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Quarterly Review
Space, Time, and Dead Reckoning
Longitude, by Dava Sobel (Penguin, 1998, $11.95)
Reviewed by Allan A. Kerin
Until the end of the eighteenth century sailors had no accurate measure of their position. For thousands of years a ship’s latitude, its north-south position, could be accurately determined by noting the date and the observed elevation of the sun. But longitude, or east-west position, could only be guessed by dead reckoning. A sea captain could keep careful track of his ship’s movements in his logbook and calculate his east-west position by adding each day’s progress to that of previous days to produce a total east-west distance traveled. But, imperfectly measured currents and winds could fool even the best captain, resulting in missed landfalls and groundings that were costly and often fatal. The great voyages of discovery and the first centuries of transoceanic trade took place under these conditions.
A great deal of scientific effort was expended in the sixteenth and seventeenth centuries to solve the problem of longitude. Galileo and others would provide partial solutions that would eventually contribute to an accurate method of determining longitude from astronomical observations. This method would ultimately be perfected in the late eighteenth century after decades were spent recording the complex path of the moon and the angles that it forms with the sun and various stars over its eighteen-year cycle. The goal of this method was to determine from a ship the time at a distant fixed point on the globe. The time at the distant reference point could be compared to the observed time, measured from solar observations, on board the ship to calculate how far east or west the ship was relative to the fixed point. This astronomical method (colloquially referred to as “lunars”) was difficult, time consuming and prone to calculation errors, particularly when applied from the unstable environment of a sailing ship on a rough sea. Of course if one could bring an extremely accurate clock set to the time at the distant point of reference on board ship the complex lunar calculations would not be necessary. A brilliant self-educated English clockmaker, John Harrison, spent the years 1727 to 1776, most of his long life, developing such a clock.
John Harrison spent decades developing a small mechanical clock (chronometer) filled with innovative inventions to overcome the instability caused by turbulent seas, changes in temperature and humidity, contaminating dust particles, and many other environmental factors. It is easy, and appropriate, to see him as a lone genius working to achieve his dream. However, his work was supported by generous stipends from a scientific bureaucracy, the Board of Longitude, established by the British Parliament, whose members included Isaac Newton and Edmund Haley. For years the somewhat narrow-minded and perhaps jealous members of the Board of Longitude obstructed the recognition of Harrison’s invention and the award of a £20,000 prize to him. Finally, George III, who took an intense interest in scientific matters, personally intervened to help Harrison triumph in his old age. The astronomical method that Harrison’s opponents advocated would ultimately have value as an adjunct to his invention, enabling sailors to periodically check the accuracy of their chronometers while at sea. Harrison’s chronometer was complex and expensive and hence enjoyed limited use. It was not until his successors were able to simplify its design in the early nineteenth century that mass production and almost universal use of marine chronometers became possible.
In our day of “instantaneous” communication there is no need to calculate the time at distant point. One need only call up and ask. And mechanical clocks are now obsolete. An inexpensive electronic watch can outperform the most exquisite handmade clock. And, most fundamentally, global positioning systems based on “instantaneous” communication allow us to calculate our position directly anywhere in the world. We have eliminated the technological limitations on communication that forced us to indirectly calculate the time at a point of reference. In the vast distance of outer space we are faced with the fundamental limitations on the speed of communication, but we are now less than a tenth of a second from simultaneous knowledge about any place on Earth.
Dava Sobel has written an interesting and thought-provoking book about the development of a now obsolete technology that was crucial to the economic, political, and military history of our world in the nineteenth and part of the twentieth century. The world that we live in would not have developed without it. This thoughtful, eloquent, and concise book is worth reading.
