Longitude

Longitude is a geographic coordinate used to specify an east-west position on Earth or other celestial bodies. It is expressed as an angular measurement, usually in degrees, and is often denoted by the Greek letter lambda (λ).

To visualize this, imagine imaginary semicircular lines called meridians that run from the North Pole to the South Pole. These meridians connect all points sharing the same longitude. The starting point for this measurement is the prime meridian, which is set at 0°. By international convention, the International Reference Meridian passes near the Royal Observatory in Greenwich in London. Locations to the east of this line are assigned positive longitudes, while locations to the west are assigned negative ones.

There is a deep scientific connection between longitude and the measurement of time. This connection exists because the Earth rotates on its axis. As the planet turns, different parts of the surface face the Sun at different times. Because of this rotation, a difference of 15° in longitude corresponds exactly to a one-hour difference in local time. To find a specific longitude, a person must compare their local time to an absolute measure of time. Historically, this absolute time could be found by watching a celestial event, such as a lunar eclipse, that is visible from two different locations. In more modern times, time signals could be received via telegraph or radio.

The study of longitude began with ancient Greek astronomers. In the 2nd century BC, Hipparchus developed a coordinate system based on a spherical Earth. He divided the circle into 360°, a standard still used today. He also suggested that comparing the local time of a lunar eclipse at two different places could reveal longitude. Later, in the 2nd century AD, Claudius Ptolemy created a mapping system using curved parallels to reduce distortion. Ptolemy used a prime meridian through the Canary Islands so that all longitude values remained positive. While his mapping system was sound, his data was often inaccurate. This caused him to overestimate the length of the Mediterranean by about 70%.

After the fall of the Roman Empire, interest in geography slowed in Europe, but Hindu and Muslim astronomers continued the work. They added many new locations and improved upon older data. For example, the astronomer al-Battānī used simultaneous observations of two lunar eclipses to find the longitude difference between Antakya and Raqqa. He achieved an error of less than 1°, which was the best possible using the tools of his time. He used an astrolabe to measure the altitude of a "clock star" to determine local time. By the 12th century, European interest revived as travel increased. Astronomers began using lunar eclipses to find the longitude differences between cities like Toledo, Marseilles, and Hereford.

Finding longitude at sea proved much harder than finding it on land. On land, errors were often less than 1° by the 1720s. However, sailors faced two massive problems: they needed immediate results, and the ocean environment was unstable. In a heavy ocean swell, it is very difficult to make precise observations. Furthermore, the pendulum clocks invented by Christiaan Huygens in 1657 did not work well on moving ships. To solve this, the British Parliament passed the Longitude Act in 1714. This law offered large financial rewards for methods that could determine longitude within 1° or 0.5°.

Two main solutions eventually emerged to meet these challenges. The first involved lunar distances, which used mathematical tables to compare the position of the Moon to stars. The second involved the development of the marine chronometer.

John Harrison, a Yorkshire carpenter and clock-maker, spent over thirty years building five different chronometers. These were highly accurate mechanical clocks that could keep time despite the motion of a ship. Harrison fought for many years to receive the top reward of £20,000, finally receiving an extra payment in 1773. While chronometers were initially very expensive, they became cheaper in the early 19th century and eventually replaced lunar distance methods around 1850.

As technology advanced, new ways to transmit time emerged. In 1839 and 1844, the first telegraphs were established in Britain and the United States. People realized they could use the telegraph to send time signals, which made determining longitude much easier. In the early 20th century, wireless telegraphy, or radio, allowed ships to receive time signals while at sea. For instance, signals were sent from Halifax, Nova Scotia, starting in 1907, and from the Eiffel Tower in Paris in 1910. After World War II, ships used radio navigation systems based on fixed beacons. These systems remained the standard for commercial shipping until they were replaced by GPS in the early 1990s.