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Determining latitude on land or at sea is a comparatively easy thing to do. It is simply related to the height in degrees of the sun above the horizon at midday. Determining longitude is much harder, because the earth's rotation continually changes the longitudinal position of a point on the earth's surface with respect to all celestial objects. Whereas latitude is defined absolutely with respect to the equator, longitude is only defined in a relative sense, with respect to some arbitrary reference location - the so-called prime meridian. Finding longitude reduces to the problem of measuring the difference between local time and the time at the prime meridian. On land, this can be done with reasonable accuracy by making extremely precise astronomical measurements of celestial landmarks and then comparing them to corresponding measurements that have been made at the prime meridian. At sea, though, the constant motion of a ship makes it almost impossible to make sufficiently accurate astronomical measurements. As a result, it was commonplace for early navigators to be uncertain of their position by a hundred or more miles during long sea voyages, which led to many maritime disasters.
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The challenge of improving astronomical measurements of longitude at sea attracted the efforts of Galileo and Cassini, among other celebrated scientists, but the problem remained intractable. By the beginning of the eighteenth century, navigating accurately east to west had emerged as the single most important challenge to international trade and military activity. In 1714, after a British naval fleet foundered on rocks off the coast of England because of uncertainties in its position, the British government posted a challenge with a prize of 20,000 pounds sterling for the first person to demonstrate a method of measuring longitude with an accuracy of one half degree (30 miles) or better at the end of a trans-Atlantic voyage. The size of the reward signaled its importance - in today's currency the prize would be worth nearly $5,000,000.
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The most influential contenders for this prize were astronomers, including the British royal astronomer Nevil Maskelyne. However, others recognized that a completely different approach could also work. Rather than use the positions of celestial objects as surrogate timekeepers, one could just carry a clock that kept the time at the prime meridian with high accuracy - a so-called marine chronometer. The problem, however, was that the necessary accuracy, even on land, was far beyond the capability of eighteenth century clocks. At sea it only got worse, as the rocking and pitching of a ship would completely disrupt the motion of a pendulum. Even on completely calm seas, changes in temperature and humidity would alter the performance of the clock to produce unacceptable errors.
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The following books document several landmark efforts to develop a means to measure longitude at sea and then demonstrate it during extended voyages. The pamphlet by John Harrison stands at the center of these efforts.
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[John Harrison], Principles of Mr Harrison's Time-keeper, London and Avignon 1767
Marquis de Courtanvaux, Journal du Voyage de M. le Marquis de Courtanvaux, sur la fregate l'Aurore, pour essayer par ordre de l'Academie, plusieurs instruments relatifs a la longitude, Paris 1768
Jean Richer, Observations astronomiques et physiques faites en l'Isle de Caienne, Paris 1679
GD Cassini, Divers ouvrages d'Astronomie par M Cassini, La Haye 1731
Ferdinand Berthoud, Essaie sur l'Horlogerie, Paris 1763
Leonard Sturm, Project de la resolution du fameux problem touchant la longitude sur mer, Nuremburg 1720
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