For centuries, mariners were able to determine their latitude by measuring the height of a celestial body such as a star, the sun or moon above the horizon. Knowing the time of year or the phase of the moon how to do the math, latitude could be calculated.

Recognizing that existing clocks weren’t accurate enough to help a ship’s captain calculate longitude, the British Parliament acted by providing a specification for accuracy and a large prize. The first Act of Longitude passed in 1714 offered a £10,000 prize (worth ~$2.15M today) for anyone who could develop a clock that help a mariner determine longitude within a degree. Later Acts of Longitude increased the prize amount and the requirement for clock accuracy as well as calling for other inventions.

A British carpenter by the name of John Harrison took on the challenge. His first effort, submitted in 1730 for the prize is known as H1. It has counter-oscillating weighted beams attached to springs. While it worked well enough, Harrison realized that the motion of the ship would affect the working of H1 and was therefore, not suitable.

In 1759, Harrison submitted a design and a working model that used caged roller bearings, bi-metallic strips to turn temperature changes into movement. It was called “The Third Sea Clock” (H3) and its mechanisms are still used by clock and watchmakers today.

Harrison’s H3 had another problem – size – that made it impractical for use on a ship. However, in his H4 design, submitted in 1761 for the Longitude Prize of £20,000 (worth ~$4.45M in 2019) contained innovations that solved the problem of size and improved accuracy. H4 uses fast moving balance wheels made of steel that moved back and forth at a rate controlled by a spiral spring. Each swing of the wheel was turned into movement of the hands on the clock. Bimetallic strips were used to compensate for the effect of heat on the springs. Watchmakers used Harrison’s design until the electronic watch was invented.

H4 was not much bigger than a large pocket watch and could be wound without disrupting its timekeeping ability. The watch as tested on several voyages across the Atlantic and met or exceeded the requirements set by Parliament. Unfortunately, Parliament found excuses not to pay Harrison the full prize. By the time he died in 1776, Harrison had collected a total of £23,065 for his work that made him a rich man.

Unfortunately, the early chronometers like the H4 and its successors were expensive to produce. Another Brit, John Arnold simplified Harrison’s design to maintain its accuracy but reduce its manufacturing costs. It was not until the 1830s did the cost come down so that the Royal and the U.S. Navy could equip each ship with a chronometer.

With an accurate timepiece, mariners could accurately estimate the time between moment they left a known point on earth with the angle to a star, the sun or moon and fix their longitude. Along with a sextant, a timepiece and a chart using a Mercator projection, ship captains could now accurately plot their position on any ocean