And this degree of precision was required under ever-changing conditions of temperature, humidity, rolling of the vessel, air pressure, and more. To measure longitude with any degree of certainty required a clock that was accurate to within a few seconds per day. One degree of longitude at the equator is about 60 nautical miles, or 70 'standard' mi (112 km). In one hour, it will turn 15 degrees, and it will turn one degree in four minutes. Other suggestions were to use clocks, if only they could be made sufficiently precise. Early suggestions were to use the Moon, the stars, or the position of Jupiter's satellites as clocks, but the practicality of making precise astronomical observations from the rolling deck of a ship proved insurmountable, and the positions of these bodies could not be predicted with a high degree of accuracy in that era. The longitude problem was recognized at the dawn of the Age of Discovery, in the early 1500s.

Eighty of those men died in the two weeks Centurion was delayed because her captain didn't know whether to turn east or west. By the time Centurion found port, 250 men had died. In 1741, another wrong guess led the HMS Centurion to spend two additional weeks at sea on a ship struck by scurvy. In 1707, a wrong guess led to the sinking of four British man-of-war and the loss of 2,000 men when their fleet ran aground on islands just off the coast of Britain. Longitude was guessed, based on the captain's estimate of ship's speed, currents, wind speed, and other factors, but the captains were often wrong. Longitude is much more difficult and, for centuries, proved nearly intractable.īecause sailors could not accurately determine their location east or west on a map, early navigation often consisted of sailing north or south to a specific latitude and then striking out east or west along that imaginary line until one's destination was reached. With but a smattering of math, this elevation can be turned into a latitude. As we travel away from the Equator, the Sun drops lower in the sky while the pole stars climb higher. In fact, latitude measurements require only a simple instrument that can measure the distance above the horizon the Sun appears to be at noon, or the distance above the horizon a given star is seen to be. Although the concepts of latitude and longitude have been with us for about two millennia (and we have been able to measure latitude for nearly that length of time), our ability to determine longitude is a recent accomplishment. This accessibility of accurate geographic information is unprecedented in human history.

Hikers carry GPS receivers in case they lose the trail, cars have GPS receivers to help navigate the streets of strange cities, and scientists use GPS to clock the uplift of Mount Everest. We live in a world that relies on GPS to tell us where we are at any time. In fact, as late as the 1980s, before completion of the Global Positioning System (GPS), most ships at sea knew their positions to within only a few kilometers or more, and many still located themselves by celestial observations.

It is difficult to remember that, until very recently, nobody on Earth knew where they were with this degree of precision. The entire surface of the Earth has been mapped photographically, gravitationally, and recently, with space-borne radar to an unprecedented level of detail. Today, for a few hundred dollars, virtually anyone can purchase a hand-held unit that, by detecting signals from an artificial constellation of satellites, will obligingly display one's latitude, longitude, and altitude with a precision of a few meters or less. This invention revolutionized travel by sea, with repercussions that lasted until the 1990s. This problem was finally solved by John Harrison (1693-1776), an Englishman, with his development of a highly precise clock called a chronometer. For centuries, determining one's longitude, that is, one's position east or west of Greenwich, was nearly impossible, leading to the loss of life, ships, and property. Locations on Earth are determined by a gridwork of lines, one set marking distance north or south of the equator and the other marking distance east and west of the Prime Meridian, running through Greenwich, England. The Invention of the Chronometer Overview

World cronometer update