Dava Sobel’s best-selling book Longitude: The True Story of a Lone Genius Who Solved the Greatest Scientific Problem of His Time tells the story of the 18th-century contest to find a precise way to locate a ship at sea, the clockmaker who built the first timepiece that could do so, and his battle with the astronomers whose alternate method competed for the winning prize. Replete with sea disasters, brilliant scientists, and scheming politicians, Longitude won numerous awards and was twice adapted for television. The 2010 paperback edition forms the basis for this study guide.

Plot Summary

Maps of the Earth are crisscrossed with horizontal lines of latitude and vertical lines of longitude. These lines help travelers locate themselves on the planet. Latitude has always been easy to reckon: Sailors simply note how high or low the sun, moon, and stars are in the sky and use that to determine how far north or south they are. Longitude—one’s location east or west on the Earth—is hard to determine, and for much of human history sailors routinely were delayed, lost, or shipwrecked for lack of knowing it.

Early attempts to solve this problem had mixed results. Galileo and others reasoned that sailors could use ephemerides, or tables of celestial events and motions, to find their longitude. England’s Charles II established the Royal Observatory at Greenwich to advance this project, and the resulting data helped somewhat, but problems persisted.

Clocks, meanwhile, would have helped reckon longitude if only they were more accurate—those of the era could lose 15 minutes a day—and pendulum clocks, the best available, lost their accuracy at sea from changes in weather and rolling ships. Compasses could help determine one’s position relative to magnetic north, but this method also suffered from accuracy problems. Other suggestions included regularly spaced ships firing cannons at standard hours and wounded dogs made to howl at precise times. Most of these ideas didn’t work at all.

As sea travel increased during the 1600s and 1700s, the longitude problem loomed larger. A 1707 English shipping disaster caused by poor navigation cost the lives of 2,000 sailors, and in 1714 Parliament passed a bill that offered a huge bounty to whoever could invent a means of determining one’s position accurately.

In 1730, woodworker and self-taught clockmaker John Harrison began to develop the first clock that could keep accurate time at sea. This would allow ships to always know the time back home, compare that to local time as determined by the position of the sun and stars, and know their place east or west on the globe. Harrison’s first clock was a success, but he insisted on retooling it to make it even better. The process took decades.

Meanwhile, astronomers made catalogs of the locations of stars in the night sky and the moon’s position relative to the stars at any given time. With these catalogs, and using a new sighting device called a reflecting quadrant, sailors could calculate their longitude. The process might take hours, but it was a great improvement over the guesswork of the past.

Harrison’s third clock underwent sea trials and performed brilliantly, losing only five seconds in 81 days. The astronomers who directed the Royal Greenwich Observatory disliked Harrison’s clocks, which competed for the longitude prize against their own lunar method, and they did their best to discredit and stall his inventions. As members of the board charged with judging the prize candidates, they managed to force the clock to undergo further tests. Astronomer Nevil Maskelyne requisitioned Harrison’s inventions, subjected them to abuse, and declared them unfit. The board awarded Harrison the runner-up prize.

On his second voyage to the South Seas, Captain James Cook brought a carefully crafted replica of Harrison’s fourth clock and swore by it, but to no avail. Desperate for a fair hearing, Harrison and his grown son, William, beseeched King George III for help. The king took up their cause, tested Harrison’s latest clock to his satisfaction, and convinced Parliament to award Harrison most of the balance of the prize money.

Other watchmakers began to produce similar timepieces in great numbers, and the Royal Navy stockpiled a supply of them. By the early 1800s, 5,000 chronometers were in use. Harrison’s invention proved itself in the hearts and minds of the people who most needed accuracy, and though the astronomers’ lunar method remained useful, it was principally employed to check the accuracy of the timepieces.

Harrison’s inventions languished in storage for more than a century. In 1920, ex-naval officer Rupert Gould volunteered to recondition them. The process took 12 years, but today the great timepieces stand proudly in the Greenwich Maritime Museum, where visitors can marvel at the precision mechanisms that changed the history of navigation.