The Longitude Problem

One of the great unsolved engineering problems of history — and its solution changed the world. Latitude was trivially easy; longitude was impossible for millennia. Thousands died because of this asymmetry.

Why Latitude Was Easy, Longitude Was Hard

Latitude: measure the altitude of Polaris (or the Sun at noon). That angle is your latitude. Requires only a simple instrument and a clear horizon.

Longitude: the Earth rotates 360° in 24 hours — 1° every 4 minutes. To know your longitude, you need to know what time it is at a fixed reference meridian (say, Greenwich) while simultaneously knowing your local time. The difference gives you longitude.

Every 4 seconds of clock error = 1 nautical mile of longitude error at the equator.

Before accurate portable clocks, there were no clocks that could maintain accuracy on a rolling, temperature-varying ship at sea. Pendulum clocks (which needed gravity) failed completely on ships. Spring-wound pocket watches of the era lost or gained many minutes per day.

The Price of Ignorance — The Scilly Disaster

On 22 October 1707, a British naval fleet under Admiral Sir Cloudesley Shovell returned from Gibraltar in rough weather. Unable to accurately determine their longitude, they miscalculated their position. Four ships of the line ran aground on the Scilly Isles. ~2,000 men died in one of the worst peacetime naval disasters in British history.

This and similar disasters motivated the British government to act.

The Longitude Prize (1714)

Parliament offered a prize of £20,000 (roughly £2 million today) for a method of determining longitude at sea to within 30 nautical miles — requiring a clock accurate to within 3 minutes over a transatlantic voyage.

Isaac Newton and most of the scientific establishment considered this impossible. The preferred approach was the lunar distance method — measuring the Moon's position relative to stars to determine Greenwich time — but this required ~3 hours of complex calculation per observation, not practical for ordinary navigators.

John Harrison's Solution

John Harrison — a Yorkshire carpenter and self-taught clockmaker — spent four decades proving Newton wrong.

He built five chronometers, each more refined:

• H-1 (1730s): large, spring-balanced, tested to Lisbon and back; performed well but Harrison considered it imperfect
• H-2, H-3: each an improvement; Harrison never submitted them for the official prize
• H-4 (1759): a large pocket watch design; completely different mechanism; the breakthrough

The H-4 sea trial in 1762 proved accurate to within 5 seconds over 81 days — far exceeding the prize requirements. The Board of Longitude, dominated by astronomers who preferred the lunar method, refused to pay. Harrison fought Parliament for his reward. At age 80, with King George III's personal intervention, he finally received full payment in 1773.

"Never go to sea with two chronometers; take one or three." — navigational adage (two chronometers give no way to know which is wrong; three allow majority-vote correction)

The Nautical Almanac (1767)

In parallel, Astronomer Royal Nevil Maskelyne published the first *Nautical Almanac* — tables of lunar distances and planetary positions allowing the lunar distance method to work for ordinary navigators. The Almanac became the standard reference worldwide and, because it was based on the Royal Observatory at Greenwich, helped drive the adoption of the Greenwich Meridian as the international prime meridian in 1884.

The Clock Won

By 1800–1850, as chronometers became cheaper and more reliable (other makers like Thomas Earnshaw simplified Harrison's design), they displaced lunar distances as the standard longitude method. The chronometer made accurate position-fixing accessible to any ship with a reasonably priced instrument.

GPS — which triangulates time signals from satellites to determine position — is the conceptual descendant of Harrison's insight: accurate time is position.