The brass strip where every time zone starts
Stand in the courtyard at Greenwich and you can put one foot in the eastern hemisphere, the other in the western. That brass strip is 0° longitude. Every time zone on the planet counts from here. All standard times are expressed as hours ahead of or behind the local time on that meridian.
The line runs through the Airy Transit Circle telescope. When the Sun crosses this meridian, it is midday at Greenwich. Mean noon, to be precise. From that moment, every other place on Earth knows how far ahead or behind it sits.
Without a single agreed meridian, every town set its clocks by the Sun. Bristol was 10 minutes behind London. Paris was 9 minutes 20 seconds ahead. Railways and telegraphs made that chaos unworkable. A single zero was needed.
The longitude problem and the Royal Observatory
The story starts with ships getting lost.
In the 17th century, sailors could measure latitude easily using the Sun or stars. Longitude was another matter. Without a way to know their east-west position, crews died from navigational errors, shipwrecks and scurvy.
King Charles II ordered the founding of the Royal Observatory in 1675 specifically to solve this problem. His warrant appointed John Flamsteed as the first Astronomer Royal with a clear instruction: apply the study of astronomy to perfect navigation.
The institution was built in Greenwich Park, on high ground above the River Thames. Flamsteed and his successors, Edmond Halley and James Bradley, produced star charts and tables that let mariners calculate longitude by comparing the position of the Moon against the stars with tables calculated for Greenwich. The local time at Greenwich became the reference those tables used.
By the 18th century, British ships carried chronometers set to Greenwich time. When a captain measured local noon by the Sun, he compared it to his chronometer showing Greenwich time. The difference told him his longitude. The more charts and almanacs used Greenwich as their base, the more the world’s mariners relied on it.
Britain’s maritime dominance made Greenwich the default
The technical answer is that Greenwich worked. The real answer is that Britain’s navy and merchant fleet made it work.
By the mid-19th century, roughly three-quarters of the world’s shipping charts used Greenwich as the prime meridian. The British Admiralty had standardised on it. British trade routes spanned the globe. American, German and French captains carried Greenwich-based almanacs because that was what the charts showed.
The United States had already begun using Greenwich for its own coastal surveys by the 1850s. Canada’s railways adopted Greenwich-based time zones in the 1880s. When the US called for an international conference to settle the question once and for all, the practical choice was already clear.
The 1884 conference: 25 nations vote
In October 1884, delegates from 25 nations gathered in Washington DC. The question on the table: which meridian should be the world’s zero?
The vote on the main resolution was 22 in favour, 1 against (San Domingo, now the Dominican Republic), with 2 abstentions. France and Brazil abstained. The resolution stated that the meridian passing through the centre of the transit instrument at the Royal Observatory Greenwich would be the initial meridian, or 0° longitude.
A separate resolution made Greenwich the basis for universal time. The conference also agreed that longitude would be counted east and west from this meridian up to 180°, and that the universal day would begin at midnight at Greenwich.
The vote was not unanimous, but it was decisive. The maritime and railway world had already voted with its charts and timetables.
The Paris Meridian lost
France had its own meridian, running through the Paris Observatory. French astronomers and cartographers had used it for centuries. It was the main rival to Greenwich.
At the 1884 conference, France abstained rather than vote against its own meridian. French maps continued to use the Paris Meridian for years afterward. French ships did not switch to Greenwich until 1911.
But the Paris Meridian was a line of national pride, not global practicality. It did not have the shipping charts, the railway timetables or the international adoption that Greenwich had. France eventually accepted the Greenwich standard, and the Paris Meridian became a historical footnote, one you can still find on old maps and, famously, marked by medallions embedded in the streets of Paris.
The Prime Meridian today
More than a century after the 1884 conference, the Prime Meridian at Greenwich is one of London’s most visited landmarks. Visitors queue to stand with one foot in the eastern hemisphere and the other in the western hemisphere, straddling the brass line set into the courtyard.
On a clear day, you can look north from the site across the Isle of Dogs and the O2 Arena, or south toward the Greenwich Park treetops. The view has not changed much since Flamsteed first trained his telescope on the stars.
The institution still houses the Airy Transit Circle, the telescope that defines the meridian. It is a working museum, a research institution and a UNESCO World Heritage site.
For practical purposes, the world now uses Coordinated Universal Time (UTC) as its time standard, based on atomic clocks, not the rotation of the Earth. But UTC keeps the same offset as Greenwich Mean Time. When you check the time in London, you are still checking the time against that brass line in the courtyard.
Is the Prime Meridian still at Greenwich?
Yes. The Airy Transit Circle at the Royal Observatory Greenwich remains the reference point for 0° longitude. Modern GPS systems use a slightly different reference called the International Terrestrial Reference Frame, but the difference is only about 100 metres. The historical meridian at Greenwich is unchanged.
Why was the conference held in Washington DC and not London?
The United States called the conference. The US had already adopted Greenwich-based time zones for its railways and wanted an international standard. Washington was a neutral venue, not London or Paris, and the US government hosted the event.
Did all countries agree to the Greenwich meridian in 1884?
No. France and Brazil abstained. San Domingo voted against. France continued using the Paris Meridian for another 27 years. The vote was 22 in favour, but the abstentions show that the decision was political as well as practical.
What is the difference between GMT and UTC?
GMT is the mean solar time at the Prime Meridian, based on the rotation of the Earth. UTC is Coordinated Universal Time, based on atomic clocks. For civil purposes, GMT and UTC are the same offset, UTC+0, but UTC is more precise. The UK uses GMT in winter and British Summer Time (BST, UTC+1) in summer. For a full breakdown, see GMT vs UTC.
How do you visit the Prime Meridian in Greenwich?
The Royal Observatory is open daily. The meridian line in the courtyard is free to walk if you visit the grounds. A ticket gets you inside to see the Airy Transit Circle. Greenwich is reachable by train, Docklands Light Railway or riverboat from central London.
Where can you learn more about the history of time at Greenwich?
The full story, from the 1675 founding of the Royal Observatory to the adoption of GMT by British railways in 1847 and the 1884 conference, is covered in the History of Greenwich Mean Time. That page connects GMT to the time you see on a phone today.