traveled from Paris to Siberia for the first transit. Eight years later, in Baja California, he measured the second transit from a typhus–infested Jesuit mission. Delirious with fever, he died hours after recording his final data.  Everywhere across the globe astronomers were encountering dangers and obstacles. Britain's Royal Society threatened Charles Mason and Jeremiah Dixon—later famed for the Mason-Dixon Line—with court martial for mutiny when they refused, after a bloody French attack, to continue on their way to Sumatra to record the transit. They resumed their journey but, unbeknown to the Royal Society, observed the transit in Cape Town.  Benjamin Franklin masterminded the colonists' effort from London, commissioning telescopes and other instruments for his friends at Harvard University and the American Philosophical Society in Philadelphia. On the day of the transit, the Pennsylvanian astronomer David Rittenhouse became so overexcited that he fainted, missing the beginning of the most important scientific event of his life. In Hudson Bay, one observer endured such cold that brandy froze in his glass, while a Swedish astronomer faced armed Russian rebels in a border conflict in Lapland. Russia's Catherine the Great dispatched eight expeditions across her vast empire.  During each of the two transits, around 250 official observers at more than 100 locations recorded data, transcending national boundaries. Once the results were compared, it became obvious that an optical phenomenon called the black-drop effect had distorted the results, and the 1761 calculations of the distance between the Earth and the sun varied widely—by some 20 million miles. The data improved after the 1769 transit, with astronomers putting the distance between 92.9 million and 96.9 million miles—very close to today's measurement of 92,960,000 miles.  The most important result of this effort, however, was the successful collaboration of an international community of scientists—a precedent that has served humankind well. As we look skyward this June to see a planet almost as big as our own dwarfed by the immensity of the sun, we might pause for a moment to remember the hundreds of men who watched the exact same spectacle some 250 years ago.  A version of this article appeared April 21, 2012, on page C3 in some U.S. editions of The Wall Street Journal, with the headline: The Celestial Event That Sparked A Revolution.
The event on June 5 and 6 recalls a pioneering collaboration, as the author of Chasing Venus: The Race to Measure the Heavens tells it.