# Longitude

Longitude is a geographical measurement that describes the position east or west of the prime meridian for a given location. On maps, longitude is illustrated by vertical lines called meridians of longitude. Unlike parallel latitudinal lines, merid­ians of longitude all intersect at the north and south poles. Using longitude measurements for east and west along with latitude measurements for north and south, the location of any place on the can be precisely described. While the equator provided the logical starting point for latitudinal measurements, there was no such obvi­ous place for measuring longitude. Eventually the Greenwich Meridian—on which the original British Royal Observatory was located—was internationally accepted as the prime meridian. In relation to time, longitude corresponds directly to the local time of any location and is the basis for time zone divisions.

The concept of longitude originated in ancient Greece, but the quest for accurate measurement of longitude continued until modern times. While many methods were proposed and attempted for calculating longitude, the most significant devel­opment came with the production of the first highly reliable mechanical clocks. These enabled travelers to compare the time of a known location with the observed local time in order to calculate their longitude on land or at sea. Modern satellite now enables precise latitude and longi­tude to be calculated using the global positioning system (GPS).

Longitude is closely related to time, because the local time of any place on the earth is directly rela­tive to its longitude. Each of the earth’s 24 time zones represents a section 15° wide in longitude, the distance over which local time changes by one hour (1/24th of 360°). The relationship is also his­torical; the need for an accurate way of finding longitude, especially while navigating at sea, was a primary driving force in the advancement of time­keeping technology.

Longitude was used at least as early as 300 BCE in ancient Greece. In the 2nd century CE, Claudius Ptolemy published Geography, one of the earliest developed mathematical works concerning latitude and longitude, in which he discussed an earlier theory of Hipparchus for figuring longitude. Recognizing that longitude could be calculated by observing the same event in two places and com­paring the local time of each, Hipparchus proposed that lunar eclipses could be used for this purpose. The soundness of this theory was outweighed by the fact that sundials, the only available timekeep­ing devices, would not work well when observing something that occurs only at night.

After centuries with no scientific advancements, in 1514 Johann Werner surmised a new theory for finding longitude: the lunar distance method. Werner proposed that longitude could be found by precise comparisons of the moon’s apparent posi­tion relative to the stars. This method had its prob­lems, but with necessary observation data it had potential. Precisely for the gathering of such data, the Royal Observatory was founded in Greenwich, England, in 1676.

In an age that depended upon maritime travel, the need for more accurate navigation became urgent, prompting several European countries to offer generous rewards for anyone who made breakthrough discoveries toward calculating longi­tude. John Harrison received the largest of such rewards ever given by Great Britain in 1770 CE for his invention of a highly accurate portable time­keeper called a chronometer. The chronometer enabled mariners to use a simpler, more accurate method of finding longitude. A traveler could com­pare the observed local time at noon with the time of a known location kept on a chronometer; the difference in time told the difference in longitude between the two places. With so much data already available in the almanacs published by the Greenwich Observatory, it was the natural loca­tion to which chronometers were set, and the lon­gitude at which it was located later became the prime meridian for the entire world. The chronom­eter method for identifying longitude remained standard until more advanced technology replaced it in the 20th century—first radio based systems, then GPS satellites.

L. Bean

Observatories; Time Zones