John Harrison (1693-1776), born in Foulby, England, became one of the world’s most renowned horologists. He won the Board of Longitude Prize for developing a chronometer that could be used aboard ship to measure longitude to within 0.5 degree at the end of a voyage to the West Indies. About 1720 he had designed a timepiece that included a compensating apparatus by using different metals for correcting errors due to variations in the weather. Scientifically speaking, Harrison invented a timepiece that allowed for temperature changes or distortion. The first chronometer, which weighed 65 pounds, was completed and submitted to the Board in 1735 and was tested aboard ship the following year. The accuracy of the chronometer was outstanding, but like many inventions it had its detractors. He then built three more; the fourth, in 1761, more than met the standard for the prize, as did the first one. But it wasn’t until 1773 that he was fully compen­sated by the Board.

Up to the early 18th century, ships and their cargoes, along with the mariners, were at extreme risk between ports of call because of not knowing their exact location. After all, an hourglass is not exactly the best timepiece for determining time at sea. John Harrison might well be called the “father” of time at sea, as his invention of an accurate time­piece was the final link in being able to determine longitude, something Galileo thought would be the most precise method for determining an east-west position. In 1730, John Hadley in England, and Thomas Godfrey in America, working indepen­dently, perfected the sextant, which accurately found latitude by “shooting” the sun at noon. Latitude, which is part of the earth’s coordinate system for measuring relative location, had been rather easily observed and measured for centuries by determining the sun’s angle above the horizon, early on using a cross-staff or back-staff. On a globe, latitude is shown as east-west lines that mea­sure distance or position north-south, while longi­tude is shown as north-south lines that measure distance or position east-west. The earth makes a full revolution on its axis from west to east every 24 hours, or 15° of longitude each hour, the equiv­alent of one modern-day time zone.

Knowing one’s exact longitude at sea is impor­tant for determining a ship’s position with respect to land. Shipwrecks due to position miscalcula­tions were so common that a Board of Longitude was organized in England in 1714 for the purpose of awarding the sum of £20,000 to anyone who could develop a method for accurately measuring longitude. John Harrison’s contribution to mea­suring this accurately was a timepiece, known as a chronometer, that could be used at sea with a fair amount of accuracy. The timepiece was tested on a voyage to Jamaica with his son William Harrison on board, in 1761-1762, and determined longi­tude to within 18 nautical miles. Later, in 1764, the chronometer was tested again during a voyage to Barbados with his son on board. The timepiece performed brilliantly and well within the standard prescribed by the Board of Longitude. The fifth chronometer was used by Captain James Cook during his journey across the Pacific Ocean in 1776, although Cook was killed by Hawai’ians before the voyage was completed.

Richard A. Stephenson

See also Astrolabes; Hourglass; Latitude; Time,

Measurements of; Timepieces

Further Readings

Bowditch, N. (1966). American practical navigator.

Washington, DC: Government Printing Office.

(Original work published 1802)

Christopherson, R. W. (2004). Elemental geosystems

(4th ed.). Upper Saddle River, NJ: Prentice Hall. Dunlap, G. D., & Shufeldt, H. H. (1972). Dutton’s

navigation and piloting. Annapolis, MD: Naval Institute Press.