Book Review of 'Longitude' by Dava Sobel

By Ken Christensen
Volume 9, Number 2 (Winter 1998-1999)
Issue theme: "Secure identification and immigration enforcement"

The quest for longitude (or more properly the means to locate it) seems archaic and obvious in these days of satellites and global positioning. But this ability to find out where we are and describe it so that others may follow is one of the great stories of the age of discovery. Dava Sobel has done a wonderful job of telling a complicated story in a straightforward, entertaining and blessedly brief account (the book, printed in a small format, is a quick read at 175 pages). The author introduces us to the subject of longitude and the problems inherent in "finding" it, then proceeds with the fascinating story of the great prize (£20,000, or approximately $12 million in today's money) offered by the English parliament in 1714 for a practical method of finding longitude at sea, and John Harrison, the man who spent his entire life in attempting to claim the prize.

As everyone knows, latitudes are the horizontal lines on the globe and longitudes are the vertical lines. These lines existed at least as far back as 300 BC and Ptolemy, the Greek geographer in Alexandria, established longitude and latitude on his maps in 150 AD. Latitude was easily established by measuring the path of the sun, passing directly overhead at the equator. Longitude, however, was far more arbitrary; Ptolemy copied the circle giving the earth 360° of longitude and setting the zero longitude at the Canary Islands .

The need to accurately measure longitude became imperative in the age of discovery. Even Columbus couldn't measure longitude, so he stuck to a known latitude, and if America hadn't gotten in the way, he might have made it to India after all. For centuries, sailors relied on latitude readings by tracking the sun, and "dead reckoning" for the rest (often with disastrous results from not knowing one's longitude). In 1514, Johannes Werner, the German astronomer, struck on a way to use the motion of the moon as a location finder. This involved mapping all of the skies, sun, planets and stars, then measuring the moon's relation to them at different times and at different locations. This is obviously an extremely complicated method and even more so for a sailor at sea. In 1610, Galileo hit upon a remarkable method of fixing longitude by measuring the eclipses of Jupiter's moons. Although very reliable, this method also involved complicated calculations and the most obvious disadvantage of these celestial methods was cloudy skies.

The other method of finding longitude is by time. As Sobel writes:

"To learn 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 moment. The two clock times enable the navigator to convert the hour difference into a geographical separation. Since the earth takes 24 hours to complete one full revolution of 360°, one hour marks one twenty-fourth of a spin, or fifteen degrees. And so each hour's time difference between the ship and the starting point marks a progress of fifteen degrees of longitude to the east or west. Every day at sea, when the navigator resets his ship's clock, every hour's discrepancy between them translates into another fifteen degrees of longitude."

This solution is obviously simpler than the complicated celestial observations; however, one major problem remained: no clock was accurate enough to perform the task, especially a clock at sea, exposed to different temperatures, humidities, gravity, and the pitching of waves.

Sobel breezily takes us through the history of the problem touching on the work of the great astronomers, Galileo, Gemma (?), Frisius, Ole Roemer, Tycho Brahe, Giovanni Cassubum and others. On the other side of the problem/solution worked the clockmakers (who were often astronomers also) such as Vincenzo Viviani, Galileo's protégé,. Christiaan Huygens, and Robert Hooke, all working in the 1600s. The problem became more urgent as Spain, France, and England sailed the seas exploring and conquering. Not only were there shipwrecks due to miscalculations, but by timidly sticking to common latitudes, there was more chance of crowded sea-lanes, piracy and conflict. The astronomical solution seemed the most promising, so the kings of both France and England established royal observatories, in Paris in 1666, and at Greenwich shortly afterwards. Naturally, both claimed to be at the center of the world, or 0° longitude.

Establishing the difficulty of the problem takes up the first third of the book, whereupon Sobel shifts the focus to John Harrison who solved the problem, but not easily and at the cost of great struggles and great obstacles thrown in his path. Harrison's solution was a near perfect clock - he built not one, but four, between 1737 and 1760. The nemesis who blocked his path to claiming the prize was Nevil Maskelyne, who, not surprisingly, was the royal astronomer who sought a celestial solution to the problem. Eventually, two large camps evolved - clock makers and astronomers, the latter looking down their noses at the former for being mere mechanics. Maskelyne eventually arrived at a solution with the publication of his book, The Nautical Almanac and Astronomical Ephemeris published in 1766. Although his method of plotting the moon worked, the calculations and observations were so complicated that they resulted in more errors than Harrison's simpler clock method.

Dava Sobel does a wonderful job of portraying the two men, their lives, work and conflicts. The story doesn't fall far short of the drama of Soliari and Mozart. And, as always, time and truth are the final judges. A simple scientific tale involves all the elements of human rivalry: a great prize, jealousies, competition, political intrigues, even world domination and finally, a bit of justice. The book is also a timeless tale of man's explorations, of wanting to know where he is, where he's going, and how to get back again! And, as with every great scientific pursuit, great discoveries were made accidentally along the way, such as the speed of light, the mapping of the stars of both hemispheres, the movement of stars in time, and, of course, advances in telescopes and clocks and their workings.

Dava Sobel is a former science writer for the New York Times, as well as contributor to Audubon, Discover, Life, and Omni magazines. The concluding chapter of Longitude is a very touching account of her personal visit to zero longitude in Greenwich (where this writer also straddled that magic line embedded in the pavement). The Maritime Museum in Greenwich is the actual home of Harrison's four clocks. Sobel writes, "Coming face to face with these machines at last, after having read countless accounts of their construction and trial, after having seen every detail of their insides and outsides in still and moving pictures, reduced me to tears."

This book may not reduce you to tears, but it will give you a sense of wonder and will raise your eyes from the little lines on maps that we take for granted, and raise them to larger thoughts of time and space and man's struggle to find out where he is.

About the author

Ken Christensen is an accomplished artist and purveyor of used books in Northern Michigan.