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Each line represents an equal fraction of the circumference of the globe. In Euclidean geometry, the earth is divided into 360 degrees (hence the name of this blog). The horizontals, or latitude, are measured from the Equator, which is 0 latitude, while the poles are 90 degrees north and south respectively. This is the easy part. Mariners have known how to measure latitude since ancient times, using an astrolabe or cross-staff to work out the angle between the Sun or a prominent star and the horizon, and comparing it to the same reading back home. So they knew how far north or south they were from home.
But measuring a position east or west of a given point, i.e. longitude, was quite a bit trickier. Because the Earth rotates, to work it out, you need to know the difference between local time and the time at a fixed longitude. (The ‘prime meridien’ – longitude 0 –was eventually fixed at Greenwich, London, on account of the presence of the Royal Observatory and the dominant position of the British Empire at the time, but any other point would do.)
Divide the 360 degrees of the Earth’s circumference by the 24 hours it takes to rotate, and you see that one hour’s time difference corresponds to 15 degrees of longitude.
Finding the current local time by the position of the Sun or other stars was easy enough, but until someone could make a clock that would resist the rocking of the waves and the corrosive effect of salt spray over a lengthy sea voyage, it was impossible to keep track of the time back home. Without that, sailors couldn’t work out their position. They basically had to guess on the basis of how long they thought they’d been travelling east or west.
In 1567 Philip II of Spain offered a prize of 6000 ducats for anyone who solved the problem. When the prize had not been awarded half a century later, Galileo proposed a method using the rotation of Jupiter’s four largest moons, which he had discovered and whose movements he had plotted, as a celestial clock. The Spanish weren’t interested, so he approached the Dutch; but soon he was a prisoner of the Inquisition, and the scheme got nowhere.
Nearly a century later, 1658, the Dutch astronomer Christiaan Huygens attempted to design a pendulum clock that would work at sea; the British scientist Robert Hooke tested it, but found that the rocking of the ship disrupted the pendulum. To overcome this, he proposed a pocket watch driven by a hairspring, but his idea was not developed.
In 1698–9 Edmond Halley, having noticed that compass bearings fluctuate in different parts of the globe, conducted two Atlantic voyages to map these variations. His belief that they could be used to calculate longitude was mistaken, but the findings proved an invaluable contribution to our understanding of the Earth’s magnetic field.
Then, in 1707, a catastrophic naval disaster dramatically raised the stakes. Four ships of Sir Cloudesley Shovell’s fleet, returning from a successful engagement with the French, struck rocks and sank off the Isles of Scilly. Some 1400 men are believed to have drowned; the body of the unfortunate admiral was found washed up in a cove on St Mary’s. (Legend has it that he survived shipwreck only to be murdered by a local woman for his emerald ring.)
The disaster was blamed on a navigational error, so in July 1714, the British government established a Board of Longitude and offered £20,000 to anyone who could devise a method of determining longitude within 30 nautical miles (56 km). The competition for this massive prize – more than £2 million in today’s money – is the subject of Dava Sobel’s bestselling book Longitude.
In 1736, the English clockmaker John Harrison‘s first attempt to build a marine chronometer was tested on a journey to Lisbon. It was a cumbersome instrument resembling a carriage clock, but the Board was sufficiently impressed to award Harrison a grant of £500 to develop it further.
Meanwhile, it pursued another line of investigation: lunar distance. This used the moon's apparent position in relation to a star to find the reference time. The principle had been understood since the 16th century, but there were no reliable tables of the Moon’s complex movements or instruments accurate enough to perform the necessary measurements.
All that was about to change. In 1731 John Hadley invented the octant, which used mirrors and prisms to measure the angle of a star to the horizon, and in 1759 John Bird produced the first sextant, extending the range of the instrument from an eighth to a sixth of a circle. Meanwhile, Nevil Maskelyne succeeded in calculating lunar distances a year in advance, and persuaded the government to finance the publication of his tables annually.
Harrison, meanwhile, continued experimenting, using bearings to reduce friction; coiled springs to minimize the effects of movement, and a balance spring made of two different metals to counteract the expansion and contraction caused by changes in temperature. After two further prototypes, in 1759 he completed a portable sea watch, which resembled a large pocket-watch about 5 inches in diameter (now known as the H4).
By now, Harrison was 67, so he sent his son to test it on a voyage to Jamaica in 1761. On arrival, it was just 5 seconds slow. The Board ordered the watch to be handed over to the Astronomer Royal for more tests. Unfortunately, the position was now held by Harrison’s arch-rival Maskelyne, who reported that it didn’t work. It was only after the chronometer was successfully reproduced by another craftsman, Larcum Kendall, that the clockmaker was offered a further £2500.
Meanwhile, despairing of justice from the Board, Harrison built a fifth version (H5) and presented it directly to King George III. “By God, Harrison,” the King is said to have declared after testing the watch himself, “I will see you righted!” Finally, at the age of 80, the inventor received the balance of the prize money and the recognition for his achievement.
Problem solved – or was it? While Harrison’s chronometer was more accurate than any other solution, it was a unique, hand-made instrument, and master craftsmen of his calibre were rare. Kendall’s copy (which impressed Captain Cook on his second voyage to the South Seas) had cost the huge sum of £450. While the Royal Navy could afford a few of these expensive instruments, they remained beyond the means of most seamen, who continued to calculate longitude by the moon for another century, until the arrival of factories made it possible to mass-produce chronometers at an affordable price.
To mark the 300th anniversary of the Longitude Act and commemorate Harrison’s discovery, and exhibition Ships, Clocks & Stars can be seen at the National Maritime Museum. Greenwich, until 4 January. Its exhibits include all five of Harrison's chronometers.