Twice in the eighteenth century, once in 1761 and once in 1769, the planet Venus crossed the face of the Sun as seen from Earth. These transits of Venus, visible as a small black dot slowly moving across the solar disk over a period of about six hours, were among the rarest and most eagerly anticipated astronomical events in history. Observing such events required the kind of telescopic precision that Galileo had pioneered a century earlier. They would not occur again for over a century (the next pair came in 1874 and 1882). And they offered something that no other observation could provide: a way to measure the astronomical unit, the distance from the Earth to the Sun.
To observe the transits, European nations organized the first truly global scientific expeditions. Astronomers sailed to Siberia, Tahiti, Hudson Bay, South Africa, Mauritius, and dozens of other locations. Some died. Some were stranded for years by wars. Some watched helplessly as clouds obscured the Sun at the critical moment. The transit expeditions were one of the first examples of large-scale international scientific cooperation, and their story combines heroism, tragedy, and obsessive precision in equal measure.
Halley’s Idea
The method for using a transit of Venus to measure the astronomical unit was proposed by Edmond Halley in 1716. Halley (the same man who funded Newton’s Principia and predicted the return of the comet that bears his name) recognized that observers at different locations on Earth would see Venus cross the Sun along slightly different paths. By measuring the precise timing of Venus’s entry onto and exit from the solar disk from two widely separated locations, the angular difference between the two paths could be calculated. From this angle, using the known distance between the observers and basic trigonometry, the distance from Earth to Venus could be determined. And from that, using Kepler’s third law (which gives the ratios of planetary distances), the distance from Earth to the Sun could be calculated.
The method required extreme precision. The angular differences were tiny (a few arc-seconds), which meant that the timing had to be accurate to within a few seconds over an observation lasting several hours. It also required observers at locations as far apart as possible, ideally one near the Arctic and one in the Southern Hemisphere. Halley knew he would not live to see the next transit (he was sixty years old in 1716 and the next transit was in 1761), but he published detailed instructions for future observers, effectively bequeathing the project to the next generation.
1761: The First Attempt
By the time the 1761 transit approached, the scientific community was mobilized on an unprecedented scale. France, Britain, Russia, Sweden, and other nations organized expeditions to stations scattered across the globe. Over 120 observers at more than 60 locations attempted to time the transit.
The results were disappointing. Clouds obscured the Sun at many stations. The “black drop effect” (a visual distortion that makes it impossible to determine the exact moment Venus touches the Sun’s edge) introduced systematic errors in the timing. And the Seven Years’ War (1756-1763) disrupted travel and communication, preventing some expeditions from reaching their planned stations.
The most famous casualty of the 1761 transit was Guillaume Le Gentil, a French astronomer who sailed to Pondicherry, India, to observe the event. He arrived to find that Pondicherry had been captured by the British (France and Britain were at war), and he was forced to observe from a ship at sea, where the rolling of the vessel made accurate timing impossible. His observation was useless.
Determined to succeed, Le Gentil decided to wait in the Indian Ocean for the next transit, eight years away. He traveled to Mauritius, then to the Philippines, and finally returned to Pondicherry (which had been returned to France by treaty). He built an observatory, calibrated his instruments, and waited.
1769: The Global Effort
The 1769 transit generated an even larger scientific mobilization. The Royal Society persuaded the British Admiralty to send an expedition to Tahiti, which had been identified as an ideal observation point in the Southern Hemisphere. The expedition was commanded by Lieutenant James Cook, a skilled navigator and cartographer making his first major voyage. Cook’s observation of the transit from Tahiti was one of the primary scientific goals of the voyage that would make him famous.
Cook’s transit observation on June 3, 1769, was partially successful. The black drop effect again complicated the timing, but Cook and his fellow observer, the astronomer Charles Green, obtained usable data. After the transit observation, Cook opened sealed orders from the Admiralty directing him to search for the hypothetical southern continent (Terra Australis). He sailed south and west, circumnavigated and mapped New Zealand, and charted the east coast of Australia. The transit of Venus had launched one of the greatest voyages of exploration in history.
Meanwhile, Le Gentil, who had been waiting in Pondicherry for eight years, prepared to observe the transit. On the morning of June 4, 1769 (the transit occurred on different dates depending on longitude), the sky was clear. Le Gentil set up his instruments. Then, as the transit began, a bank of clouds rolled in and obscured the Sun for the entire duration of the event. Le Gentil saw nothing.
He had spent eleven years traveling and waiting, had endured tropical diseases, near-shipwreck, and the hostility of warring nations, and had missed both transits. When he finally returned to Paris in 1771, he discovered that he had been declared legally dead, his wife had remarried, his relatives had divided his property, and his seat in the Academy of Sciences had been given to someone else. Le Gentil’s story is the most extreme example of scientific dedication and cosmic bad luck in the history of astronomy.
The Result
Combining the observations from 1761 and 1769, astronomers calculated the solar parallax (the angular size of the Earth as seen from the Sun) and from it the astronomical unit. The best results, compiled over the following decades, yielded a value of approximately 153 million kilometers, within about 2% of the modern value (149.6 million km).
This was the first reasonably accurate measurement of the scale of the solar system. Before the transit observations, astronomers knew the relative distances of the planets (from Kepler’s third law) but not the absolute distances. Knowing that Mars is 1.52 times as far from the Sun as Earth is not the same as knowing that the Earth-Sun distance is 150 million kilometers. The transit observations provided the missing scale factor, converting the relative map of the solar system into an absolute one.
With the astronomical unit in hand, astronomers could calculate the distances to the planets, the size of the Sun, and (using stellar parallax, first measured in the 1830s) the distances to the nearest stars. The entire distance ladder of modern astronomy, from the Earth to the farthest galaxies, rests on foundations that include the transit of Venus measurements.
The Human Cost of Precision
The transit expeditions exacted a heavy toll. Several observers died of disease during their journeys. Others were stranded for years by wars or shipwrecks. Le Gentil’s eleven-year ordeal is the most dramatic example, but many others suffered hardship, illness, and professional disappointment in the pursuit of a few seconds of precise timing.
The expeditions were also among the first examples of international scientific cooperation on a global scale. Despite the Seven Years’ War, French and British astronomers shared data and coordinated their observations. Catherine the Great of Russia funded expeditions to Siberia. The King of Denmark supported observations in Norway. Science, for perhaps the first time, operated as a truly international enterprise, with nations that were at war cooperating for the advancement of knowledge.
This tradition of international cooperation in astronomy continues today. The observatories, space agencies, and telescope networks that map the universe are international collaborations, funded by nations that may disagree on politics but agree on the value of understanding the cosmos. The transit of Venus expeditions helped establish this tradition, demonstrating that some questions are too large and too important for any single nation to answer alone.
The Last Transits
Transits of Venus occur in pairs separated by eight years, with the pairs separated by alternating gaps of 121.5 and 105.5 years. The 1761 and 1769 transits were followed by transits in 1874 and 1882, which were observed with improved instruments and produced more accurate results. The most recent pair occurred in 2004 and 2012. The next transit of Venus will not occur until December 11, 2117.
By the time of the 2004 and 2012 transits, the astronomical unit had long since been measured by other methods (radar ranging to Venus, beginning in the 1960s, provides accuracy to within a few meters). The transits were observed for their scientific interest in studying Venus’s atmosphere and for the sheer beauty of the event, rather than for their utility in measuring distances. But the memory of the eighteenth-century expeditions gave the modern transits a poignancy that a purely technical observation would lack. When Venus crossed the Sun in 2012, astronomers around the world paused to remember the observers who had risked their lives for the same sight 250 years earlier.
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