The History of Longitude Navigation by Marine Chronometer
and Astronomical Methods

I INTRODUCTION

Since ancient times, navigation across open oceans was unreliable and dangerous, until latitude and longitude measurements were made directly from the moving ship. Coordinates of latitude and longitude are necessary for safe navigation on the ocean. Latitude is simply the angle north or south of the equator as measured using the center of the earth as the vertex. Latitude is quite easily determined from altitude measurements of the sun and/or stars using a sextant; in the northern hemisphere, the north star (Polaris) provides the observer’s latitude directly. But longitude remained unknown for many centuries, even after the Vikings and Columbus sailed across the Atlantic ocean.

Longitude is simply the difference between the observer's local (sundial) time and the local (sundial) time at Greenwich, England. As any sundial shows, the earth turns at 15 degrees per hour (360 degrees per day), so this time difference is easily converted into degrees of the earth's longitude. Both local and Greenwich time must be known onboard the observer ship to determine longitude, so finding longitude is much more difficult than finding latitude. Either a marine chronometer or various astronomical methods may be used to determine these two times, and therefore longitude.

To utilize accurate clocks for the measurement of longitude while sailing the high seas was first proposed by Gemma Frisius, in 1530. Christiaan Huygens’ tried his pendulum clock as such a marine clock in 1664, and in 1675 invented a marine timepiece with his spiral-spring balance wheel, but ocean tests showed that these lacked the accuracy required for marine use. With a highly refined version, which had technical improvements including a remontoire and temperature compensation, John Harrison demonstrated the first sufficiently accurate marine chronometer in 1762. We therefore will start with Harrison’s contributions, and follow with the previous and subsequent history of the three main methods of longitude measurement.

II HARRISON'S GRASSHOPPER

John Harrison was trained as a carpenter, and had built several wooden-gear clocks before building marine chronometers, to compete for the 20,000 pound prize offered by the British Board of Longitude. His earliest wooden gear longcase clocks (circa 1712-1715) contained a metal escape wheel and recoil anchor escapement. When contracted to make a turret clock at Brocklesby Park, he first designed and installed a large wooden-gear clock with a metal recoil anchor escapement [1-7]. It included a Huygens-type pendulum with Huygens’ cycloidal cheeks [7], so it is clear he was acquainted with Huygens' work of the previous century. His clock wheels had wood grain aligned radially, as was standard in wooden-gear mine-pumps, water mills and wind mills. The use of the metal recoil anchor was also quite standard in wooden-gear clocks of the time, as in the German cuckoo clocks. But, as it was exposed to the weather, the escapement friction problems he encountered were much more serious than in his longcase versions, as it needed regular cleaning and oiling, without which the clock would stop. As this would obviously be a regular occurrence, the escapement left much to be desired. With his brother, James, he quickly set out to design an improvement, with articulating legs to provide frictionless action, to directly replace the cam action of the recoil anchor [2,5,7]. This was the first model of Harrison's grasshopper escapement [7].

III HARRISON’S MARINE CHRONOMETERS

After this turret clock, Harrison made, with his brother, James, several very accurate wooden-gear regulators before 1730 with roller pinions, and with his new temperature-compensated grid-iron pendulum, with adjustable cycloidal cheeks [8-10]. For these he claimed an accuracy of 1 second per month. The success of Harrison's first marine chronometer which he built shortly thereafter (subsequently labeled "H1" by Gould) proved he was an early pioneer, who built on, and improved on, the pendulum-based, and spiral-spring balance wheel based experimental marine chronometers of Huygens, as well as the experiments of Graham, Thackery, and Sulley [9]. He cleverly adapted and redesigned Huygens' components like the pendulum, the weight remontoire, counter-rotating balance wheels, verge-driven spiral-spring balance wheel, and epicyclic maintaining power [2], as well as Sulley's anti-friction roller wheels. That he carefully studied the works of his predecessors, shows he was a well-read horologist, rather than a lone carpenter working in isolation in a country village.

Harrison designed his H1, H2, and H3, to contain a counter-rotating balance, his own temperature-compensation curb, and a grasshopper escapement, H2 and H3 driven by his new spring remontoire, which kept both the oscillator-spring error, and the escapement error, constant. Rather than a grasshopper, his final chronometer, H4 (and its copy H5), however, utilized a high precision (almost dead-beat) form of verge, with diamond pallets, Huygens' spiral-spring balance wheel, spring-remontoire, and Harrison’s maintaining power with fusee. The grasshopper was the escapement he chose for H1, H2 and H3, as it was superior to the recoil and dead beat anchor, in the sense that it suffered no power loss due to friction. It therefore exhibited no variations in power loss due to friction. Of course, the regular flexing of the composing springs and weights would exchange some energy on every cycle, some of which is expelled from the escapement after each leg kicks free; but, unlike friction losses, this loss was, demonstrably, very constant. The remaining remontoire power was delivered directly to the oscillator, all of which allowed his spring-driven chronometers to exhibit very high accuracy.

IV HARRISON and the BOARD

Harrison used his grasshopper, the first friction-free escapement, quite similar to a detent escapement, as a component in his first three marine chronometers. This fascinating era of horology [9] involving the development of marine chronometers is recounted in ref. [10], but much of this text is obviously written from the point of view of Harrison's version of his dealings with the British Board of Longitude. Recently, an assessment of this book has been provided in ref. [11], which, with reference to works by Derek Howse [12] and others, illustrates some of the other sides to the story. More recently, the core of [10] has been recast into the form of heroic fairy-tales in children's storybooks [13]. David Landes [14] provides an extremely detailed history of clocks and watches, including the longitude problem, in all its various aspects. In particular, he includes the important marine chronometer research and development on the continent (Europe).

Nevil Maskelyne displayed fair judgment and finesse in many areas, and he was eminently qualified for his positions as Astronomer Royal and member of the Board of Longitude, which he held for many of these important years of development of the marine chronometer and lunar-distance method. By action, if not by actual intent, the board was to use the longitude prize as a carrot" offered to the competitors, yet moving it ahead as they advanced toward it. Even the prize rules were changed periodically. The board might be described as the world's first official research and development agency [10] although, similarly, certain of Huygens’ research was funded by the Dutch East Indies Company, a century earlier. Contrary to most popular accounts in articles and books, the British longitude prize was actually never won by anyone [15], and the Board of Longitude, while Thomas Young was secretary, voted its own termination, by persuasion of board member George Airy, in 1828 [16].

Harrison is portrayed as the only clockmaker pursuing marine chronometers at the time [10]. Recent television movies follow just about the same structure and restricted theme, in which the only researcher is Harrison, and the only country is England [17,18]. These are all simplified local stories about an important English inventor. Similarly, the Longitude symposium of 1993 (the tricenteniary of Harrison’s birthday) was mainly a celebration of Harrison’s contributions. While Harrison was certainly a critical player, whose impressive results (especially with H1) strongly encouraged many more horologists to design and make marine chronometers, he was far from the only one. Rather than a lone self-taught carpenter working in isolation, most of his technical results were clever adaptations and improvements on results of earlier and contemporary horologists, of which he was certainly aware. It is important to remember that the story of longitude, including the marine chronometer, cannot be told completely without including the important contributions and developments which occurred on the continent (Europe) [14]. Nearly all major concepts and methods originated on the continent, but came to fruition in England, mainly because of the Board of Longitude and associated research awards.

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