- - Friday, May 25, 2012

By Andrea Wulf
Alfred A. Knopf, $26.95 336 pages

Very occasionally Venus glides between the sun and the Earth, looking like a black dot passing across the sun. Such peregrinations are known as transits of Venus, and they are among the rarest of predictable astronomical phenomena. They come in pairs separated by eight years; 121.5 years later there’s another pair separated by eight years, and then 105.5 years pass before the 243-year cycle starts again. Venus’ last transit was in June 2004, and its next will be on June 5 and 6 of this year. Anyone who wants to see Venus make its trans-solar expedition had better catch it this time around because it won’t do it again until December 2117.

In 1716, Edmund Halley, who had predicted the appearance of the comet now named after him, explained that if astronomers stationed around different parts of the globe were to observe the duration of the transit of Venus in 1761 they would be able to calculate the distance between the sun and the Earth by mathematically comparing their results. In 1760, the official astronomer to the French navy, Joseph-Nicolas Delisle, took up Halley’s challenge and began organizing the multiple necessary observations.

Innumerable obstacles stood in the astronomers’ way. Some observations would have to be made in the Arctic, and some as far south as possible in the Southern Hemisphere. It took Jean-Baptiste Chappe d’Auteroche five months of traversing terrible terrain in appalling weather to reach Tobolsk, the ideal observation spot in Siberia. Astronomers heading south fared no better. Charles Mason and Jeremiah Dixon, who later established the Mason-Dixon line, were drafted to go to Bencoolen in Sumatra, but only got as far as Cape Town. Other astronomers similarly found themselves in places far distant from those they had planned.

One reason for this was that the 1761 transit occurred during the height of the hostilities of the Seven Years War, and while French and British scientists collaborated with each other, their armies and navies often stopped them from reaching their observation posts.

Once the scientists were in place, they faced further problems. Many had to build observatories, and all needed to establish their longitude, but at the time there was no universally agreed way to do this. Then, after all their work, their ability to observe the transit depended entirely on the weather. Rain, fog, smoke and clouds prevented many of them seeing anything. Their perilous journeys had been for naught. Even those who saw all or part of the transit had problems with timing that meant that the distance between the sun and the Earth was still debatable.

Eight years later the scientists were better prepared and more successful. European governments and monarchs provided funds and facilities; astronomers agreed on practices; and the war had ended so they could disperse themselves around the globe more easily. Even so, there were problems. The Spanish refused to allow British observers into their territory of California for fear they would be spies.

Capt. James Cook discovered an ideal southern island - Tahiti - from which to observe Venus, but while his team was setting up its observatory, their telescopes were stolen. Maximilian Hell, who hated cold weather, had to establish his observatory in the Arctic Circle and spend the winter there to be ready for the transit on June 3, 1769.

In “Chasing Venus,” Andrea Wulf highlights the story of this 18th-century international scientific project by concentrating on the scientists who made the lengthiest journeys. (Many more observed it from their homes). Her feeling for personality and her attention to both the scientific records and to the astronomers’ journals brings their exploits to life as both scientific exploration and adventurous derring-do.

Today worldwide scientific collaboration is taken for granted, and computers and telecommunications make sharing information easy. Ms. Wulf shows how different things were in the 18th century, when it took three months to travel from Paris to St. Petersburg; when ships could be becalmed for weeks; when scientists had to travel with tons of instruments and set them up. She is also a friendly guide to the mathematics of the transit, so the problems that inspired this colossal 18th-century effort - and its benefits - are clear. A list of all the observers and extensive notes and bibliography add essential historical and scientific ballast to her enticing tale.

Less successful is the use of many 18th-century maps, figures and charts. Most are too small to be useful. The images of scientific instruments and locations are often uninteresting, usually because the captions do not sufficiently elucidate them. The portraits of the astronomers and those who helped their project along - including Benjamin Franklin and Catherine the Great - also vary in quality and importance. Nonetheless, “Chasing Venus” effectively dramatizes an important moment in the history of science while making June’s forthcoming transit more accessible to the nonscientist.

• Claire Hopley is the author of “The History of Tea and Tea Times” (RW Publishing).

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