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Longitude... it plays a vital role in many areas of everyday life, and yet, it was considered impossible to measure back in the 18th Century, when it was desperately needed for navigation.

The Basics

The Earth is a sphere and conventional co-ordinate systems, such as the Cartesian co-ordinates1, don't fit, as they are only suitable for flat surfaces and can't be used to specify points on Earth's curved surface. So new types of co-ordinates had to be invented called longitude and latitude.


Defining the latitude was easy because the origin is given by nature in the guise of the equator. The equator is the line which has the greatest distance from Earth's axis of rotation. It was generally agreed upon that the equator was to be 0° of latitude. The other degrees of latitude, called parallels, were defined as a series of shrinking concentric rings, oriented just like the equator, which grow smaller and smaller and reach zero size at the poles. The poles are where we have 90° of latitude.


The lines of longitude, or meridians, are defined in another way. They are lines that loop from the North Pole to the South in great, equal-sized semicircles. That is, they all have the same length, but the distance between them changes - from 68 miles per degree of longitude on the equator to zero at their points of intersection, the poles.

Finding the Position

In order to navigate a ship safely, a seaman needs to know its position on the surface of the Earth, that is, its co-ordinates in longitude and latitude. The latitude is easily obtained by measuring the height of the sun at noon, or the position of certain stars at night. This is a very old method, and even Columbus used it in attempting to sail to India, but he discovered America instead.

Longitude can't be obtained that easily. The method that is considered the most reliable and practicable is by using a chronometer which keeps the local time at a point of known longitude, say, the home port of the ship. By judging the local time of the ship2, the navigator computes the time difference between home port and ship. Because good old Earth does one full rotation (360°) in 24 hours, one hour corresponds to 360 /24 = 15°. One hour time difference from the home port means 15° longitude difference (east or west).

Having determined latitude and longitude in the described ways, the ship's captain knew where he was and in which direction he had to sail.

The Quest for Longitude

Ancient Attempts

The problem of defining co-ordinates for Earth's surface is an old one. The first known occurrence of latitude and longitude is at least 3BC. The ancient Greek cartographer and astronomer Ptolemy introduced these imaginary lines to his 27-map world atlas, around 150AD. The origin of latitude he used was the equator, which he determined by observing the motions of the celestial bodies, while nature had not provided him with a mark for 0° longitude. He chose the Canary and Madeira Islands for this purpose, rather arbitrarily.

Problems with Longitude

As already stated above, finding latitude on sea was as easy as comparing the length of the day with astronomical tables provided for this purpose. Longitude, which depends on knowing the exact time on two places on Earth, was impossible to discover, because no clock or timekeeper was precise enough on land, let alone on a rolling ship with strongly varying temperatures, atmospheric pressures, and slight changes of the Earth's gravity, which made clocks run faster or slower, rendering them unusable for navigating purposes.

The Old Royal Observatory

Great Britain was well aware of this problem. After a series of accidents at sea, the reigning monarch, Charles II, took action. In 1676, the Royal Observatory in Greenwich was founded, with the explicit purpose for the astronomers to:

Apply the most exact Care and Diligence to the rectifying of the Tables of the Motions of the Heavens, and the Places of the fixed Stars, so as to find out the so-much desired Longitude at Sea, for perfecting the Art of Navigation.

The first Astronomer Royal was John Flamsteed, a young man who wanted to predict the moon's motion against the stars and spent all his life in mapping their positions. His excellent star catalogue was published in 1725. However, it was no solution for the longitude dilemma, because the movement of the moon wasn't fully understood at the time. A nation of seafarers, with the strongest navy of the time, Great Britain still wasn't able to keep track of its ships' positions. This was brought to the attention of the general public in 1707, when four English warships returning victorious from a sea battle with the French fleet at Gibraltar, misjudged their longitude and sailed northward, colliding with the Scilly Islands. Two thousand men died, among them Admiral Sir Clowdisley Shovell.

The Longitude Act

The government was forced to act. In 1714, it passed the 'Longitude Act', stating that a reward of £20,000, a king's ransom, was to be awarded to anyone who found a practicable solution to the longitude problem.

From then on, many curious and dubious so-called solutions were sent to the Longitude Board, only to be rejected. Some of them are so curious that they deserve special mention:

  • The Wounded Dog Method
    In Paris, there was an alchemist who claimed he had invented a magical healing powder. When applied to a piece of clothing that had been in contact with a wound, it would cause that wound to close, but also considerable pain to the injured person. The idea was simple. A wounded dog was taken on board the ship. Every day at noon in Greenwich, a reliable person applied the powder to a strip of clothing that had been in contact with the wound, causing the dog to howl up with pain. Thus, the navigator would know the exact time at Greenwich, rendering a timekeeper obsolete. Of course, the dog had to be wounded every day, so that the wound couldn't heal.

  • The Anchored Ships Method
    Another proposal was to anchor many ships at strategic positions in the oceans. Based approximately seven miles apart from each other, the ships could announce the exact time by firing their cannons.This was rejected, mostly because of the cost involved, but also because the ships weren't safe from pirates, and the sailors on the ship would be worse off than a lighthouse keeper.

  • The Solar/Lunar Eclipse Method
    Astronomy was able to predict eclipses of the Sun and the Moon fairly precisely at the time, so it was suggested that such events be used to determine the exact time. The problem was that they occurred much too rarely to be of any use.

  • The Jupiter's Moons Eclipse Method
    The idea for this solution is Galileo Galilei's. He observed Jupiter's moons, noticing that eclipses occurred very often, several times a day. He figured that if it was possible to predict the events with sufficient precision, they could be used to discover exact time. This method was considered the main rival of the time keeper method, and was used successfully on land, but at sea, it had several drawbacks. The rolling of the ship made it nearly impossible to keep a telescope on Jupiter for any period of time, and even when the sea was calm and the sky clear, it took as much as four hours to do the necessary computations. That is why this method couldn't succeed.

John Harrison

John Harrison, the man who solved the longitude problem, was born in 1693, the fifth child of a carpenter. Little is known about his childhood. He became a clockmaker, though apparently he didn't have a teacher. All he knew, he had taught himself, yet he had acquired quite a reputation as a watchmaker. He had also built a tower clock. When he first heard about the Longitude Prize around 1726, he realised that making his fine clocks seaworthy could make him rich and famous. The rest of his life he spent developing marine timekeepers.

He had an inventive mind, and thought of ways to render a pendulum, which was useless on a rolling ship, obsolete. He also needed to find a way to make a clock less subject to temperature differences and how to minimize friction in his clockworks. In 1737, he presented his first marine timekeeper, Harrison-1 or H-1 for short, to the Longitude Board. The test journey to the West Indies proved that the clock entitled Harrison to claim the prize, but he was too much of a perfectionist to do so. He said that he could make the clock run even more precisely, and that he would come back with the second clock. He only took a little amount of money to cover his expenses.

In 1741, he presented H-2, but he was disgusted with it. He said that he could do much better and asked for more money and time. After being granted both, he retreated to his workbench for 18 years. At the end of this period, in 1758, he came up with H3, which he considered his masterpiece. However, it was never tested on sea - first because of the Seven Years War (nobody would risk losing this precious item in a sea battle), and then because Harrison had already made its successor - H-4, completed in 1760.

H-4 was very different from its predecessors. Harrison called it 'The Watch'. It was the first that was designed to be small and easy to handle, in contrast to H-1 to H-3. As required by the Longitude Act, the Watch was tested on a transatlantic voyage at the end of 1761 and the beginning of 1762. The results achieved were sensational - on the journey across the ocean, with fine sailing weather, it lost as little as five seconds. On the return journey, with heavy storms and high waves, the loss was less than two minutes. This was precise enough (according to the Longitude Act) to claim the £20,000.

The Board of Longitude, however, was reluctant and asked Harrison to explain the mechanism. He refused. Then the Board decided that a second trial voyage should be made, and in 1764, H-4 sailed to the West Indies, performing three times better than required by the Longitude Act. Now, there was no denying Harrison's deserving the reward. Nevertheless, the Board set up a couple of conditions. Harrison was to have half the prize, £10,000, for all four timekeepers he had constructed, and for H-4's plans. If he wanted the rest of the money, he had to build two duplicates of H-4, to prove that it could be duplicated.

In 1770, John Harrison finished the first copy, H-5. He was getting older, and it was to be doubted that he could make another watch. Then, in 1772, his son William wrote an emotional letter to King George III, describing his father's hardship. The King agreed to test H-4 himself. After a year of successful trial at the Royal Observatory in Greenwich, he ordered Harrison to be awarded another £8,750. That still wasn't the full sum, but Harrison was content. He died on his 83rd birthday, 24 March, 1776.

Sources of Information

  • Encyclopaedia Britannica
  • Microsoft's Encarta
  • Much of the information is taken from Dava Sobel's excellent book, Longitude, which is a recommendable read to anyone interested in the topic.
  • Last but not least, the website of Greenwich, the Maritime Museum and the Old Royal Observatory were very helpful.
1Cartesian co-ordinates are the 'normal' co-ordinates that you may have used in school, two axises (x and y, for example) form a right angle and points are specified by giving their offset from a point of origin in the x and y directions.2Local noon is when the sun reaches the highest point of its path.

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