Longitude: The True Story of a Lone Genius Who Solved the Greatest Scientific Problem of his Time

by Dava Sobel

King Louis XIV of France, confronted with a revised map of his domain based on accurate longitude measurements, reportedly complained that he was losing more territory to his astronomers than to his enemies.

Danish astronomer Ole Roemer made a startling discovery: The eclipses of all four Jovian satellites would occur ahead of schedule when the Earth came closest to Jupiter in its orbit around the sun. Similarly, the eclipses fell behind the predicted schedules by several minutes when the Earth moved farthest from Jupiter. Roemer concluded, correctly, that the explanation lay in the velocity of light.

Roemer used the departures from predicted eclipse times to measure the speed of light for the first time in 1676. (He slightly underestimated the accepted modern value of 300,000 kilometers per second.)

The most we can hope a watch to do is mark that progress. And since time sets its own tempo, like a heartbeat or an ebb tide, timepieces don’t really keep time. They just keep up with it, if they’re able.

The clocks of the early sixteenth century weren’t equal to the task. They were neither accurate nor able to run true against the assault of changing temperature on the high seas.

However, the distinction for completing the first working pendulum clock fell to Galileo’s intellectual heir, Christiaan Huygens, the landed son of a Dutch diplomat who made science his life.

By 1660, Huygens had completed not one but two marine timekeepers based on his principles.

To circumvent this problem, Huygens invented the spiral balance spring as an alternative to the pendulum for setting a clock’s rate, and had it patented in France in 1675.

Huygens found himself under pressure to prove himself the inventor of a new advance in timekeeping, when he met a hot-blooded and headstrong competitor in the person of Robert Hooke.

In Gulliver’s Travels, for example, the good Captain Lemuel Gulliver, when asked to imagine himself as an immortal Struldbrugg, anticipates the enjoyment of witnessing the return of various comets, the lessening of mighty rivers into shallow brooks, and “the discovery of the longitude, the perpetual motion, the universal medicine, and many other great inventions brought to the utmost perfection.”

Harrison completed his first pendulum clock in 1713, before he was twenty years old.

Its movement and dial—signed, dated fossils from that formative period—now occupy an exhibit case at The Worshipful Company of Clockmakers’ one-room museum at Guildhall in London.

One would imagine that Harrison grew up well aware of the longitude problem—just as any alert schoolchild nowadays knows that cancer cries out for a cure and that there’s no good way to get rid of nuclear waste.

The tower clock that Harrison completed about 1722 still tells time in Brocklesby Park. It has been running continuously for more than 270 years— except for a brief period in 1884 when workers stopped it for refurbishing.

“Mr Graham began as I thought very roughly with me, and the which had like to have occasioned me to become rough too; but however we got the ice broke . . . and indeed he became as at last vastly surprised at the thoughts or methods I had taken.”

H-1 now lives and works (with daily winding) in an armored-glass box at the National Maritime Museum in Greenwich, where it still runs gamely in all its friction-free glory, much to the delight of visitors.

As a result of this same failed attempt to measure stellar distances, Bradley arrived at a new, true value for the speed of light, improving on Ole Roemer’s earlier estimate.

He also determined the shockingly large diameter of Jupiter, and detected tiny deviations in the tilt of the Earth’s axis, which he correctly blamed on the pull of the moon.

Some horologists consider the Jefferys timepiece the first true precision watch. Harrison’s name is all over it, metaphorically speaking, but only John Jefferys signed it on the cap. (That it still exists, in the Clockmaker’s Museum, is something of a miracle, since the watch lay inside a jeweler’s safe in a shop that took a direct bomb hit during the Battle of Britain, then baked for ten days under the building’s smoldering ruins.)

Lying in state now in an exhibit case at London’s National Maritime Museum, H-4 draws millions of visitors a year.

To run it would be to ruin it.

H-4 may look forward to a well-preserved life of undetermined longevity. It is expected to endure for hundreds of years, if not thousands—a future befitting the timepiece described as the Mona Lisa or The Night Watch of horology.

The June 6, 1761, transit of Venus also paired (Charles) Mason with (Jeremiah) Dixon on a successful observing run at the Cape of Good Hope—several years before the two British astronomers drew their famous boundary line between Pennsylvania and Maryland.

The prize should have gone to John Harrison then and there, for his Watch had done all that the Longitude Act demanded, but events conspired against him and withheld the funds from his deserving hands.

H-5 is a thing of beauty in its simplicity. It now occupies center stage at the Clockmakers’ Museum in Guildhall, London,

Harrison had used his grasshopper escapement in the big sea clocks, then turned to a brilliant modification of the old-fashioned verge escapement in H-4. Mudge won lasting acclaim for his lever escapement, which appeared in nearly all mechanical wrist- and pocket watches manufactured through the middle of the twentieth century, including the famous Ingersoll dollar watch, the original Mickey Mouse watch, and the early Timex watches.

Thus the total world census of marine timekeepers grew from just one in 1737 to approximately five thousand instruments by 1815.

This same long voyage of the Beagle introduced its official naturalist, the young Charles Darwin, to the wildlife of the Galápagos Islands.

In 1884, at the International Meridian Conference held in Washington, D.C., representatives from twenty-six countries voted to make the common practice official. They declared the Greenwich meridian the prime meridian of the world.

Since time is longitude and longitude time, the Old Royal Observatory is also the keeper of the stroke of midnight. Day begins at Greenwich. Time zones the world over run a legislated number of hours ahead of or behind Greenwich mean time (GMT).

I finished this, with a gale lashing the rain on to the windows of my garret, about 4 P.M. on February 1st, 1933—and five minutes later No. 1 had begun to go again for the first time since June 17th, 1767: an interval of 165 years.”