The Longitude Problem

The history of latitude and longitude pillar mentioned the longitude problem as the central geodetic challenge of the 17th and 18th centuries. This article tells the full story — the problem, the prize, the hero, and the long-disputed payoff.

Why longitude was hard

Latitude could be measured at sea by the 1500s. A sextant (or its predecessors) measured the angle between the horizon and the sun at solar noon, or between the horizon and Polaris (the North Star, very close to the celestial north pole). The arithmetic gave latitude to within ~10 minutes of arc (~18 km), good enough for most navigation.

Longitude required a different approach. The Earth rotates 360° in 24 hours = 15° per hour. A ship's longitude could be derived from the time difference between local solar noon (when the sun is highest overhead at the ship) and the simultaneous time at a known reference meridian (e.g., Greenwich). If reference time was 3 PM when local solar noon occurred at the ship, the time difference was 3 hours → 45° west of the reference.

The catch: how do you know what time it is in Greenwich when you're three months into a voyage in the middle of the Atlantic?

This requires a chronometer — a clock that:

1. Keeps accurate time at sea, despite ship motion, temperature changes, humidity, and saltwater.
2. Was set to reference time before departure.
3. Continues to keep that reference time accurately throughout the voyage.

No such clock existed in 1700. Land-based pendulum clocks were accurate (a few seconds per day) but the pendulum needed gravity to swing in a regular arc; ship motion disrupted the swing pattern, ruining accuracy. Some 17th-century attempts to use shipboard pendulum clocks yielded errors of minutes per day — far too inaccurate for longitude.

A 1-second timekeeping error translates to ~28 km of longitude error at the equator. Over a 3-month voyage with seconds-per-day accumulating drift, the longitude error exceeded the entire width of an ocean.

The cost in lives and ships

Without accurate longitude, ships ran aground because captains thought they were further from shore than they were. Famous examples:

• HMS Association (1707): Sir Cloudesley Shovell's fleet of British warships ran aground on the Isles of Scilly off Cornwall after navigational error attributed to longitude. 1,500 sailors died. The disaster prompted the British Parliament to take longitude seriously.
• General loss rates: 17th-century estimates suggest 5–10 % of British naval ships lost per year to navigational error, much of it longitude-related.
• Trade economics: the Spanish, Dutch, and British trading companies all suffered substantial losses to longitude error. The economic incentive to solve the problem was enormous.

The 1714 Longitude Act

The British Parliament passed the Longitude Act on 8 July 1714, offering prizes:

• £20,000 for a method accurate to half a degree of longitude (~50 km at the equator). Roughly £3 million in modern money.
• £15,000 for 2/3 of a degree.
• £10,000 for 1 degree.

The act created the Board of Longitude to administer the prize. Eligible methods had to be tested on a voyage to the West Indies and back. The Board would assess accuracy on arrival and decide who got paid.

Similar prizes were offered by other nations: France, Spain, Holland, Venice. The British prize was the largest and became the most famous.

Competing methods

Many approaches were tried. The two main contenders:

Lunar distance

Observe the moon's angular distance from selected stars at a known moment of local time. Compare the observation to published tables of predicted lunar distances at the reference meridian. The difference yields the longitude.

The lunar-distance method required:

• Accurate published tables of lunar positions (the British Nautical Almanac, founded 1767, was specifically produced to support this).
• A precision sextant for measuring lunar angles.
• A skilled navigator able to do ~30 minutes of computation per observation.

Lunar distance worked in principle but was operationally fragile — clouds prevented observations, the math was error-prone, the tables needed updating. It was used in practice for several decades alongside chronometers.

Marine chronometer

Build a clock so accurate that it could be set to reference time at departure and still tell that time on arrival. Then read local solar noon with a sextant and take the difference.

This is the approach John Harrison pursued.

Galileo's moons

Galileo had proposed (~1610) using Jupiter's moons — the timings of their eclipses are predictable from astronomical tables. Observation at sea proved impractical: the moons of Jupiter are visible only through a telescope, and steady telescope observation on a moving ship was impossible.

Magnetic variation

The magnetic compass's deviation from true north varies geographically. Some 17th-century theorists thought this variation could yield longitude. In practice, the variation is too irregular and too slowly changing to be useful for longitude.

John Harrison and the chronometer race

John Harrison (1693–1776) was a Yorkshire carpenter who taught himself clockmaking. From 1730 to 1761 he developed four marine chronometer designs:

H1 (1735)

A 35-kg brass clock with grasshopper escapement and bimetallic temperature compensation. Tested on a voyage to Lisbon in 1736; performed well enough that Harrison received funding for further work.

H2 (1741)

An improved 39-kg design. Harrison decided H2's basic architecture wasn't the right approach; he abandoned it and started H3.

H3 (1759)

A 27-kg design with bimetallic strip temperature compensation and caged-ball-bearing escapement. Took 19 years to develop. Performed adequately but Harrison still wasn't satisfied.

H4 (1761)

A revolutionary design: a 1.45-kg pocket-watch-sized chronometer with a diamond-pallet escapement, bimetallic temperature compensation, and a balance spring of unusually high quality. Tested on a 1761–1762 voyage from England to Jamaica:

• Departure: Portsmouth, 18 November 1761.
• Arrival: Port Royal, Jamaica, 19 January 1762.
• 81 days at sea.
• Time loss: ~5 seconds total, translating to longitude accuracy under half a degree — better than the £20,000 prize threshold.

The Board of Longitude's long battle

Harrison expected to receive the £20,000. The Board of Longitude, dominated by astronomers who preferred the lunar-distance method, was reluctant. They:

• Required additional sea trials.
• Demanded Harrison disclose the H4's mechanism in full (so other clockmakers could replicate it).
• Awarded partial payments rather than the full prize.
• Argued the H4's success was a fluke; further trials with replicas were needed.

Harrison fought for decades. By 1773, he was 80 years old and still arguing. King George III intervened personally, expressing public outrage at the Board's treatment of Harrison. Parliament passed a special act awarding Harrison the bulk of the prize (~£8,750, on top of partial payments already made).

Harrison died in 1776 — the year of the American Declaration of Independence — having received approximately £23,000 in total across his career. The Board of Longitude disbanded in 1828. Marine chronometers based on Harrison's design dominated naval timekeeping through the 1800s.

Why the chronometer approach won

Several factors:

• Operational simplicity: a navigator with a chronometer and sextant could measure longitude in 5 minutes of routine observation, day or night, in any weather (the sextant needs the horizon visible, but the chronometer doesn't need stars).
• Independence from celestial conditions: lunar distance required clear viewing of moon and stars; chronometer plus solar-noon worked any clear day.
• Reduction in skill required: once the chronometer was set, the navigator's job was reading a clock and taking a sun-sight. Lunar distance required 30+ minutes of trigonometric calculation per fix.
• Manufacturing scaling: once Harrison's design was understood, other clockmakers (Larcum Kendall, John Arnold, Thomas Earnshaw) produced affordable copies. Marine chronometers became commodity instruments by ~1800.

By the early 1800s, every British naval ship and most merchant vessels carried a chronometer. The longitude problem was operationally solved.

Modern relevance

Today's GPS receivers solve the longitude problem electronically by reading time signals from atomic-clock-equipped satellites. The principle is the same Harrison used: precise reference time, compared to local observation, yields longitude. The chronometer is in orbit; the receiver is the navigator's arithmetic. The /learn/how-gps-works pillar covers the modern implementation.

Marine chronometers remain in use on commercial vessels as backup to GPS — a legacy of Harrison's 18th-century work that survived two world wars and the satellite revolution.

Common misconceptions

“Longitude was unmeasurable until 1714.” It was unmeasurable at sea. Land-based longitude could be calculated for stationary observatories using lunar eclipses and astronomical observation. The sea problem was the practical-portable-time-keeping bit.

“Harrison invented the marine chronometer single-handed.” Many predecessors and contemporaries worked on the problem. Harrison's contribution was the specific designs and the operational accuracy demonstration. Larcum Kendall, John Arnold, and Thomas Earnshaw refined the basic approach into commercial production.

“Lunar distance was inferior to the chronometer.” They coexisted for decades. Lunar distance was the backup method when a chronometer failed or wasn't available. Some 19th-century navigators used both as cross-checks. The chronometer won by being practical; lunar distance remained viable but operationally inferior.

“The £20,000 was paid promptly.” It took Harrison 50+ years of work and royal intervention to collect most of it. The Board of Longitude was hostile to his approach. The story is a case study in scientific- prize politics.

“The 1714 Act was the first scientific prize.” It was unusual but not the first. Various European sovereigns had offered prizes for specific scientific discoveries earlier. What made the Longitude Act unusual was the size of the prize, the structured assessment process, and the durable institution (Board of Longitude) that administered it.

“The longitude problem is solved.” Technically yes, multiple times over. But the institutional legacy persists: the British Nautical Almanac, founded for the lunar-distance method, is still published. Marine chronometers are still made. The Royal Observatory at Greenwich still exists. The story is woven into modern maritime and astronomical infrastructure.

“Dava Sobel's book is fiction.” It's narrative non-fiction, based on archival research at the National Maritime Museum and the Royal Society. Some dramatisation of personalities, but the technical history is accurate. The 1995 publication brought Harrison's story to a mass audience and motivated the major Royal Observatory exhibition that displays the H1–H4 chronometers to this day.

“Modern sailors don't need to understand the longitude problem.” Most don't use chronometer methods — GPS replaced them. But the underlying time- based longitude approach is still taught as part of celestial-navigation backup procedures for commercial mariners; in a GPS-denied scenario (jamming, failure), a sextant plus chronometer is still the canonical fallback.