Longitude: The True Story of a Lone Genius Who Solved the Greatest Scientific Problem of His Time   Behind John Harrison stands his 1726 pendulum clock while his H4 watch lies to his left on the table Author Dava Sobel Genre(s) Popular Science Publisher Walker Publication date 1995 Pages 208 ISBN 9780802715296 OCLC Number 183660066

Longitude: The True Story of a Lone Genius Who Solved the Greatest Scientific Problem of His Time is a best-selling book by Dava Sobel about John Harrison, an 18th Century clockmaker who created a chronometer which was sufficiently accurate to be used to determine longitude at sea—an important development in navigation. The book won the 1997 Whitbread prize and was made into a period drama.^[1][2]

[edit] Problem of longitude

Map of Earth
Longitude (λ)
Lines of longitude appear vertical with varying curvature in this projection; but are actually halves of great ellipses, with identical radii at a given latitude.
Latitude (φ)
Lines of latitude appear horizontal with varying curvature in this projection; but are actually circular with different radii. All locations with a given latitude are collectively referred to as a circle of latitude.
The equator divides the planet into a Northern Hemisphere, a Southern Hemisphere and has a latitude of 0°.
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Determining longitude on land was fairly easy compared to the task at sea. A stable surface to work from, a comfortable location to live in while performing the work and the ability to repeat determinations over time made for great accuracy. Whatever could be discovered from solving the problem at sea would only improve the determination of longitude on land.

Determining latitude was hard for the sun's declination for the day.^[3] For longitude, early ocean navigators had to rely on dead reckoning. This was inaccurate on long voyages out of sight of land and these voyages sometimes ended in tragedy as a result.

In order to avoid problems with not knowing one's position accurately, navigators have, where possible, relied on taking advantage of their knowledge of latitude. They would sail to the latitude of their destination, turn toward their destination and follow a line of constant latitude. This was known as running down a westing (if westbound, easting otherwise).^[4] This prevented a ship from taking the most direct route (a great circle) or a route with the most favourable winds and currents, extending the voyage by days or even weeks. This increased the likelihood of short rations^[5], scurvy or starvation leading to poor health or even death for members of the crew and resultant risk to the ship.

Errors in navigation have also resulted in shipwrecks. Motivated by a number of maritime disasters attributable to serious errors in reckoning position at sea, particularly spectacular disasters such as that which took Admiral Cloudesley Shovell and his fleet, the British government established the Board of Longitude in 1714.

"The Discovery of the Longitude is of such Consequence to Great Britain for the safety of the Navy and Merchant Ships as well as for the improvement of Trade that for want thereof many Ships have been retarded in their voyages, and many lost..." and announced the Longitude Prize "for such person or persons as shall discover the Longitude."

The prizes were to be awarded to the first person to demonstrate a practical method for determining the longitude of a ship at sea. Each prize, in increasing amounts, were for solutions of increasing accuracy. These prizes, worth millions of dollars in today's currency, motivated many to search for a solution.

Britain was not alone in the desire to solve the problem. France's King Louis XIV founded the Académie Royale des Sciences in 1666. It was charged with, among a range of scientific activities, the improvement of maps and sailing charts and advancement of the science of navigation. From 1715, the Académie offered one of the two Prix Rouillés specifically for navigation.^[6] Spain's Philip II offered a prize for the discovery of a solution to the problem of the longitude in 1567; Philip III increased the prize in 1598. Holland added to the effort with a prize offered in 1636.^[7] Navigators and scientists in most European countries were aware of the problem and were involved in finding the solution. Due to the international effort in solving the problem and the scale of the enterprise, it represents one of the largest scientific endeavours in history.

[edit] Knowing Longitude

A blue plaque marks the house at Red Lion Square where Harrison lived and died.

As Dava Sobel explains, "to know one's longitude at sea, one needs to know what time it is aboard ship and also the time at the home port or another place of known longitude-at that very same moment. The two clock times enable the navigator to convert the hour difference into geographical separation. Since the earth takes 24 hours to revolve 360 degrees, one hour marks 1/24 of a revolution or 15 degrees. And so each hour's time difference between the ship and starting point marks a progress of fifteen degrees of longitude to the east or west. Every day at sea, when the navigator resets his ships clock to local noon when the sun reaches its highest point in the sky, and then consults the home port clock, every hour's discrepancy between them translates into another fifteen degrees of longitude. One degree of longitude equals four minutes of time the world over, although in terms of distance, one degree shrinks from 60.15 nautical miles [Earth's circumference being: 21,653.521 nautical miles, or 24,901.55 statute miles at the Equator], to virtually nothing at the poles. Precise knowledge of the hour in two different places at once - a longitude prerequisite so easily accessible today from any pair of cheap wristwatches - was utterly unattainable up to and including the era of pendulum clocks. On the deck of a rolling ship such clocks would slow down, or speed up, or stop running altogether. Normal changes of temperature encountered en route from a cold country of origin to a tropical trade zone thinned or thickened a clocks lubricating oil and made its metal parts expand or contract with equally disastrous results. A rise or fall in barometer pressure, or the subtle variations in the Earth's gravity from one latitude to another, could also cause a clock to gain or lose time."[1]

Harrison's final triumph - the compact chronometer

Before the 1700s ocean navigators could not find an accurate way of determining longitude. This failure caused ships to miss their destination, many times crashing into rocks and killing their crews. A practical solution came from a simple carpenter, John Harrison, who solved one of the most difficult problems of his time by creating an accurate chronometer. The best scientists of the time, including Isaac Newton, thought it impossible. Harrison spent four decades perfecting a watch that would ultimately earn him the prize established by the Longitude Act of 1714, thanks to the recognition and influence of King George III of England.[2]

[edit] See also

[edit] References

[edit] External links

• Booknotes interview with Sobel on Longitude, January 17, 1999.