Finding east-west position took centuries and cost thousands
Longitude fixes east-west position as an angular measurement in degrees. A 15° difference equals one hour of local time. Hipparchus grasped the link in the 2nd century BC, yet reliable sea navigation waited until Harrison's chronometers and Mayer's lunar tables won British Longitude Act prizes.
Longitude specifies east-west position on Earth's surface, usually in degrees denoted by lambda. Meridians are semicircles from pole to pole connecting equal longitudes. The prime meridian, conventionally the International Reference Meridian through Greenwich, defines 0°; positive values lie east, negative west. Latitude on the same meridian gives a full position.
Earth's rotation ties longitude to time: 15° corresponds to one hour. Ancient Greek astronomers developed the concept. Hipparchus divided a spherical Earth into 360° with a prime meridian through Alexandria. Ptolemy used the Canary Islands so all longitudes read positive, though his Mediterranean length erred by about 70%.
At sea, accurate longitude remained elusive for centuries. Huygens patented the pendulum clock in 1657 and telescopes became precision instruments, so land surveys often got within a degree, but a pitching deck defeated both tools. Galileo's idea of reading time from Jupiter's moons shared the problem, since it required a steady telescope. Britain's 1714 Longitude Act offered rewards up to £20,000. John Harrison built five chronometers over decades; Tobias Mayer's lunar-distance tables, edited by Nevil Maskelyne, offered a rival method. Lunar distances dominated after 1790; chronometers replaced them after 1850.
Telegraphs from 1839 let observers compare times across continents. The United States Coast Survey mapped chains through the Americas and as far as Japan and China between 1874 and 1890. Wireless signals from Halifax in 1907 and the Eiffel Tower in 1910 let ships reset chronometers. Radio navigation after World War II served shipping until GPS in the early 1990s. Every method except magnetic declination compares local time at two places and multiplies the hour difference by fifteen. Columbus tried the older eclipse trick twice, timing lunar eclipses from Saona Island on 14 September 1494 and from Jamaica on 29 February 1504.
Source: Longitude
