The Magazine

Venus Observed

The global race to measure Earth’s distance from the sun.

Aug 20, 2012, Vol. 17, No. 45 • By JOSHUA GELERNTER
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In 1677, the British astronomer-royal Edmond Halley used a very large telescope to watch Mercury eclipse a small disc of the sun. Mercury “transiting” the sun is an unusual event, and very hard to see. Halley only spotted the little planet because he was the finest astronomer of his generation. And, being the finest astronomer of his generation, he got an idea from Mercury’s transit: If Mercury were closer and larger, its passing in front of the sun could be used to work out the distance between the sun and the Earth. Fortunately, a closer, larger Mercury exists: the planet Venus. 

Sir Edmund Halley

Sir Edmund Halley

Mercury transits happen about once every 10 years; Venus transits happen twice a century, in pairs 8 years apart. Halley calculated that Venus would next pass directly between the Earth and sun the year he turned 105, but, despite his best efforts, he died in his eighties.

Mark Anderson here describes the largest science experiment ever undertaken: In 1761, every astronomer in the world rushed to find a good spot to watch the Venus transit that Halley had predicted. By comparing observation records from two spots on the Earth, as far away from each other as possible, Europe’s scientists knew they could triangulate the distance from Earth to Venus and Venus to the sun. Since thousands of years of observation had determined the relative distances between the sun and the planets, knowing the Earth-to-sun distance would enable the drawing of an accurate map of the entire solar system.

This prospect so excited Europe’s big thinkers that Britain, France, Russia, and Austria happily collaborated on the project despite the fact that they were, at the time, fighting each other in the Seven Years’ War. The French and Austrians sent their best astronomers to Siberia for one of the two sets of observations required; the English went to South Africa for the other. The Siberian trip was a triumph, but both of the African observations were thwarted by clouds. 

Europe’s scientists buckled down: They’d get one more chance, eight years later, to see Venus transit the sun. After that, it would be a hundred-year wait before it happened again. Elaborate preparation was made and Europe’s belligerent powers pulled together. With Denmark’s help, Austria sent astronomer-priest Maximilian Hell to Vardo, at the northern tip of Norway. Spain helped France send Jean-Baptiste Chappe d’Auteroche to California. The English sent Captain James Cook with a hundred-man entourage to the just-discovered island of Tahiti.

The Frenchman Chappe was then the greatest living astronomer, but he also found time during his trip to correspond with Benjamin Franklin on fossils and theology, and to lay the groundwork for Europe’s first telegraph network. Father Hell and his assistant, Joannes Sajnovics, made breakthroughs in variable gravitation and bioluminescence during their expedition, and discovered that the Finns and Hungarians share a language. Captain Cook’s trip cured scurvy, invented seltzer, and almost killed the entire crew when they crashed into then-undiscovered Australia. And the Venus data they all brought back allowed the 95-million-mile distance to the sun to be measured to 99.8 percent accuracy. It was the best year for science since Isaac Newton got plunked by an apple.

The Day the World Discovered the Sun is part science, part adventure—and when Anderson isn’t getting bogged down with flowery prose, his writing is clear and tells a fast-moving story. His superior summary of the most important and least discussed decade of Enlightenment science is made even better by its timing: In 1769, the French astronomer-royal César-François Cassini de Thury was in charge of using Jean-Baptiste Chappe’s, Captain Cook’s, and Father Hell’s data in order to work out the distance to the sun. This was the final step, and he understood the weightiness of his job: In gathering the Venus data, vast sums of money had been spent and hundreds of lives (including the lives of some of our main characters) had been lost. He knew that another transit wouldn’t happen for another century—and that, even then, it wouldn’t be as easily observed. “The transit of Venus [won’t] be nearly as advantageous as it was in 1769,” Cassini wrote, “until 2012.” That transit—the last of this century—was visible in North America on the evening of
June 5; thousands watched and were thrilled. I climbed to the roof of my apartment building about five minutes before it started, but the sky was cloudy, and stayed cloudy. I hope the weather’s nicer next time around, in 2117.

Joshua Gelernter is a writer in Connecticut.