Last summer's transit of Venus, when our sister planet could be seen as a tiny black circle crossing the massive fiery disc of the Sun, was an engaging lesson in astronomy. For those in the right spot and the inclination to arise at 4.30am, the solar system was transformed into a giant orrery.
If the 2012 transit was a graphic illustration of what we already know, the unusually adjacent transits of 1761 and 1769 provided a vital opportunity to advance scientific knowledge. Andrea Wulf's thrilling book describes an international effort to observe the transit around the world even though the participating nations were at war.
Edmund Halley first identified the opportunity offered by the transits in 1716, but since he would have been 104 in 1761, it was a Frenchman, Joseph Nicolas Delisle, who urged scientists into action. As Wulf points out, "The interests of science transcended national boundaries." Surprisingly perhaps, her account is an absorbing, even exciting yarn.
The best spots for observation tend to be at the extremities of the world. Inaccessible even today, they took months of arduous travel to reach in the 18th century. By comparing the times taken for the transit of Venus from a number of locations, it was possible to discover the distance of the Earth from the Sun, as Wulf explains, "using relatively simple trigonometry".
For the 1761 transit, British astronomers went to the Cape of Good Hope and St Helena (despite the island being "infested" with clouds). French astronomers took sightings at Tobolsk in Siberia and the island of Rodrigues in the Indian Ocean.
A third would have gone to the French outpost of Pondicherry but was obliged to view the transit onboard a swaying ship since the British invaded the port shortly before his arrival.
The expeditions mounted for the 1769 transit involved Captain Cook sailing to Tahiti and the Frenchman Chappe d'Auteroche making observations while dying of typhus in Baja California.
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