The Enduring Fascination of the Transit of Venus

July 17, 2012


Drawing of the transit of Venus as predicted to occur in London, India, St. Helena, and Bencoolen on June 6, 1661; published in James Ferguson’s book on the subject. From Chasing Venus.
Chasing Venus: The Race to Measure the Heavens. By Andrea Wulf, Knopf, New York, 2012, xxvi+304 pages, $26.95.

In Providence, Rhode Island, where I grew up, there is a short, quiet street named Transit Street, a few blocks south of the Brown University campus. The name dates from 1769, when the astronomer Benjamin West and Joseph Brown, a merchant, attended by a crowd of interested observers, measured the transit of Venus across the sun's disk. Their report was published in the first volume of the Proceedings of the American Philosophical Society, the first American scientific journal. The group in Providence was part of a scientific enterprise of unprecedented international scope. Hundreds of scientific groups all around the world observed the transit. Scientific expeditions had set out months or even years earlier, travelling hundreds or thousands of miles and enduring all the hardships of 18th-century exploration to destinations ranging from Yakutsk, Lapland, and Hudson's Bay to Pondicherry, Baja California, and---for Captain Cook on his first voyage---Tahiti.

The purpose was to measure the distance to the sun. Kepler's theory had determined the orbits of the planets with great precision in terms of astronomical units---the mean distance from the sun to the earth. But how long, in absolute terms, was the astronomical unit? Kepler had computed a lower bound of 14,000,000 miles, Edmund Halley of 69,000,000 miles, but these were very rough estimates.

It was Halley who, fifty years before the event, had proposed that astronomers use the transit of Venus to resolve the question. Having observed a transit of Mercury from the island of St. Helena in 1677, Halley realized that a transit of Venus would take different amounts of time when observed from different latitudes and that, by measuring the difference, it would be possible to determine the distance to the sun. The greater the difference in latitude, the greater the divergence in time---and hence the more precise the calculation. Transits of Venus occur in pairs eight years apart, at intervals of more than a century. The most recent occurrence had been in 1639, and was observed by the astronomer Jeremiah Horrocks. The next pair would be in 1761 and 1769.* Halley would no longer be alive by then, but in 1716, he wrote an essay calling on the scientific community, when the time came, to send expeditions north and south to carry out the observations needed for the measurement.

Joseph Nicholas Delisle, the official astronomer to the French Navy, took the lead on the 1761 project. The circumstances were adverse; the Seven Years' War was raging, and France and England, the leading naval powers and the leading countries in science, were on opposite sides. Indeed, ships carrying several of the scientists were attacked en route. Nonetheless, French scientists travelled to Pondicherry, Rodrigues, and Tobolsk; British scientists travelled to St. Helena and the Cape of Good Hope; a Swedish scientist made his way to eastern Finland, and John Winthrop of Harvard went to Newfoundland, the only part of the western hemisphere in which any part of the transit would be visible. When the measurements were collected, tabulated, and combined, the new estimates were 77.1 million to 98.7 million miles---not as precise as hoped, but a tremendous improvement.

Circumstances in 1769 were much more favorable. The Seven Years' War had ended, though international relations were still tense. Catherine the Great had come to power and gave her full support to the enterprise with her characteristic élan; seven scientific teams were sent out to points all over Russia, after meeting with her for her personal blessing. Captain Cook and Joseph Banks set sail for Tahiti, William Wales for Hudson's Bay, Jean-Baptiste Chappe d'Auteroche for Baja California. When in the end the data was collected, the estimate was 93,726,000 miles, within 400,000 miles of the true figure, with an estimated uncertainty of about 4,000,000 miles.

Andrea Wulf gives a vivid account of the difficulties, hardships, and dangers of these long expeditions through unknown territories, each carrying half a ton of delicate astronomical equipment. William Wales had to set out for Hudson's Bay a year before the event and over-winter there---and he hated the cold. In 1761, travelling to Siberia, Chappe d'Auteroche risked his life crossing frozen rivers whose ice barely supported the weight of his carriage, just before the spring thaw. In 1769, he gave his life to the enterprise by landing in Baja California despite an epidemic of typhus. He died there of typhus, but not before he had succeeded both in measuring the transit and in computing the longitude.

And, of course, after all that, a cloudy day could render it all useless. Joseph LeGentil left Brest in March 1760 for Pondicherry. Before he could get there, Pondicherry had fallen to the British, and LeGentil found himself, on June 6, 1761, on a boat in the middle of the Indian Ocean. He gamely tried to make his observations from there, but the rolling of the ship made it impossible to keep the sun in view. Opting to remain in the region until 1769, he made his way to Manilla, where cloudy days were rare, and made his preparations there. The French Academy sent a message ordering him to go to Pondicherry (back in French hands) instead. In Pondicherry on June 3, 1769, the skies were overcast for the first time in months. LeGentil disconsolately made his way back to Paris; when he arrived in August 1771, he found that his heirs had had him declared dead and that the Academy had removed him from its payroll.

Even when measurements could not be obtained, though, many of these expeditions made tremendous contributions to scientific, geographic, and anthropological knowledge. The results of Captain Cook's voyage and Joseph Banks's botanical collections from the South Pacific are well known. None of Catherine the Great's teams ended up getting usable measurements of the transit, but they returned with an extraordinary collection of reports on parts of Russia almost as unknown as Tahiti. János Sajnovic, who travelled to Lapland in 1769, discovered that the Finnish and Hungarian languages are closely related.

Like Andrea Wulf's previous book, Founding Gardners, about the horticultural enterprises of the American founders, Chasing Venus is a fascinating read, wonderfully well written, a vivid portrayal of the world of 18th-century science and its interactions with the wider intellectual and political world of the time. It is extensively researched, with a bibliography of about 500 items, mostly primary sources, in seven languages (English, French, German, Latin, Russian, Dutch, and Swedish), and beautifully illustrated with reproductions of contemporary drawings and portraits. Wulf makes the excitement of the enterprise and the heroic courage and dedication of its participants come dramatically alive.

Thinking of the Church in the 12th century and science in modern times, Kenneth Clark wrote,

"Where some way of thought or human activity is really vital to us, internationalism is accepted unhesitatingly."

The enterprise to measure the transits of Venus in the 1760s was one of the most extra-ordinary manifestations of that noble thought.

*The most recent pair occurred on June 8, 2004, and June 5/6, 2012; the next will be in 2117.

Ernest Davis is a professor of computer science at the Courant Institute of Mathematical Sciences, NYU.


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