Chronometer
When Christopher Columbus arrived in America in 1492, he thought he had reached the shores of Asia. He probably would have been able to avoid the error and anticipate the achievement if he only had an accurate timepiece and an accurate star chart. With the two combined, he could have reliably charted his course. Sailors had long been able to determine latitude (north-south position). By using a quadrant to measure the altitude of the sun or the polestar--the brightest star that appears nearest to either celestial pole at a certain latitude. But latitude is only half the data needed. Longitude and latitude intersect to pinpoint position. While on land longitude could be measured and checked against reference points, at sea navigators had to relate the positions of the stars and other objects in the sky with time to determine where they were on the ocean. The necessary celestial measurements were made and eventually published in the early 1700s by such astronomers as John Flamsteed (1646-1719), England's astronomer royal, and the Frenchman Nicolas Louis de Lacaille (1713-1762), among others. Yet since early clocks did not work accurately at sea, navigation remained treacherous. As far back as 1533, the German geographer Rainer (Regnier) Gemma Frisius (1508-1555) suggested that a navigator could determine his location with an accurate timepiece. In 1656, Christiaan Huygens (1629-1695) invented a clock that used a weight-and-pendulum system. But a clock that relied upon gravity as its driving force could not be very portable and certainly would not work on a ship being tossed about at sea. Development of an accurate timepiece for navigators, or marine chronometer, was of such importance to the seafaring English that the British Parliament posted a 20,000 pound award for whomever could invent one of sufficient accuracy. A carpenter's son and a self-taught carpenter, John Harrison (1693-1776) invented a chronometer that was successfully employed on a voyage to Lisbon in 1736. The English Board of Longitude did not give Harrison the recognition due him, however, until he developed his fourth chronometer, which proved to be accurate to within five seconds on a voyage to Jamaica in 1761-62. Still, it took a fifth improvement and a personal appeal to King George III before Harrison received his prize.
Also in 1761, another Englishman, Nevil Maskelyne (1732-1811), drew upon the work of Lacaille and others to determine his location on a voyage to St. Helena by relating the position of the moon with lunar navigation tables. These tables were later distributed to mariners. But this method was too complex to become a substitute for the chronometer. The Frenchman Pierre Le Roy (1717-1785) created a chronometer in 1763 that used an adjustable gyration radius to compensate for temperature changes, which affect the accuracy of timepieces. Appointed Horologer de la Marine in 1764, Ferdinand Berthoud (1727-1807) improved on Harrison's and Le Roy's devices and wrote 10 volumes on the subject of marine chronometers. In the late 1700s, John Arnold (1736?-1799) improved and simplified the chronometer by combining a helical balance spring with a pivoted detent escapement and was able to achieve amazing accuracy. About the same time, Thomas Mudge (1717-1794) devised a detached leaver escapement which allowed for uniform movement independent of external influences. His design became the basis for watch making during the nineteenth century. Also in the late eighteenth century, Thomas Earnshaw (1749-1829) constructed a spring-dent escapement and a temperature-compensated or "bimetallic" balance that became the pattern of marine chronometer construction for more than a century. Despite its increasing accuracy, the chronometer could not be truly accurate until a standard system of marking off meridians of longitude was set. This took place in 1884, when an international convention agreed that the observatory at Greenwich, England, should become the Prime Meridian, or starting place, with a longitude of 0 degrees. All timepieces originally were referred to as chronometers. As time passed the term acquired its current meaning: any precise time-keeping instrument used in navigation. The need for chronometers became less important in the 1920s when broadcasting time by radio signals became widespread. Later satellite locating systems further lessened their importance, though many navigators keep them on board for backup use. By the late twentieth century, quartz digital chronometers were gradually replacing their mechanical ancestors. While at sea, beyond the sight of land, a marine navigator will take star sightings through a sextant. With a chronometer to time the readings, the ship's location can be calculated and charted.
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