The Prime Meridian
The prime meridian explained — 0° longitude through Greenwich since 1884, the IERS Reference Meridian 102 m east of the Airy Transit, and the 8 countries crossed.
By Steve K.. Published . Last updated .
The prime meridian is the great half-circle from pole to pole that defines 0° longitude — the east-west reference for all geographic coordinates. The 1884 Washington Conference adopted the Airy Transit Circle at Greenwich; the modern operational 0° is the IERS Reference Meridian, 102.478 m east of the historic brass strip.
The prime meridian is a conventional line: there is no physical reason it must pass through Greenwich rather than Paris, Cádiz, Washington or Tokyo. The history of "where 0° longitude is" is the history of which great power's observatory was authoritative at the time. This pillar runs the chronology, the 1884 vote, the 1984 shift from the Airy Transit to the IERS Reference Meridian, the 8 countries the meridian crosses on land, and the relationship between the prime meridian and Greenwich Mean Time.
Why there is no "natural" prime meridian
The equator is set by Earth's rotation axis — the unique parallel perpendicular to the spin axis. There is no analogous geometric reason for any particular meridian to be "the" zero. Every meridian is geometrically equivalent; the choice is conventional.
| Pre-1884 prime meridian | Country / use | Offset from Greenwich | Retired |
|---|---|---|---|
| Greenwich Airy Transit | UK, then global (1884) | 0° (definition) | 1984 (IERS Ref. Meridian replaced for high-precision use) |
| Paris Observatory | France | 2°20′14.025″ E (+2.3372°) | 1911 (UK time adopted) / 1978 (final) |
| Washington Naval Observatory | USA (briefly) | 77°03′02″ W (−77.0506°) | 1884 |
| Pulkovo Observatory | Russia / USSR | 30°19′39″ E (+30.3275°) | 1884 (international) / 1925 (internal) |
| Hierro / Ferro (Canary Islands) | Ptolemy, much of Europe | 17°39′ W (−17.65°) | 1884 |
| Tenerife (Pico de Teide) | Ptolemaic atlases | 16°38′ W (−16.63°) | obsolete |
| Madrid (Royal Observatory) | Spain (some maps) | 3°41′ W (−3.683°) | 1850s |
| Naples | Some Italian charts | ~14°15′ E | 19th c. |
| Rio de Janeiro | Brazilian maps (1880s) | ~43°10′ W | 1884 |
| Tokyo (Akihito / Tsukiji) | Japan internal | +139°44′40.9″ E | 1888 |
By 1880 every navigation power and many smaller countries used a different prime meridian. International maritime charts had to be re-projected or re-annotated for each market. The 1884 conference was called to fix this.
The 1884 International Meridian Conference
| Conference detail | Value |
|---|---|
| Convening date | 1 October 1884 |
| Closing date | 22 October 1884 |
| Location | Diplomatic Hall, US Department of State, Washington DC |
| Convened by | US President Chester A. Arthur |
| Nations attending | 25 (US, UK, France, Germany, Spain, Italy, Japan, Russia, Sweden, Switzerland, Brazil, Chile, Colombia, Hawaii, Mexico, Netherlands, San Domingo, Salvador, etc.) |
| Resolution 1 (adopt Greenwich) | 22 yes, 1 no (San Domingo), 2 abstain (France, Brazil) |
| Resolution 2 (universal day from Greenwich midnight) | Adopted by majority |
| Resolution 3 (mean midnight Greenwich start) | Adopted |
| Resolution 5 (study of decimal time / circle) | Not adopted |
| France retained Paris meridian until | 1911 |
| Soviet Union retained Pulkovo internally until | 1925 |
The Greenwich choice was practical: 72% of world shipping already used Greenwich-based charts by 1884 (British naval supremacy + Nautical Almanac dominance), so the adoption minimised re-engraving costs. France abstained partly out of principle (their Paris meridian had been a scientific standard since 1667) and partly out of a hope that the conference would adopt a "neutral" line; they ratified Greenwich in 1911. San Domingo (Dominican Republic) voted against because it used the Hierro meridian.
The Airy Transit Circle
| Property | Value | Source |
|---|---|---|
| Instrument | Airy Transit Circle (transit telescope mounted on east-west axis) | Royal Museums Greenwich |
| Commissioned by | George Biddell Airy, Astronomer Royal | 1851 |
| Manufacturer | Ransomes & May (with optics by Troughton & Simms) | 1851 |
| Operation | Observed star transits along the meridian to determine local sidereal time and the meridian's direction | 1851-1954 |
| Decommissioning | 1954 (operations moved to Herstmonceux Castle 1957) | RMG |
| Modern status | Tourist attraction; brass strip in courtyard marks the line | — |
| Brass strip latitude | ~51°28′40″ N | Coordinate of Greenwich |
The Airy Transit Circle defined longitude 0° from 1851 through the 1884 international agreement. Its observations could determine a star's transit time across the local meridian to within ~0.05 seconds — sufficient for navigation chronometer calibration but ~5,000× less precise than modern Very Long Baseline Interferometry.
The 1984 shift: from Airy to IERS Reference Meridian
In 1984 the international geodesy community switched from the Airy Transit's astronomical definition to a satellite-and-quasar definition based on VLBI and satellite laser ranging. The new zero came out 102.478 m east of Airy.
| Era | Defining technique | Where 0° is on the ground | Accuracy |
|---|---|---|---|
| 1884-1984 | Mechanical sighting through Airy Transit Circle | Brass strip in Royal Observatory courtyard | ~0.05 s of sidereal time (~few metres at the equator) |
| 1984-present | VLBI + satellite laser ranging + DORIS | 102.478 m east of the brass strip; crosses gift shop | Sub-centimetre |
The shift has nothing to do with the historical meridian moving; it's a consequence of moving from a local-vertical definition (susceptible to deflection-of-the-vertical from local gravity anomalies) to a geocentric definition (free of those biases). The IERS Reference Meridian is what every GPS receiver reports.
Two distinct prime meridians therefore exist at Greenwich today: the historic Airy line (a brass strip in the courtyard, preserved as a tourist site) and the IERS line (~102 m east, through the observatory's gift shop). GPS reports the IERS line; the brass strip is now wrong by ~102 m if measured against modern coordinates.
The 8 countries crossed by the prime meridian
The prime meridian crosses 8 countries on land (and several large stretches of ocean).
| Country | Continent | Approximate prime-meridian crossing length on land (km) | Notable landmark on the line |
|---|---|---|---|
| United Kingdom | Europe | ~325 | Greenwich (Royal Observatory) — Cleethorpes (Lincolnshire) |
| France | Europe | ~330 | Crosses Normandy, near Bordeaux |
| Spain | Europe | ~625 | Castellón province; Cuenca, Albacete |
| Algeria | Africa | ~1,300 | Crosses central Algerian Sahara |
| Mali | Africa | ~500 | Crosses Tessalit region |
| Burkina Faso | Africa | ~150 | — |
| Togo | Africa | ~50 | — |
| Ghana | Africa | ~200 | Crosses Volta region; exits near Tema (just east of Accra) into the Gulf of Guinea |
The prime meridian and the equator intersect at (0°, 0°) in the Gulf of Guinea, about 600 km south of Accra. The closest land to that intersection is Ilhéu das Rolas in São Tomé and Príncipe; the actual (0°, 0°) point is open ocean.
Prime meridian and time
The historic motivation for Greenwich was timekeeping, not cartography. Mean solar time at the prime meridian became "Greenwich Mean Time" (GMT), which became the time reference for British navigation, which by the late 19th century was the global standard.
| Time scale | Definition | Relation to prime meridian |
|---|---|---|
| Greenwich Mean Time (GMT) | Mean solar time at the Greenwich meridian (Airy line) | 0° longitude by definition; civil time at the prime meridian |
| Universal Time (UT1) | Mean solar time at IERS Reference Meridian | Differs from GMT by <1 ms due to the 102 m shift |
| Coordinated Universal Time (UTC) | Atomic time steered to stay within 0.9 s of UT1 via leap seconds | International civil standard since 1972 |
| GPS time | Started at midnight UTC 1980-01-06; no leap seconds since | 18 seconds ahead of UTC in 2026 |
| British Summer Time (BST) | GMT + 1 hour, summer only | UK observes; Greenwich is on BST in summer, GMT in winter |
| Z (Zulu) time | Synonym for UTC in aviation/military | Same time scale as UTC |
Civil GMT is the same as UTC to within a second; the technical distinction is that UTC is atomic-clock-based and GMT is solar-rotation-based. For all but the most precise scientific purposes they're interchangeable.
The antimeridian
The antimeridian is the meridian opposite the prime meridian: 180° east longitude = 180° west longitude. It runs mostly through the Pacific Ocean and crosses very few landmasses.
| Antimeridian crossing | Region | Status |
|---|---|---|
| Russia (Chukotka Peninsula) | Asia/Pacific border | Crosses Wrangel Island and the easternmost mainland |
| Fiji (Taveuni) | Oceania | Antimeridian passes through eastern Fiji; line marked on Taveuni |
| Antarctica | Mostly ocean ice extension | Crosses Ross Ice Shelf area |
| Aleutians | US/Russia border region | Approximate; not exact |
| Kiribati Line Islands | Oceania | Kiribati moved its IDL east of the line in 1995 to keep the country on one date |
The antimeridian and the International Date Line are related but not identical. The IDL is a political-administrative line that approximately follows the antimeridian, with zigzags around the Aleutians, Kiribati, Samoa and other island groups to keep each on a single calendar day. The antimeridian itself is a geometric great circle.
Common misconceptions
Related
- Why Greenwich Is the Prime Meridian— The historical and political path that led to the 1884 choice
- The 1884 International Meridian Conference— The Washington conference that fixed the prime meridian by treaty
- Meridians of Longitude— The family of half-circles the prime meridian anchors
- The International Date Line— The companion line at the antimeridian, ±180° from the prime meridian
- GMT vs UTC— How modern civil time relates to Greenwich Mean Time
- The Equator— The latitude counterpart — fixed by rotation, not by treaty
- What Is Longitude— The angular axis the prime meridian anchors at 0°
- My Location— Your current longitude — measured east or west of the prime meridian
- Methodology— How content is sourced and verified
Frequently asked questions
What is the prime meridian?
The prime meridian is the great half-circle from the North Pole to the South Pole that defines 0° longitude. Longitudes east of it are positive (up to +180°), longitudes west are negative (down to −180°), and the two extremes meet on the opposite side of Earth at the antimeridian. Unlike the equator, which is fixed by Earth's rotation, the prime meridian is a chosen convention.
Where exactly does the prime meridian pass?
The modern prime meridian — the IERS Reference Meridian used by GPS, GIS, and every modern coordinate system — passes through the British Royal Observatory in Greenwich, then south through France (near Le Havre and Villers-sur-Mer), Spain (Castellón), Algeria, Mali, Burkina Faso, Togo, and Ghana, before crossing the Atlantic and continuing south through the ocean to Antarctica. It crosses the equator just south of Ghana in the Gulf of Guinea, at the point traditionally called ‘Null Island’ (0°N, 0°E).
Why is the GPS prime meridian 102 metres east of the Greenwich brass line?
The brass strip in the courtyard of the Royal Observatory marks the historical Airy Transit Circle, set in 1851 and adopted as the prime meridian by the 1884 International Meridian Conference. The modern IERS Reference Meridian is defined not by a ground instrument but by the IERS Reference Frame from VLBI, SLR, and GPS observations. The IRM happens to lie about 102.5 metres east of the Airy line (a shift of about 5.3 arcseconds). The offset reflects a deflection of the vertical at Greenwich — the local plumb line tilts slightly from the geocentric reference — plus accumulated definitional changes since 1884.
Did everyone use Greenwich before 1884?
No. National prime meridians varied widely through the 19th century. France used Paris, Spain used Cadiz and later Madrid, Russia used Pulkovo, the United States used Washington, the Netherlands used Amsterdam, and many older charts referenced Ferro (El Hierro) in the Canary Islands. The 1884 International Meridian Conference in Washington, DC adopted Greenwich by a vote of 22 to 1 with two abstentions; France was the most prominent holdout and did not formally adopt Greenwich until 1911.
What is the antimeridian?
The antimeridian is the great half-circle exactly opposite the prime meridian, at longitude ±180°. It is also called the 180th meridian. The International Date Line broadly follows it but bends to avoid splitting Russia, Alaska's Aleutian Islands, Kiribati, Samoa, Tonga, and New Zealand. The antimeridian is geometrically interesting because it is the one place where two valid longitude values (+180° and −180°) describe the same point — a source of off-by-one bugs in geocoding software.
Is GMT the same as the prime meridian?
Greenwich Mean Time (GMT) was historically defined as the mean solar time at the Greenwich prime meridian — the time the Sun would cross the meridian, averaged over the year. Modern civil time uses Coordinated Universal Time (UTC), which is an atomic time scale kept within 0.9 seconds of GMT via occasional leap seconds. UTC retains Greenwich as its longitude reference; every time zone is offset from UTC in whole hours, half hours, or quarter hours.
How many countries does the prime meridian cross?
Eight countries on land: the United Kingdom (Cleethorpes to Greenwich), France (Normandy through Bordeaux), Spain (Castellón province), Algeria (central Sahara), Mali, Burkina Faso, Togo, and Ghana (exits near Tema, just east of Accra, into the Gulf of Guinea). The meridian also crosses the Atlantic Ocean, the Arctic Ocean, and Antarctica. The closest land to (0°, 0°) — where it intersects the equator — is São Tomé and Príncipe, about 600 km north of the intersection point.
Why does the prime meridian go through Greenwich?
Practical politics in 1884. The Greenwich Royal Observatory had published high-quality lunar-distance tables for navigation for over a century, and 72% of world shipping already used Greenwich-based charts by 1884. At the International Meridian Conference, 22 nations voted to adopt Greenwich; 1 voted against (San Domingo); 2 abstained (France, Brazil). France retained the Paris meridian internally until 1911 for political reasons; the Soviet Union kept Pulkovo until 1925.
When was Greenwich chosen as the prime meridian?
The 1884 International Meridian Conference in Washington DC adopted Greenwich as the prime meridian on October 13, 1884, after a 3-week diplomatic conference (October 1-22). The conference convened 25 nations at the invitation of US President Chester A. Arthur. The Greenwich Airy Transit Circle (installed 1851 by George Biddell Airy) was the specific instrument named in the resolution — its meridian became 0° longitude internationally.
Sources
- IERS — IERS Conventions (2010), TN 36 — IERS Reference Meridian · https://www.iers.org/IERS/EN/Publications/TechnicalNotes/tn36.html · Accessed .
- Royal Museums Greenwich — The Prime Meridian at Greenwich · https://www.rmg.co.uk/stories/topics/prime-meridian-greenwich · Accessed .
- BIPM — Coordinated Universal Time (UTC) · https://www.bipm.org/en/time-ftp/utc · Accessed .
- US Naval Observatory — Earth Orientation Department — astronomical references · https://www.usno.navy.mil/USNO/earth-orientation · Accessed .
- NIST — Time and frequency from A to Z, GMT · https://www.nist.gov/pml/time-and-frequency-division/popular-links/time-frequency-z-glossary · Accessed .
- 1884 Conference — Proceedings of the International Meridian Conference (Washington, October 1884) · https://www.gutenberg.org/ebooks/17759 · Accessed .
- NGA — World Geodetic System 1984 (NGA.STND.0036) · https://earth-info.nga.mil/index.php?dir=wgs84&action=wgs84 · Accessed .
Cite this article
APA format:
Steve K. (2026). The Prime Meridian. Coordinately. https://coordinately.org/learn/the-prime-meridian
BibTeX:
@misc{coordinately_theprimemeridian_2026,
author = {K., Steve},
title = {The Prime Meridian},
year = {2026},
publisher = {Coordinately},
url = {https://coordinately.org/learn/the-prime-meridian},
note = {Accessed: 2026-06-05}
}