Name: Prime Meridian
Address: Royal Observatory, Blackheath Ave, London SE10 8XJ, UK
Telephone: 020 8312 6608
Rating: 4.5

Historic meridian line demarcating the earth's western & eastern hemispheres, plus a small museum.

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The Prime Meridian is located in Greenwich, London, and is officially known as the International Meridian.

Location, Location, Location

The Prime Meridian marks the line of 0° longitude, the starting point for measuring longitude around the world.

Starting Point

The Prime Meridian is based on the location of the Airy transit circle telescope at the Royal Observatory, Greenwich, installed by Astronomer Royal George Biddell Airy.

Astronomer Royal

The Greenwich Meridian was internationally adopted as the Prime Meridian in 1884 at the International Meridian Conference.

International Agreement

The Prime Meridian divides the Eastern and Western Hemispheres of the Earth.

Hemisphere Division

The Prime Meridian is the basis for Greenwich Mean Time (GMT), the time standard by which the world was historically regulated (now largely superseded by Coordinated Universal Time (UTC)).

Greenwich Mean Time

Visitors can stand with one foot in the Eastern Hemisphere and the other in the Western Hemisphere by straddling the Prime Meridian line at the Royal Observatory.

Straddling the Line

Prime Meridian

The Prime Meridian, a line of zero degrees longitude, finds its terrestrial anchor at the Royal Observatory in Greenwich, England. More specifically, it's not the grand building itself, but rather the Airy Transit Circle telescope housed within that defines the meridian's location. This instrument, designed by Sir George Biddell Airy, the seventh Astronomer Royal, was crucial for establishing accurate timekeeping and navigational standards. Interestingly, the modern Prime Meridian, used in GPS and mapping, is not exactly aligned with the original Airy Transit Circle. Due to advancements in technology and a shift to satellite-based measurements, the "modern" Prime Meridian is located approximately 102 meters east of the original. This subtle discrepancy highlights the evolving nature of scientific measurement and the challenges of defining a fixed point on a dynamic planet. While many visit to straddle the symbolic line, few realize they are, in reality, standing slightly west of the true, technologically defined, zero longitude.

Introduction

The Greenwich Meridian's historical evolution is intrinsically linked to the Prime Meridian in England, United Kingdom, specifically at the Royal Observatory, Greenwich. Initially, various countries established their own prime meridians, often running through their national observatories. England's meridian, passing through Greenwich, gained prominence over time. The first Greenwich meridian, named for John Flamsteed, was established in 1676, followed by three subsequent meridians as newer instruments were introduced.

In 1851, Sir George Biddell Airy established the fourth Greenwich meridian, which ran through the Airy Transit Circle, an instrument used for precise astronomical observations. This meridian became the internationally recognized prime meridian in 1884, chosen at the International Meridian Conference in Washington, D.C., solidifying Greenwich's role in global timekeeping and navigation.

However, the modern standard, the IERS Reference Meridian, superseded the Greenwich meridian in 1984. It is based on the Greenwich meridian but lies approximately 102 meters to the east. Despite this shift, the historical Greenwich meridian remains significant. The Royal Observatory features a marker symbolizing the prime meridian's location. The adoption of the Greenwich Meridian facilitated the standardization of longitude and time, leading to the establishment of Greenwich Mean Time (GMT). While GMT has been replaced by Coordinated Universal Time (UTC) as the international time standard, it continues to be used as a time zone in several countries.

Greenwich Meridian's Historical Evolution

The IERS Reference Meridian (IRM) serves as the modern prime meridian, differing slightly from the historical Greenwich Meridian defined by the Airy Transit Circle at the Royal Observatory. While the Greenwich meridian was the international standard from 1884 to 1974, the IRM, maintained by the International Earth Rotation and Reference Systems Service, has since become the standard. The IRM's adoption stems from advancements in technology, transitioning from optical instruments to techniques like lunar laser ranging and very-long-baseline interferometry. This shift ensures the meridian's plane passes through the Earth's center of mass, unlike the Airy transit, which is affected by local vertical deflection.

The IRM lies approximately 5.3 arcseconds east of Airy's transit circle; at Greenwich's latitude, this equates to a 102-meter difference. This offset is primarily due to the IRM's geodetic-based longitude, referencing the Earth's center of figure, contrasting with the astronomical longitude used for the Airy transit circle. Despite this difference, the IRM remains closely tied to Greenwich, with the historic observatory serving as a crucial reference point. The European portion of the Eurasian plate, where the Royal Observatory is located, moves northeast about 2.5 cm per year relative to the IERS system. The GPS receivers show that the marking strip for the prime meridian at Greenwich is not exactly at zero longitude but at approximately 5.3 seconds of arc to the west of the meridian, meaning that the meridian appears to be 102 metres east.

IERS Reference Meridian

The Greenwich Meridian in England serves as the prime meridian, the 0° longitude line, crucial for global longitude and timekeeping standards. Historically, various nations used different meridians, complicating navigation and time synchronization, with France using the Paris meridian and others choosing their capital cities. The standardization became critical with the advent of transcontinental railroads, which demanded consistent timekeeping to avoid scheduling errors across different local time standards.

In 1884, an international agreement designated the Greenwich Meridian as the official prime meridian, resolving the previous ambiguities. This decision facilitated the establishment of Coordinated Universal Time (UTC), the standard for time zones worldwide, which are measured relative to Greenwich. The adoption of Greenwich Mean Time (GMT) across Great Britain in the 19th century by railway companies and later legally, marked a significant step toward standardized time.

British mariners relied on GMT to calculate longitude using chronometers set to Greenwich time, linking time difference to longitudinal distance. Though the physical marking of the prime meridian at the Royal Observatory, Greenwich, has a slight offset from zero longitude due to localized gravity effects, it remains the symbolic and historical origin of global time and longitude measurements. Today, while UTC has superseded GMT as the international civil time standard, Greenwich's legacy endures as the foundation of modern timekeeping and navigation.

Longitude and Timekeeping

The adoption of the Greenwich Meridian as the international standard prime meridian in 1884 stemmed from a growing need for unified longitude and timekeeping, particularly driven by advancements in communication and transportation during the 19th century. Before this agreement, various maritime countries used their own prime meridians, often running through national observatories. The Royal Observatory at Greenwich had already established its prime meridian for Great Britain by 1851.

The International Meridian Conference in Washington, D.C., was convened to address this issue, with the Greenwich Meridian ultimately chosen due to the observatory's reputation for reliable navigational data and its existing widespread informal recognition. While 22 out of 25 countries voted in favor of Greenwich, France abstained and continued using the Paris meridian until 1911. The selection of Greenwich was also influenced by its satisfaction of scientific criteria, including possessing a first-rate astronomical observatory linked to other observatories and a network of triangles in the surrounding country.

The adoption of the Greenwich Meridian also facilitated the establishment of Greenwich Mean Time (GMT) as the world's time standard, leading to the international 24-hour time zone system. Although GMT was later replaced by Coordinated Universal Time (UTC) as the international time standard in 1972, the legacy of Greenwich as the prime meridian continues. The modern prime meridian, the IERS Reference Meridian, is based on the Greenwich meridian, but differs slightly from it, lying approximately 102 meters to the east.

Prime Meridian

Did You Know?

Location, Location, Location

Starting Point

Astronomer Royal

International Agreement

Highlights from Reviews

Location