Because sequels and adaptations infest publishing as they do movie-making, please note that The Illustrated Longitude is neither. It consists of Dava Sobel's original text, this time swaddled in 178 illustrations intended to supplement Sobel's prose with the texture of a bygone era sumptuous pictures of the people and the instruments that populate this novelistic story. When Longitude first appeared in 1995, Sobel's lucid, insightful little book had a single imperfection: a scarcity of illustrations. Now Sobel has joined forces with William J. H. Andrewes, the man who first invited her to write about the Longitude Symposium he was hosting in 1994 and thereby inspired what proved to be a surprisingly popular book. In a sense, Andrewes is curator of the exhibition that now accompanies Sobel's text. For centuries, the difficulty of determining longitude was a barrier to accurate navigation. Longitude is not like latitude. Latitude is not an arbitrary line. Measured outward from the Equator, its lines remain genuinely parallel and equidistant all the way around the globe. For this reason, determining latitude is a relatively simple procedure. Meridians of longitude, however, converge at the Poles, and therefore the distance between them varies. Because of this variation, mariners lacked but desperately needed a reliable way to determine longitude at sea. The only way to measure longitude was by the discrepancy between the time aboard your ship and the simultaneous time at, say, your home port.

Any sailor could determine latitude, and knowing latitude and the time difference he could calculate longitude and therefore know his precise location on the globe.

But such calculations required accurate time-keeping. Unfortunately 18th-century pendulum clocks lacked sea legs. The roll and sway of a ship at sea confused their mechanisms and rendered them useless in the determination of longitude.

Commerce and imperialism depended upon the successful resolution of this problem. In 1714, the English Parliament's offer of an award (equaling roughly $12 million nowadays) fueled the search for a solution. While others tried to find the answer in better maps of the sky, a clockmaker named John Harrison decided that what was needed was a truly reliable chronometer, and he set out to invent it. He succeeded fabulously. The Illustrated Longitude is the history of how clocks and watches developed, how our concepts of time changed, how Britannia came to rule the waves, and how one determined human being altered the course of history.

Michael Sims is the author of Darwin's Orchestra (Henry Holt).

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