Globes and maps are often covered by a grid of lines running north-south and east-west. These lines, which represent degrees of “latitude” and “longitude”, are used to identify a position on the Earth’s surface. Mariners used the noonday Sun’s position over the horizon to calculate north-south distances for latitude. Longitude measurement systems that could be used on a moving ship to determine an east-west position remained unknown in the early 1700s. Without longitude, accurate charting and fixing a position while far out at sea relied on estimates of speed and distance travelled, known as dead reckoning. But explorers and kings wanted precision.
In 1714, the British Parliament offered the enormous prize of £20,000 for the solution to this problem. The Board of Longitude was established to judge and reward the submissions from mathematicians, clockmakers, and scientists. The Earth could be divided, like a circle, into 360 degrees and it took 24 hours for it to rotate, making one day. It was known that local time shifted one hour for every 15 “degrees” of longitude travelled: 15 degrees east “lost” an hour and 15 degrees west “gained” an hour. If the difference in the local time in two locations was known, then the distance and longitudinal measures could be calculated. Local time at sea could be determined using the sun and moon, but how could time at a homeport be used for comparison when clocks did not function properly with the rocking and jostling of the ocean?
It was a “pocket watch” that solved the problem of longitudinal measurement. An English joiner named John Harrison worked throughout his life to perfect a series of precision clocks that were spring-driven and counterbalanced. They could, therefore, keep time even during the constant movement endured at sea. His clocks, known as chronometers, went on sea trials (in fact, a copy was tested by Captain Cook while on expedition in 1775). Harrison’s design was officially recognized in 1773 based on measurements of time relative to the Royal Observatory at Greenwich. They had found a system to measure longitude, and we continue to use “Greenwich Mean Time” today.
