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For every 15° that one travels eastward, the local time moves one hour ahead. Similarly, travelling 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 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 Commisioners 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.

John Harrison (1693-1776)


John Harrison, Inventor of the Compound Pendulum & of several Time Keepers by Thomas King (artist) and P. L. Tassaert (engraver), 1768
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 longcase 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.

He married his first wife, Elizabeth, in 1718. She died just eight years later and he remarried within six months, to another Elizabeth.

During the latter part of his early career, he worked with his younger brother James. Their first major project was a revolutionary turret clock for the stables at Brocklesby Park, seat of the Pelham family. The clock was revolutionary because it required no lubrication. 18th century clock oils were uniformly poor and one of the major causes of failure in clocks of the period. Rather than concentrating on improvements to the oil, Harrison designed a clock which didn't need it. It was radical thinking of this sort that would be important later on, when he tackled the problem of designing a marine timekeeper.

During the mid-1720s, John and James designed a series of remarkable precision longcase 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, 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.

To solve the longitude problem, Harrison would have to devise a portable clock which kept time to the same accuracy as these precision regulators...