Longitude

Lore & Background

The concept of longitude was first developed by ancient Greek astronomers. Hipparchus 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 a prime meridian through the Canary Islands, though his data often led to errors such as a 70% overestimate of the Mediterranean's length. After the fall of the Roman Empire, Hindu and Muslim astronomers improved on Ptolemy's data; al-Battānī used simultaneous lunar eclipse observations to determine longitude between Antakya and Raqqa with an error of about 1° 30' to 2°.

Reader's Guide

The determination of longitude has been a critical problem in navigation and geography. The relationship between longitude and time—where 15° corresponds to one hour—underpins all methods. Early methods relied on lunar eclipses, but errors were large, as seen in Christopher Columbus's attempts. The invention of the telescope and pendulum clock in the 17th century improved land-based accuracy, but at sea the challenges remained. Two successful methods emerged: lunar distances, made practical by Tobias Mayer's tables and Nevil Maskelyne's nautical almanac, and John Harrison's chronometers. The telegraph in the 19th century allowed time signals for land-based longitude determination, and wireless telegraphy in the early 20th century enabled ships to check chronometers. Radio navigation systems after World War II became standard until replaced by GPS in the early 1990s.

Did You Know?

• A difference of 15° longitude corresponds to a one-hour difference in local time.
• Hipparchus proposed determining longitude by comparing local time of a lunar eclipse at two places.
• John Harrison built four distinct marine timekeepers (H1–H4) over more than three decades, supported by the Board of Longitude.