Longitude

Longitude is an angular measurement, typically given in degrees and symbolized by the Greek letter lambda (λ), that defines how far east or west a point lies on Earth or another celestial body. Imaginary semicircular lines called meridians, which run from pole to pole, connect all points sharing the same longitude. By international convention, the prime meridian—marking 0° longitude—passes near the Royal Observatory in Greenwich, southeast London. Locations east of this line have positive longitude values, while those to the west are negative.

Because Earth rotates, longitude and time are closely linked. A 15° difference in longitude corresponds to a one-hour shift in local time, as the Sun’s position in the sky changes. Determining longitude involves comparing local time with an absolute time reference. Historically, that reference might have come from a celestial event visible at two places (like a lunar eclipse) or from a time signal sent by telegraph or radio. The idea is simple, but finding a reliable method for doing this at sea took centuries and demanded the work of many great scientists.

A location’s north-south position along a meridian is given by latitude, which approximates the angle between the equatorial plane and the ground’s normal at that spot. Longitude is usually measured using the geodetic normal or the direction of gravity. Slight differences can occur between astronomical longitude and ordinary longitude due to vertical deflection—small variations in Earth’s gravitational field (similar to how astronomical latitude can differ).

**History**

Ancient Greek astronomers first developed the concept of longitude. In the 2nd century BC, Hipparchus used a spherical Earth model divided into 360°, with his prime meridian passing through Alexandria. He also proposed determining longitude by comparing the local times of a lunar eclipse at two different places, showing he understood the link between longitude and time. In the 2nd century AD, Claudius Ptolemy created a mapping system using curved parallels to reduce distortion, gathering data from Britain to the Middle East. He placed his prime meridian through the Canary Islands so all longitudes would be positive. Though his system was sound, his data were often poor, causing him to overestimate the Mediterranean’s length by about 70%.

After the Roman Empire fell, European interest in geography waned. Hindu and Muslim astronomers kept these ideas alive, adding new locations and improving Ptolemy’s data. For instance, al-Battānī used simultaneous observations of two lunar eclipses to find the longitude difference between Antakya and Raqqa with an error under 1°—considered the best possible with naked-eye observation and an astrolabe to measure a “clock star’s” altitude.

In the later Middle Ages, as travel increased and Arab scholarship reached Europe through Spain and North Africa, geography revived in the West. In the 12th century, astronomical tables for several European cities were prepared based on al-Zarqālī’s work in Toledo. The lunar eclipse of September 12, 1178, was used to establish longitude differences between Toledo, Marseilles, and Hereford. Christopher Columbus tried using lunar eclipses to find his longitude twice: first near Saona Island on September 14, 1494, and then in Jamaica on February 29, 1504, likely using astronomical tables. His results were off by 13° and 38° west, respectively. Between 1514 and 1627, Portuguese and Spanish measurements of longitude in the Americas and Asia had errors ranging from 2° to 25°.

The telescope, invented in the early 1600s, was initially just for observation but became an accurate measuring tool over the next fifty years. Christiaan Huygens patented the pendulum clock in 1657, improving accuracy about thirty times over earlier mechanical clocks. These two inventions revolutionized observational astronomy and cartography. On land, from the telescope and pendulum clock’s development until the mid-1700s, the number of places with reasonably accurate longitudes grew steadily—often within a degree, and almost always within 2° to 3°. By the 1720s, errors were consistently under 1°. At sea, however, the situation was far worse. Two problems proved stubborn: navigators needed immediate results, and the marine environment made accurate observations difficult—ocean swells made pendulum clocks unreliable.

**The Chronometer**

To solve navigation problems, several European maritime powers offered prizes for a method to determine longitude at sea. The most famous was Britain’s Longitude Act of 1714, which offered rewards for solutions accurate to within 1° and 0.5°. Rewards were eventually given for two approaches: the lunar distance method, made practical by Tobias Mayer’s tables and developed into a nautical almanac by Astronomer Royal Nevil Maskelyne; and the chronometers built by Yorkshire carpenter and clockmaker John Harrison.