Prime meridian
Zero longitude line dividing Earth into Eastern and Western Hemispheres.
A prime meridian is a meridian (a line of longitude) in a geographic coordinate system at which longitude is defined to be 0°. On a spheroid, a prime meridian and its anti-meridian form a great ellipse, dividing the body into the Eastern and Western Hemispheres. Unlike the equator, the prime meridian is astronomically arbitrary; Earth's current international standard is the IERS Reference Meridian, derived from but differing slightly from the Greenwich Meridian.
- type
- geographic coordinate system reference line
- notable_historic_meridians
- Paris, Berlin, Copenhagen, Ujjain, El Hierro
Lore & Background
The notion of longitude was developed by the Greek Eratosthenes (c. Ptolemy (c. In the 4th century CE, the Surya Siddhanta described a prime meridian passing through Avanti (Ujjain) and Rohitaka (Rohtak) in India. By the 18th century, most European countries used their own capital's meridian (e.g., Paris, Berlin, Copenhagen, Greenwich).
Reader's Guide
The prime meridian is fundamental to global navigation, timekeeping, and mapping. The development of accurate star charts by John Flamsteed and the marine chronometer by John Harrison improved longitude determination at sea. The prime meridian's definition affects all geographic coordinate systems, and its history reflects the interplay of astronomy, politics, and technology.
Did You Know?
- Ptolemy's prime meridian was based on the Fortunate Isles, corresponding to about 20°W today.
- The Surya Siddhanta (4th century CE) described a prime meridian passing through Ujjain and Rohtak in India.
- The IERS Reference Meridian passes through Earth's center of mass, unlike the Greenwich Meridian affected by local gravity.
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