The term “chronometer” denotes an especially precise and high-quality clock or watch. But a timepiece cannot legally be termed a “chronometer” unless it has proven its accuracy according to a standardised measuring procedure and has been accordingly awarded a rate certificate by an officially recognised rate-measuring authority. Perusal of the history books reveals the extraordinary importance which these precise time-measuring instruments formerly had for seamen. Toward the end of the 17th century, imprecise navigation often obliged mariners to spend many additional weeks at sea, and maritime accidents were an almost daily occurrence. The loss of precious human lives and valuable cargo was immense.

In many instances, good luck was largely responsible for bringing renowned captains such as Dom Vasco da Gama, Fernando Magellan or Sir Francis Drake to their destinations. All of these daring mariners faced the same problem on their voyages of discovery: they could only guess at their exact position on the high seas because no accurate method had yet been invented to measure longitude. The so-called “problem of longitude” couldn’t be solved without precise timekeeping. If Britain’s Admiral Shovell could have relied on an accurate timekeeper and thus on precise navigation, his fleet would not have run aground in dense fog near the Isles of Scilly in 1707 – and one of history’s worst naval accidents, which claimed the lives of more than 1,900 men, could have been prevented.

This tragedy prompted the English crown to offer a reward of 20,000 pounds sterling for the development of a measuring procedure that could be used to precisely determine longitude at sea. The most promising method was the mechanical one, which involved measuring the time difference between the time in a vessel’s home harbour and the time onboard the ship. The difference between these times corresponds to the difference between the longitudes of the two locations. The call for a precise time-measuring instrument therefore grew progressively louder. Only if ships were equipped with exact, sturdy and practical chronometers could their navigators accurately determine the vessels’ positions at sea – and thus rule the waves

The H4 chronometer, with which Harrison won the prize,
was surprisingly small compared to his three previous timekeepers.