For every 15 degrees that one travels eastward, the local time moves one hour ahead. Similarly, traveling West, the local time moves back one hour for every 15° of longitude. Therefore, if we know the local times at two points on Earth, we can use the difference between them to calculate how far apart those places are in longitude, east or west.
This idea was very important to sailors and navigators in the 17th century. They could measure the local time, wherever they were, by observing the Sun, but navigation required that they also know the time at some reference point, e.g. Greenwich , in order to calculate their longitude. Although accurate pendulum clocks existed in the 17th century, the motions of a ship and changes in humidity and temperature would prevent such a clock from keeping accurate time at sea. King Charles II founded the Royal Observatory in 1675 to solve the problem of finding longitude at sea. If an accurate catalogue of the positions of the stars could be made, and the position of the Moon then measured accurately relative to the stars, the Moon's motion could be used as a natural clock to calculate Greenwich Time. Sailors at sea could measure the Moon's position relative to bright stars and use tables of the Moon's position, compiled at the Royal Observatory, to calculate the time at Greenwich . This means of finding Longitude was known as the Lunar Distance Method. In 1714, the British Government offered, by Act of Parliament, £20,000 (an immense fortune in those days) for a solution which could provide longitude to within half-a-degree (2 minutes of time). The methods would be tested on a ship, sailing over the ocean, from Great Britain to any such Port in the West Indies as those Commissioners choose without losing their Longitude beyond the limits before mentioned and should prove to be tried and found Practicable and Useful at Sea.
A body known as the Board of Longitude was set up to administer and judge the longitude prize. They received more than a few weird and wonderful suggestions - like squaring the circle or inventing a perpetual motion machine. The phrase 'finding the longitude' became a sort of catchphrase for the pursuits of fools and lunatics. Many people believed that the problem simply could not be solved.
The longitude problem was eventually solved by a working class joiner from Lincolnshire with little formal education. John Harrison took on the scientific and academic establishment of his time and won the longitude prize through extraordinary mechanical insight, talent and determination.
Harrison was born in Foulby, near Wakefield , in Yorkshire in 1693 but his family moved to Barrow, in Lincolnshire , when he was quite young. His father was a carpenter, and John followed in the family trade. He built his first long case clock in 1713 at the age of 20. The mechanism was made entirely from wood, which was not a curious choice of material for a joiner. Three of Harrison's early wooden clocks have survived; the first (1713) is in London , at the Worshipful Company of Clockmakers' Collection in Guildhall; the second (1715) is in the Science Museum; the third (1717) is at Nostell Priory in Yorkshire . During the mid-1720s, John and his brother, James, designed a series of remarkable precision long case clocks to see how far they could push the capabilities of the design. By inventing a pendulum rod made of alternate wires of brass and steel, Harrison eliminated the problem of the pendulum's effective length increasing in warmer weather and slowing the clock. As a result, Harrison's regulators from this period achieved an accuracy of one second in a month - a performance far exceeding the best London clocks of the day.
A second trial of H4 was arranged and William departed for Barbados aboard the Tartar on 28 March 1764. As with the first trial, William used H4 to predict the ship's arrival at Madeira with extraordinary accuracy. The watch's error was computed to be 39.2 seconds over a voyage of 47 days; three times better than required to win the £20,000 longitude prize. The Board of Longitude, however, implied that the watch was a fluke and would not be satisfied unless others of the same kind could be made and tested. Harrison would be paid £10,000 as soon as he disclosed his secrets and handed over his mechanisms to the Astronomer Royal, with the remaining £10,000 being paid when other timekeepers of the same type, accurate enough to find longitude to within 30 miles, were made.
In order to qualify for the second half of the prize, Harrison had to make at least two more watches and have them tested. The Board of Longitude insisted that he make these copies of H4 himself, but took the original away for testing at the Royal Observatory. Nevil Maskelyne, who had been appointed Astronomer Royal in 1765, remained unconvinced that a watch could be more reliable than the lunar distance method for finding Greenwich Time.
John Harrison (then in his seventies) and William worked on a fifth timekeeper (H5), while Kendall made good progress on his copy of H4. Kendall 's watch, now known as K1, was completed in 1769 and inspected in early 1770 by the same panel that had examined H4. William Harrison was also present and admitted that the copy was exceptional.
The Board of Longitude was asked to consider H5 and K1 as the two copies of H4, but told John and William, in no uncertain terms, that both copies of H4 should be made by Harrison. John, now 79 years old, made an appeal to the highest authority in Britain. On 31 January, 1772 an approach was made to King George III via a letter to his private astronomer at Richmond , Dr Stephen Demainbray. William was summoned for an interview with the King himself, at which time the King is said to have remarked, "... these people have been cruelly wronged ..., and By God, Harrison, I will see you righted!"
H5 was put on trial by the King himself in 1772, and performed superbly. The Board of Longitude, however, refused to recognize the results of this trial, so John and William petitioned Parliament. They were finally awarded £8750 by Act of Parliament in June, 1773. Perhaps more importantly, John Harrison was finally recognized as having solved the longitude problem.
In the meantime, Captain Cook had set out on his second voyage of discovery with K1, Kendall 's copy of H4. He returned in July, 1775, after a voyage of three years which ranged from the Tropics to the Antarctic. The daily rate of K1 never exceeded 8 seconds (corresponding to a distance of 2 nautical miles at the equator) during the entire voyage, and Cook referred to the watch as "Our faithful guide through all the vicissitudes of climates". It is not known for certain whether Harrison knew of this success, but Cook's voyage proved beyond doubt that longitude could be measured from a watch. John Harrison died almost one year after Cook's return, on 24 March, 1776 in his house at Red Lion Square, London. It was his 83rd birthday.