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

As a child, the author was fascinated by the horizontal lines of latitude and the vertical lines of longitude that overlie the Earth on maps. In 150 CE, Ptolemy placed the Equator at zero degrees latitude so that it girdles the planet around its waist; he put the prime meridian of longitude at the Canary Islands. This north-south line has since moved several times and now runs through Greenwich in London: “The placement of the prime meridian is a purely political decision” (4).

The Earth spins through 360 degrees, and each hour it moves 15 degrees. At sea, when the sun reaches its highest point in the sky, sailors set their ship’s clock to local noon time and then consult a second clock with the time back at the home port. The difference in hours, multiplied by 15 degrees, tells them how far west or east they are from home.

The pendulums of early clocks were unable to swing correctly on a rolling ship; temperature and other changes also affected their accuracy. Early seagoing explorers and traders basically were lost until they bumped into land. This could prove deadly. As the Middle Ages gave way to the early modern era, European rulers began seeking a solution.

Latitude is easy to determine by observing the stars, and astronomers tried to do the same with longitude without success. Many other possible solutions also failed to help. Governments offered high bounties for a solution.

John Harrison, an English clockmaker, solved the problem by constructing a portable clock with a nearly frictionless, rust-proof, non-oiled, maintenance-free mechanism “that would carry the true time from the home port, like an eternal flame, to any remote corner of the world” (8). He faced resistance from the award committee, which wanted to give the prize to an astronomer, but after 40 years of struggle an aged Harrison and his supporters finally triumphed.

On their way home from a battle near Gibraltar in 1707, four of the five ships in English admiral Sir Clowdisley Shovell’s squadron, lacking accurate timepieces and struggling through a dense fog, ran aground and sank on the Scilly Isles at the southwest tip of England. Two thousand men died; only Clowdisley and one other survived and washed ashore, where a local resident promptly murdered the admiral for his lovely emerald ring.

Captains in the 15th through 17th centuries routinely reckoned their location with a compass, an estimate of time traveled, and logs dropped overboard to gauge ship speed. These methods were too crude to prevent disasters; navigational delays sometimes cost ships their crews to nutritional deficiencies such as scurvy, caused by lack of vitamin C. Most ships confined themselves to well-known trade routes, where pirates exacted a toll.

In 1714, the English Parliament passed the Longitude Act, which offered a reward of £20,000 to whoever solved the navigation problem. By 1736, John Harrison’s early chronometer reduced the navigation error to 60 miles on a ship sailing from Lisbon to London. In 1841 that same ship, the Centurion, sailed around the southern tip of South America and into the Pacific Ocean, but it didn’t have the chronometer. Storms and currents, plus weeks of guesswork sailing, caused such delays that when they finally reached the safe haven of Juan Fernandez Island, the sailors were so weak from scurvy that more than 500 died.

For centuries people have been able to tell time of day and day of year by keeping track of the sky—specifically, the motions of the sun, moon, and stars. Now and then scientists would know of an astronomical event—an eclipse, for example—in advance and predict its time for a home location, so sailors who witnessed the same event some hours earlier or later could figure out just how far west or east of home port they were. These events are rare, however; a daily event was needed.

In 1514 astronomer Johannes Werner suggested mapping the moon’s motion across the night sky and recording each passage of a star in a table for use as a precise time stamp. When the moon passed a given star at a given time at, say, Berlin, a ship’s crew could note their local time, calculate the time difference to Berlin, and reckon their longitude. Werner’s idea was ahead of its time, as no such accurate calculations of moon and stars were yet available.

In 1610 Galileo discovered the moons of Jupiter and their highly predictable orbits; he created tables called ephemerides that listed the precise times of those moons’ eclipses—about 1,000 a year—hoping he could sell copies to sea captains who could use them nightly in a way similar to the much rarer lunar and solar eclipses. His plan foundered at sea because Jupiter was available during only part of the year, and because it was very hard to observe the planet and its moons through a telescope on the deck of a moving ship. It did work on land, where mapmakers used the technique to greatly improve the accuracy of maps.

Astronomer Ole Roemer noticed that the timing of the Jupiter moon eclipses differed by several minutes depending on how close Earth was to Jupiter. He realized that light doesn’t move instantly but travels at a specific speed, which in 1676 he calculated fairly accurately, though at somewhat less than the true value of 300,000 kilometers per second.

Around that time, England’s King Charles II, whose fleet of merchant ships was the world’s largest, paid for the construction of the Greenwich Royal Observatory under the direction of astronomer John Flamsteed. Flamsteed would collect observations of the moon’s motion through the stars in the manner of Johann Werner’s 1514 proposal, compiling ephemerides that sailors could use to reckon their longitude. Flamsteed’s work was published 50 years later in 1725. It helped, but the dream of a proper timepiece persisted.

In the 1500s, clocks existed but weren’t very accurate, which made them useless for keeping track of distance from home. Galileo, who famously gleaned the principles of pendulums by watching a church lamp swing back and forth, tried to develop a pendulum clock. It was astronomer Christiaan Huygens, an assistant to Giovanni Cassini at the Paris Observatory and the discoverer of Saturn’s rings, who produced the first pendulum clocks for sea vessels.

These clocks kept good time on calm seas but became inaccurate on wave-tossed waters. Huygens replaced the pendulums with springs, but scientist and builder Robert Hooke claimed credit for the invention; neither completed the spring clock’s development. Astronomers mocked the inventors, believing “[T]he answer would come from the heavens” (40).

While inventors struggled to design a better clock throughout the late 1600s and early 1700s, eccentrics proposed outlandish ideas for solving the longitude problem. One such idea was “powder of sympathy” (42), a medicine that would cure a person at a distance when applied to an item belonging to them. A wounded dog aboard ship would supposedly yelp when a person back at home port applied the powder to the dog’s original bandage at exactly noon. The difference in hours between that moment and the ship’s local time, multiplied by 15, would give the ship its longitude.

This technique was no crueler than using a “forestaff” to locate the sun and gauge its position: Sailors who sighted the sun through the staff’s eyepiece tended to lose sight in one eye over the years. The forestaff was replaced by the backstaff, which allowed sailors to gauge the sun’s height by facing away from it and studying shadows the staff cast.

Another approach was to use a compass, which points to magnetic north (in the Arctic Circle but not directly over the North Pole), and compare the result to a sighting of Polaris, a star that hovers directly over the Pole. As the ship moves east or west, the difference between the two sightings changes, which gives the ship’s crew a significant clue about their longitude. Compasses, though, weren’t much more accurate than the clocks of the era.

One idea was to post ships at regular intervals along trade routes and have them fire off cannon shells at regular intervals for other ships to see or hear. This project foundered because it was hard to anchor a ship in the middle of a deep ocean, and because sight and sound are notoriously unreliable across stretches of water and sea air. It would also have required thousands of sailors to live alone at sea for weeks or months at a time.

The ships’ cannonade idea did attract the attention of London’s maritime interests; they petitioned Parliament in 1714 to do something about the longitude problem. Parliament acted, setting a reward for the solution.