History: How Earth shaped up

Subjects

Andrew Robinson enjoys an account of the first expedition to the equator to calibrate latitude.

Measure of the Earth: The Enlightenment Expedition That Reshaped Our World

Basic Books: 2011. 376 pp. $28, £15.99 9780465017232 | ISBN: 978-0-4650-1723-2

What was the world's first international scientific expedition? Scientist-explorers of the Enlightenment, such as Alexander von Humboldt, would have answered without hesitation: the French government's eight-year geodesic mission to the Spanish colony of Peru, beginning in 1735. The mission — the subject of Measure of the Earth — calculated the precise length of a degree of latitude at the equator, thereby helping to define Earth's shape.

The 1730s French geodesic mission revealed that Earth bulges at the equator. Credit: ROYAL ASTRON. SOC./SPL

Today, the geodesic mission's fame has faded. Until this book, no modern history of the mission has been published in English (unpublished academic studies exist). It was virtually ignored in Ken Alder's prizewinning account of the surveying of the arc of the meridian and the origins of the metric system, The Measure of All Things (Free Press, 2002). Neil Safier's Measuring the New World (University of Chicago Press, 2008) focused instead on the Amazon explorations of its best-known member, Charles-Marie de La Condamine. But in 2006, the French geodesic mission was the basis for one episode in a BBC television series, Voyages of Discovery, for which the historical consultant was science writer and naval historian Larrie Ferreiro. In Measure of the Earth, Ferreiro puts this important, dramatic and gruelling trip back on the map.

The geodesic mission was organized by the French Academy of Sciences, under the sponsorship of the French minister of the navy acting in wary collaboration with his Spanish counterpart in Madrid. Spain sent two naval officers, also astronomers, to learn from and keep an eye on the French.

Along with a similar French scientific mission to the Arctic Circle in 1736–37, the expedition determined that a degree of latitude is shorter at the equator (where it measures a length of 56,753 toises, or 110.54 kilometres) than at Paris (57,060 toises) or towards the North Pole (57,437 toises). Thus it proved that Earth was not a sphere but an oblate spheroid: it bulges slightly at the equator and is slightly flattened at the poles.

This asymmetry explains why the force of gravity is slightly less near the equator than in Paris. This was first observed by a puzzled French astronomer measuring the period of a pendulum clock in Guyana in 1672, and seized on by Isaac Newton in his Principia Mathematica of 1687. Newton predicted the oblate shape of Earth from his theory of gravity. However, his idea was strongly contested in France by followers of philosopher René Descartes, whose theory of vortices in the ether had led them to the view that Earth must be prolate, like an upright egg. The geodesic mission thus settled a long scientific debate, and at the same time improved the accuracy of navigation by ships.

“The expedition had reckoned without obstacles including yellow fever, local love affairs and brigandry.”

By 1735, the basic geometric and astronomical principles of trigonometric surveying were well understood. To the government and scientists in Paris, it seemed straightforward to apply these accurately among the peaks of the Andes, over a period of a year or two. But the expedition members had reckoned without a myriad of obstacles, large and small, including precarious mountain transport, extreme weather, earthquakes, altitude sickness, yellow fever, local love affairs and brigandry. They suffered the theft of their triangulation signals by local Indians, damage to instruments, the withdrawal of their French funding, Spanish colonial politics and the ripples from European wars, as well as the murder of their surgeon at a bullfight.

There were also purely scientific challenges, such as stellar aberration: the then little-understood phenomenon of tiny variations in the location of the stars used to fix latitude, which we now know arises as a result of Earth's motion around the Sun.

The most serious problem was the expedition's lack of leadership. Internecine squabbling nearly scuppered the project. The precious results announced to the French Academy in 1744 by expedition member and geophysicist Pierre Bouguer were those measured by himself and La Condamine, without the knowledge of the official expedition leader, Louis Godin, who decided to remain in South America as a professor of mathematics in Lima. Measure of the Earth accomplishes its mission with skill and devotion, although it lacks some necessary diagrams. Its intermixing of politics and science is particularly fascinating. It also shows, unintentionally, the astonishing development of international scientific cooperation since the days of the Enlightenment pioneers.

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Correspondence to Andrew Robinson.

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