The Royal Observatory Greenwich, home of the Prime Meridian

The Prime Meridian at Royal Observatory Greenwich: Longitude Zero Explained

A metal line fixed across the Royal Observatory Greenwich’s courtyard in 1851 became the world’s Prime Meridian in 1884, zero degrees longitude, the reference every map and coordinate still measures from.

Visit

From £24, Royal Observatory Greenwich admission includes the meridian courtyard until 2 November 2026. Royal Observatory Greenwich Tickets.

Location

Blackheath Avenue, London SE10 8XJ, in the Observatory’s own courtyard at the top of Greenwich Park.

Visit duration

Royal Museums Greenwich’s own guidance suggests 1.5 to 2 hours for the full Observatory visit. Plan your visit.

Coordinates

0° longitude, fixed by the Airy Transit Circle’s cross-hairs from 1851 to 1954.

The Line That Splits East from West

The Prime Meridian is the line of zero degrees longitude, the reference every other line of longitude counts east or west from, the same way the equator anchors zero degrees latitude on any map or globe. At the Royal Observatory Greenwich, that line is not an abstraction: it runs as a real metal strip let into the courtyard paving, fixed there since 1851 and confirmed as the world’s meridian in 1884. For Flamsteed House, the Great Equatorial Telescope and the rest of the hill’s story, see Royal Observatory Greenwich’s overview.

Longitude Zero: Quick Facts About the Prime Meridian Line

The metal line in the courtyard marks zero degrees longitude, fixed by the Airy Transit Circle’s cross-hairs between 1851 and 1954.

Confirmed as the world’s Prime Meridian by the International Meridian Conference in Washington D.C. in 1884, after nearly two-thirds of the world’s shipping was already navigating by Greenwich-based charts.

It is the third of three historic meridian lines drawn across this same courtyard: Halley’s from around 1725, Bradley’s from 1750, then Airy’s from 1851, each replacing the last with a more accurate instrument.

A green laser has marked the meridian across the night sky since 1993, upgraded in 1999, and has been seen from as far as 36 miles away on a clear night.

Modern GPS places true zero longitude about 102 metres east of the historic line, because satellites measure from the Earth’s rotation axis rather than the Observatory’s local gravity.

Three Lines Before the One That Stuck

Royal Observatory Greenwich buildings in Greenwich Park

Long before Airy, two earlier Astronomers Royal drew their own meridian lines across this same courtyard. Edmond Halley marked the first from around 1725, timing star transits through open roof hatches with a pendulum clock. James Bradley replaced it with a more accurate line in 1750, precise enough that the Ordnance Survey adopted it as zero degrees for its own maps in 1801, a use that outlived Bradley’s own instruments by decades.

George Biddell Airy, the seventh Astronomer Royal, superseded both. His Transit Circle recorded its first star on 4 January 1851 and kept working, almost without a break, until 1938, with one final observation in 1954. When the world needed a single meridian in 1884, it was the cross-hairs in this one telescope’s eyepiece that delegates in Washington agreed to call zero degrees.

A Green Line Across the Sky, and a Line That Moved

Greenwich Park below the Royal Observatory Greenwich

Since 1993, upgraded in 1999, a green laser called 0 degrees has projected the meridian out from a point above the old Transit Circle, switched on every evening the Observatory closes until 11pm. Artists Peter Fink and Anne Bean designed it to make an invisible line something visitors can actually watch arc over London; on a clear winter night it has been seen from up to 36 miles away.

The brass line itself has not been perfectly accurate for decades, though. GPS measurements place true zero longitude about 102 metres east of Airy’s courtyard line, because satellites measure from the Earth’s rotation axis, while Airy aligned his telescope to the local direction of gravity. The line has not moved; the gap comes from what “straight down” means at that exact spot on an imperfectly round planet.

Three Things That Share the Meridian’s Story

Flamsteed House at the Royal Observatory Greenwich

The Airy Transit Circle, still in its room

Still mounted where Airy installed it, this telescope made its first observation on 4 January 1851 and kept recording star transits, almost without a break, until 1938, with one final observation in 1954. Its eyepiece cross-hairs are the exact point delegates in Washington agreed to call zero degrees in 1884.

Royal Observatory Greenwich above Greenwich Park

The green laser, on every clear night

Titled 0 degrees and switched on every evening the Observatory closes until 11pm, this laser has marked the meridian across London’s sky since 1993, upgraded in 1999. On a clear winter night it has been seen from up to 36 miles away with the naked eye.

Royal Observatory Greenwich in Greenwich Park

The Shepherd Gate Clock, at the gates

Fixed outside the Observatory gates in 1852, this 24-hour dial was the first clock to show Greenwich Mean Time directly to the public. A German bomb damaged its face on 15 October 1940; the replica dial fitted afterward is the one visitors read today, now driven by a quartz movement inside.

Royal Observatory Greenwich: Admission Ticket

Highlights

Experience standing on the Prime Meridian, dividing eastern and western hemispheres.

Discover the rich history behind Greenwich Mean Time (GMT) established here.

View remarkable astronomical instruments and exhibits including John Harrison’s timekeepers.

Time your visit to see the Time Ball drop, a historical time signal at 1 PM daily.

Enjoy panoramic views from this iconic observatory site overlooking Greenwich.

Royal Museums Greenwich Day Pass

Highlights

Entry to four iconic attractions in London’s Greenwich area.

Explore the historic tea clipper, Cutty Sark.

Visit the Royal Observatory and stand on the Meridian Line.

Discover Britain’s maritime heritage at the National Maritime Museum.

Experience 17th-century elegance at the Queen’s House.

See the Line Before First Light Closes the Courtyard

The Meridian Courtyard stays open, with the rest of the Observatory, only until 2 November 2026, then closes for the First Light works and is not due back until summer 2027. Book a timed slot now to stand across the line while the courtyard is still open, one foot in each hemisphere, before it shuts for the works.

Questions About the Greenwich Prime Meridian

What is the Prime Meridian?

It is the line of zero degrees longitude, the point every other line of longitude is measured from, east or west, the same way the equator anchors zero degrees latitude. On a map or globe, the two lines together form the grid a place’s exact coordinates are read from.

Why is the Prime Meridian at Greenwich?

The International Meridian Conference in Washington D.C. chose it in 1884, largely because nearly two-thirds of the world’s shipping already navigated using Greenwich-based charts. The exact line was fixed by the Observatory’s Airy Transit Circle, first used in 1851.

Can I stand on the Prime Meridian line at the Royal Observatory Greenwich?

Yes, on an Observatory admission ticket, in the courtyard, but only until 2 November 2026. The Meridian Courtyard then closes for the First Light works and is not due back until summer 2027.

Is the brass line at Greenwich exactly zero degrees longitude?

Not quite, by modern measurement. GPS places true zero longitude about 102 metres east of the historic line, because satellites measure from the Earth’s rotation axis rather than the local direction of gravity Airy used to align his telescope.

What are the three meridian lines in the Observatory’s courtyard?

Edmond Halley drew the first from around 1725, James Bradley replaced it with a more accurate line in 1750, and George Airy superseded both with the Transit Circle in 1851. Airy’s is the one the world adopted in 1884.

Can you see the Prime Meridian at night?

Yes. A green laser called 0 degrees, installed in 1993 and upgraded in 1999, projects the meridian across the sky every evening the Observatory closes until 11pm, visible from up to 36 miles away on a clear night.

What is the Shepherd Gate Clock, and how does it relate to the Prime Meridian?

It is the 24-hour dial fixed outside the Observatory gates in 1852, the first clock to show Greenwich Mean Time directly to the public. It sits within a few metres of the meridian itself, reading the same time the line's own name gave the world.