The early navigators were able to sail along lines of latitude using the meridian passage of the sun or sights of Polaris (the North Star). Neither of these techniques required an accurate timepiece. Measuring longitude at sea is a much more difficult proposition.

By the end of the seventeenth century the availability of accurate pendulum clocks made the calculation of longitude on land feasible and even accurate. For obvious reasons pendulum clocks cannot operate on a moving platform. It is in a feverish atmosphere of exploration, discovery and charting of discoveries that the British parliament passed the Longitude Act in 1714. A sum of £20,000 was offered as a prize to the “person or persons who discover a Method of Finding Longitude at sea”. This sum would be equivalent to several millions today.

A Board of Longitude was established to adjudicate on who was the winner of the prize. The winner had to be able to demonstrate he/she could establish longitude within half a degree (equivalent to 30nM on a great circle). Expectations were initially low that such a feat was possible but they had not counted on the ingenuity and persistence of John Harrison.

Harrison designed and built H4. In 1763 it was used to navigate to Barbados and back. It passed the test with flying colours but the Longitude Board needed to be convinced that it’s performance was more than a fluke. They needed to know that H4 could be replicated and maintain its accuracy across the range. The adjudication process dragged on for years frustrating the now elderly Harrison who was now supported by his son William. Eventually in 1773 Parliament overruled the Longitude Board and gave the well deserved prize to the Harrisons. With this the maritime chronometer became a key instrument in every navigator’s toolkit.

Today we take accurate timekeeping for granted. A mobile phone or good watch will provide time to a degree of accuracy undreamt of three hundred years ago. He have the Harrisons and many other fine watchmakers who improved on their design to thank for “finding longitude “.