Longitude

Lore & Background

The concept of longitude was first developed by ancient Greek astronomers. Hipparchus (2nd century BC) used a coordinate system that assumed a spherical Earth, divided into 360°, with his prime meridian passing through Alexandria. He proposed determining longitude by comparing local time of a lunar eclipse at two places.

Claudius Ptolemy (2nd century AD) developed a mapping system using curved parallels and used a prime meridian through the Canary Islands. After the fall of the Roman Empire, Hindu and Muslim astronomers continued these ideas; al-Battānī used simultaneous observations of two lunar eclipses to determine longitude between Antakya and Raqqa with an error of less than 1°. In the 12th century, the lunar eclipse of September 12, 1178 was used to establish longitude differences between Toledo, Marseilles, and Hereford.

The Chronometer

Christopher Columbus made two attempts to use lunar eclipses to discover his longitude, with large errors of 13° and 38° W. The telescope and pendulum clock (patented by Christiaan Huygens in 1657) revolutionized observational astronomy and cartography. The British Longitude Act of 1714 offered rewards for solutions within 1° and 0.5°. Rewards were given for lunar distances (using tables of Tobias Mayer developed into a nautical almanac by Nevil Maskelyne) and for chronometers developed by John Harrison. Harrison built five chronometers over more than three decades, supported by the Board of Longitude, and finally received an additional payment in 1773 after Parliament intervened.

The first working telegraphs were established in Britain in 1839 and in the US in 1844, and were quickly used to transmit time signals for longitude determination. The United States Coast Survey (renamed the United States Coast and Geodetic Survey in 1878) established chains of mapped locations through Central and South America, the West Indies, and as far as Japan and China in 1874–90. Wireless time signals for ships were transmitted from the Eiffel Tower in Paris from 1910.

The Geometry of East-West Position

Longitude is the angular coordinate that pinpoints where a location sits along the east-west axis of a sphere, whether that sphere is Earth or another celestial body. It is conventionally written in degrees and marked with the Greek letter lambda. Imaginary semicircular arcs called meridians stretch from pole to pole, linking every point that shares the same longitude value. The zero-degree reference, the prime meridian, is fixed by international convention near the Royal Observatory in Greenwich, south-east London.

Values east of that line are positive; values west are negative. A location's north-south placement along any given meridian is described separately by latitude, roughly the angle between the equatorial plane and the local vertical. In practice, longitude is usually referenced to the geodetic normal or the direction of gravity, though small gravitational irregularities can cause the astronomical longitude to deviate slightly from the ordinary geodetic value. Together, these two coordinates form the backbone of every map, navigation chart, and satellite positioning system we rely on today.

Longitude, Time, and the Sun

Because the Earth rotates, a point's longitude is intimately tied to its local solar time. A shift of fifteen degrees in longitude translates into a one-hour difference in when the Sun crosses the local meridian, a direct consequence of each location's changing angle relative to the Sun throughout a day. This relationship means that if you can compare your local solar time against a known absolute reference, you can back-calculate your east-west position.

In earlier centuries, that absolute reference might come from a celestial event visible from two separated places simultaneously, such as a lunar eclipse. In later eras, the reference could be a time signal carried by telegraph or radio. The underlying principle is deceptively simple, yet turning it into a dependable, repeatable method of finding one's longitude at sea proved extraordinarily difficult, demanding centuries of ingenuity and the work of some of history's most brilliant scientific minds.

From Alexandria to the Age of Exploration

The intellectual roots of longitude stretch back to ancient Greece. He also sketched a method for finding longitude by comparing the local time of a lunar eclipse at two sites, revealing an early grasp of the longitude-time link. Claudius Ptolemy, two centuries later, refined mapping with curved parallels and gathered positional data from Britain to the Middle East, choosing a prime meridian through the Canary Islands so all values stayed positive. After Rome's collapse, European interest waned, but Hindu and Muslim astronomers kept the tradition alive.

Al-Battānī, for instance, used simultaneous naked-eye observations of two lunar eclipses to measure the longitude gap between Antakya and Raqqa to within one degree. In the later Middle Ages, European scholars revived the discipline through contact with Arab learning in Spain and North Africa. Columbus himself attempted lunar-eclipse longitude fixes during his second and fourth voyages, though his results carried errors of 13 and 38 degrees respectively.

The Clock That Solved the Sea

For centuries, determining longitude on open water remained the great unsolved problem of navigation. At sea, however, ocean swell made precise observations far harder, and pendulum clocks simply could not keep reliable time in a rolling ship. Two approaches ultimately earned recognition: the lunar-distance method, made practical by Tobias Mayer's tables and Nevil Maskelyne's nautical almanac, and the marine chronometers crafted by Yorkshire carpenter and clockmaker John Harrison.

Reader's Guide

Because of the Earth's rotation, a difference of 15° longitude corresponds to a one-hour difference in local time. Comparing local time to an absolute measure of time allows longitude to be determined.

The absolute time might be obtained from a celestial event visible from both locations, such as a lunar eclipse, or from a time signal transmitted by telegraph or radio. A location's north-south position along a meridian is given by its latitude, which is approximately the angle between the equatorial plane and the normal from the ground at that location. Longitude is generally given using the geodetic normal or the gravity direction; astronomical longitude can differ slightly because of vertical deflection.