Why Greenwich Is the Prime Meridian

The /learn/the-prime-meridian pillar covers what the prime meridian is; this article covers why it is at Greenwich. Unlike the equator, which is set by Earth's rotation and would be the same regardless of which culture defined the coordinate system, the prime meridian is a free choice. Any meridian could serve as 0° with no loss of geometric generality. The choice that was made in 1884, and that has been quietly ratified every time a coordinate system is implemented since, was driven by a specific set of practical and historical pressures that all pointed to Greenwich.

This article covers the pre-1884 landscape of competing national prime meridians, the candidates considered at the conference, the practical case that won, France's 27-year holdout (and the curious legal language that kept the holdout alive even after formal adoption), and the modern IERS Reference Meridian that has slightly drifted from the original Airy Transit line.

The pre-1884 patchwork

Through most of the 19th century, national prime meridians varied widely. Per the Royal Museums Greenwich archives, the major national references included:

• Greenwich (UK): used by Britain since the late 17th century.
• Paris (France): used by France since the 17th century, anchored on the Paris Observatory on the rue Cassini.
• Cadiz then Madrid (Spain): the Spanish Royal Observatory.
• Pulkovo (Russia): the imperial Russian observatory near St. Petersburg, established in 1839.
• Washington (US): the US Naval Observatory in Washington, DC, used for federal mapping from 1850.
• Amsterdam (Netherlands).
• Ferro / El Hierro (Canary Islands): conventionally 20° west of Paris by a 1634 French royal decree, used on continental European maps that wanted to give every European longitude as a positive number.

Each meridian had its proponents and its body of charts. A given sea route between, say, Le Havre and Boston might be charted using the Paris meridian on French charts and the Greenwich meridian on British charts — and the captain crossing the Atlantic with both sets of charts in his cabin had to convert continuously, adding or subtracting the ~2°20′ that separated Paris from Greenwich.

The patchwork was a problem for navigation and a problem for telegraphic coordination. Standardisation was already overdue when the 1884 conference was convened.

The 1884 conference candidates

The 1884 International Meridian Conference proceedings record substantial debate over candidate meridians. The main candidates considered:

Greenwich. Supported by Britain, the United States (despite the US Naval Observatory's own Washington meridian), and most of the British Commonwealth attendees. The practical case rested on existing chart investment.

Paris. Supported by France and (initially) by some Latin American delegations. The case was scientific: the Paris Observatory had a long tradition of astronomical and geodetic work. The Paris meridian was as well-defined as Greenwich.

A “neutral” meridian. France in particular pushed for a meridian unconnected to any national observatory, proposing candidates that crossed only ocean — the Bering Strait was floated, as was a mid-Atlantic position. The argument was that a neutral meridian would avoid giving any nation symbolic preference. The counter-argument was that any neutral meridian would require all nations to convert their charts, where Greenwich would require only the conversions of non-British, non-American charts.

Other observatories. Washington (US — interestingly, the US did not push its own meridian), Berlin/Potsdam, Pulkovo, and a few others were named in the discussions but did not receive serious support.

The Italian delegation proposed Jerusalem as a symbolic choice referencing the historical role of the Holy Land in cartography; the proposal did not gain traction.

The practical case

The decisive argument was made by US delegate Cleveland Abbe (meteorologist), British delegate Sir Frederick Evans (hydrographer of the Royal Navy), and several others: the world's charts already overwhelmingly used Greenwich.

Specific figures cited at the conference: about 72% of world shipping tonnage was using Greenwich-referenced charts; the British Admiralty alone published or licensed charts covering the great majority of the world's navigable waters; the Nautical Almanac published since 1767 from the Royal Observatory provided the authoritative astronomical data used by navigators worldwide. The Royal Museums Greenwich records the Almanac's continuous publication and its long adoption by foreign navies, including the US Navy, the German Navy, and most Latin American merchant fleets.

Switching the prime meridian to Paris would have invalidated all those charts and required a generation of navigators to be retrained. Switching to a neutral mid-ocean meridian would have required both the British and the French to convert. Switching to Greenwich required only the French and a few smaller players to convert. The arithmetic of practical adoption favoured Greenwich heavily.

The vote on 13 October 1884 was 22 in favour, 1 against (the Dominican Republic / San Domingo), 2 abstentions (France and Brazil). Per the /learn/the-1884-international-meridian-conference support, the Dominican delegate offered a complex objection about American hegemony; the French and Brazilian abstentions reflected political reservations about the British meridian.

France's 27-year holdout

France formally rejected the conference decision. The Paris meridian remained the legal reference for French civil time, French maps, and French maritime charts until 1911.

The 1911 French legislation was unusual. It defined French legal time as “Paris Mean Time retarded by 9 minutes 21 seconds”. Mathematically, this is exactly Greenwich Mean Time — the longitudinal difference between Paris (2°20′14″ E) and Greenwich corresponds to a 9-minute, 21-second time difference. But the legislation avoided explicitly naming the British meridian, preserving French national prestige by phrasing the same time in Parisian terms.

The retarded-Paris language remained the French legal formulation until 1978, when France finally amended its civil-time law to refer to UTC directly. For 67 years, French civil time was officially GMT under another name.

The persistence of the formulation is a small monument to how slow international scientific conventions can be to override national pride.

The Royal Observatory's position

Greenwich's reputation came from two centuries of work before 1884. Per the Royal Museums Greenwich archives:

• 1675: The Royal Observatory founded by Charles II, with the explicit mission of solving the longitude problem at sea (covered in the /learn/the-longitude-problem support). John Flamsteed became the first Astronomer Royal.
• 1721: Edmond Halley succeeded Flamsteed.
• 1742–1762: James Bradley discovered stellar aberration and nutation; established astrometric standards.
• 1765–1811: Nevil Maskelyne established the Nautical Almanac (first edition 1767) using Greenwich Mean Time. The almanac was the primary tool by which navigators worldwide computed their longitude.
• 1835–1881: George Biddell Airy installed the Airy Transit Circle (1851), which became the line of reference for the Greenwich meridian.

By 1884, Greenwich was not only a national observatory but the de facto international observatory for navigation. The Royal Observatory at Greenwich Observatory's scientific lineage was unbroken since the 17th century; the Nautical Almanac had been the navigator's working reference for over a century.

The 1884 vote dissected

The voting record at the conference is preserved in the proceedings. Twenty-five delegations were present; the vote on the Greenwich resolution was:

| Position | Delegations | |---|---| | In favour (22) | Austria-Hungary, Chile, Colombia, Costa Rica, Germany, Guatemala, Hawaii, Italy, Japan, Liberia, Mexico, Netherlands, Paraguay, Russia, El Salvador, Spain, Sweden, Switzerland, Turkey, UK, US, Venezuela | | Against (1) | Dominican Republic (San Domingo) | | Abstaining (2) | France, Brazil |

Notable inclusions: Russia voted in favour despite having its own Pulkovo Observatory; Italy voted in favour despite earlier Jerusalem-meridian advocacy; Spain voted in favour despite its own Madrid observatory. In each case the practical-chart argument won over national-observatory prestige.

The Dominican delegate Manuel de Jesus Galván objected on grounds that included American hegemony, the asymmetric power of the British maritime empire, and the absence of consultation with smaller nations. The objection was eloquent but stood alone — even other small Latin American states voted for Greenwich because their trade went through Greenwich-charted ports.

France's abstention was unsurprising given the country's strong meridian tradition and the political alignment of the Third Republic in 1884. Brazil's abstention was less clearly motivated; some accounts suggest it was tactical, hoping to keep options open during the regional debates that followed.

Beyond navigation: time zones and civil life

The 1884 decision had consequences well beyond the immediate question of charts. The /learn/how-time-zones-were-created support covers how the time-zone system grew from the prime-meridian decision over the next several decades. Greenwich Mean Time became the civil time reference for the British Empire, then for the Commonwealth, then by 1972 for the international UTC system that replaced it.

Every modern UTC offset — the +5:30 of India, the −8 of Pacific Time, the +14 of Kiribati — measures from the meridian fixed in 1884. The pervasiveness of the choice is hard to overstate: every clock that displays a UTC offset, every map that uses a longitude value, every database timestamp that records a time zone, every GPS reading anywhere on Earth — all of these refer ultimately to the Greenwich meridian, by way of the 1884 conference vote.

The modern IERS Reference Meridian

The prime meridian was formally redefined in the late 20th century. Per the IERS Conventions, the modern 0° longitude is the IERS Reference Meridian (IRM), defined by the IERS Reference Frame derived from VLBI, SLR, and GPS observations rather than by any ground instrument.

When the IRM was first defined in the 1980s, the intention was to keep it aligned with the historical Airy Transit Circle at Greenwich. The alignment turned out to be approximate rather than exact: the IRM lies about 102.5 metres east of the Airy line at the latitude of Greenwich (about 5.3 arcseconds of longitude difference). The offset is the deflection of the local vertical at Greenwich — the direction a plumb line hangs differs from the geocentric normal to the WGS 84 ellipsoid by that amount.

The Airy Transit Circle was aligned with the local plumb line; the IRM is aligned with the geocentric reference frame. The two references diverge by the deflection at the Greenwich location.

For everyday navigation the offset is invisible. For high-precision geodesy it matters; a modern GPS receiver placed on the famous brass strip in the Greenwich Observatory courtyard reads approximately 0°0′5.3″W, not exactly 0°. The brass strip marks the historical Airy line, not the modern IRM. The /learn/the-prime-meridian pillar covers this distinction in more depth.

Sources

• Royal Museums Greenwich, The Prime Meridian at Greenwich
• Proceedings of the International Meridian Conference (1884)
• IERS Conventions (TN 36)
• USNO history — Washington meridian and US position at the 1884 conference.
• Royal Museums Greenwich, The Nautical Almanac — Maskelyne's 1767 publication and its international adoption.

For closely related material, see /learn/the-prime-meridian for the pillar, /learn/the-1884-international-meridian-conference for the conference itself, and /learn/how-time-zones-were-created for the time-zone system that grew out of the prime-meridian decision.