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

A chronometer is a clock designed for high accuracy. Developed by John Harrison as a marine timekeeper that could withstand weather changes and rolling seas, chronometers made possible quick, accurate determination of a ship’s position (specifically, its longitude) at sea. The term continues to refer to precision timepieces, including highly accurate wristwatches.

Ephemerides (eh-fuh-MER-ih-deez) are “timetables of astronomical movements” (25): They keep track of the motions of stars, planets, the sun, and the moon across the sky. On any given day, the tables will show where important celestial objects will be, along with the precise time of eclipses and lunar occultations (when the moon passes in front of a star or planet). This helped seafarers determine their location, and ephemerides were used extensively before the invention of a reliable chronometer.

A clock’s escapement controls the speed of the mechanism. It is “the part that count[s] the heartbeats of the clock’s pacemaker” (71). John Harrison invented a new type of escapement, the “grasshopper,” that replaced the pendulums that couldn’t function properly on a rolling ship at sea.

A forestaff is a device used at sea for estimating the sun’s elevation in the sky and, therefore, the local time of day. Forestaffs required sailors to sight the sun by staring directly at it through an eyepiece; this caused many crew members to lose sight in one eye over time. Backstaffs solved this problem and eventually replaced forestaffs. (See “backstaff”)

19th-century ships and their crews relied widely on England’s system of longitude. At a special international conference in 1884, nations began to formally adopt the meridian of longitude that runs through London’s Greenwich Observatory as the standard for determining time in the rest of the world—the “prime meridian.” Until 1967, this was referred to as Greenwich Mean Time, or GMT; since then, it’s been called Coordinated Universal Time (UTC). San Francisco, for example, is eight hours behind UTC, and its time is listed as UTC-8. (For technical reasons, scientists use slightly different systems called UT0, UT1, and UT1R.)

John Harrison’s first attempt at a sea clock, Harrison’s No. 1, also known as H-1, is an elaborate contraption that weighs 75 pounds and displays four dials—one each for hour, minute, second, and day. This clock proved itself in trials held by England’s Board of Longitude, and that committee was prepared to give the longitude prize to Harrison, but he insisted instead on perfecting his timepiece. Over the next two plus decades, he built clocks H-2 through H-5. His highly accurate sea clock H-4, a new standard in miniaturization at three pounds and five inches wide, passed the trials and eventually won the longitude prize. H-4 made possible the mass production of portable marine timekeepers and changed the way seafarers navigate.

Horology is the art and science of timekeeping. Horologists are fascinated with timepieces—everything from pendulum clocks to chronometers to atomic clocks—and how best to track time. They also study the history of timekeeping and clockmaking.

On maps, the Earth is divided by horizontal lines of latitude and vertical lines of longitude. For a ship at sea in the 1700s, figuring out latitude, which varies from 0 degrees at the Equator to 90 degrees at the North and South Poles, was a fairly simple process: The height of the sun, moon, or stars in the sky on a given day told sailors how far north or south they were. Longitude, though, was another problem entirely. Without knowing the time at their home port, a ship had no real idea how far east or west they were. An extremely precise clock, set to home time, could be compared to local time to give a location within a few tens of miles east or west. Another method used books filled with tables of the moon’s position against the stars at a given time in London or Paris; comparing these numbers to the moon’s position at local time could provide a ship with its longitude. Today, satellites can calculate an object’s position on the planet to within a few feet.

Established by the English Parliament’s Longitude Act of 1714, the longitude prize of £20,000 (a few million US dollars today) would be awarded to whoever developed a method for determining a ship’s longitude to within half a degree of longitude, or 34 miles at the Equator. The two main competitors were John Harrington’s precision sea clocks and the Greenwich Observatory’s lunar method, which relied on books filled with data and careful sighting of celestial objects with a quadrant or sextant. (See “Reflecting quadrant”)

The lunar method proved popular before sea clocks became widely and inexpensively available, but by the mid-1800s most ships had sea clocks and could calculate their longitudes quickly within a few tens of miles. Harrison won most of the longitude prize; his clocks opened a new era of precision timekeeping that has advanced to the point where today’s positions can be determined to within a few feet.

Developed over many decades during the 1700s, the lunar method located longitude through a complex series of steps. These involved careful sightings, using a quadrant or sextant, of the moon’s position against the stars and comparing the findings to books called ephemerides, which contained tables of the moon at various positions and their corresponding times above London or Paris. This gave the ship the correct time in London, and comparing that to the ship’s local time provided its longitude. Ships of the late 1700s and early 1800s most often used the lunar method, but when precision chronometers became widely available, seafarers switched to them because that method was quicker and less prone to error.

In Longitude, movement refers either to the mechanism inside a clock or watch that generates the precise motion of the hands on a timepiece’s dials, or it refers to the relative motions of the moon, sun, and Earth used in determining a ship’s location.

This was a “miraculous powder, discovered in southern France by the dashing Sir Kenelm Digby, [that] could purportedly heal at a distance” (41). Applied to, for example, a bandage once used by an injured person, the powder was supposed to cure that person. This led some to suggest injuring a dog, bandaging the cut, and then removing the bandage and placing the dog aboard a ship at sea. An assistant would then apply powder of sympathy to the bandage back at home each day at noon, the dog onboard the ship would yelp, and the crewmen would know instantly the time back home and, deducting local time, estimate their location east or west of home port. One problem with this technique was the need to keep re-injuring the dog; the other problem was that it simply didn’t work.

(see “Greenwich Mean Time”)

Also called an octant for its shape (a pie-slice of a circle) or Hadley’s quadrant in honor of one of its inventors, the reflecting quadrant used twin mirrors that made it possible to sight and measure the distance between two celestial objects—the moon and a bright star, for example. Because the moon moves across the sky about 13 degrees a day, its position relative to the stars will shift as the hours pass, making it possible for someone with a quadrant to home in on the precise moment that the moon will be in a given position. The observer could then refer to tables of such measurements and the times they occur above home port, using the time difference between home and ship to calculate longitude. This made the sky into a celestial clock more accurate than any timepiece until John Harrison perfected his sea clock. The more accurate sextant later replaced the quadrant. Both devices also could determine local time of day with good precision, without which even a highly accurate timepiece couldn’t determine longitude.

Commissioned in 1675 by England’s King Charles II to provide data that could help determine a ship’s longitude, the Royal Observatory, Greenwich (today called Royal Greenwich Observatory, or RGO) proved important in the development of the “lunar method.” Data collected for this method was assembled into books, called ephemerides, for use by ships at sea. The Observatory’s directors championed this method in their competition against John Harrington’s sea clocks for the longitude prize. The RGO still performs some astronomical work; its Maritime Museum contains John Harrison’s famous clocks.

“Sea clock” was an early name for John Harrison’s precision timepieces that could help determine a ship’s location at sea. Accurate clocks or watches later were called chronometers—a name still used today, especially for high-quality wristwatches.