Greenwich and the prime meridian: why world time starts in London

World time is counted from Greenwich, in south-east London, because the Royal Observatory there had become the reference for navigation by the 19th century. In 1884 the International Meridian Conference chose the Greenwich meridian as 0° longitude, and Greenwich Mean Time became the base for the world's time zones. Today the atomic standard UTC has taken over, but it still tells the same time as GMT.

Royal Observatory founded	1675, by Charles II
First Astronomer Royal	John Flamsteed
Time ball first dropped	1833 (every day at 1 p.m.)
Airy Transit Circle defines 0°	1851
GMT becomes legal time in Great Britain	1880
International Meridian Conference	October 1884, Washington, D.C.
GPS zero longitude	About 102 m east of the historic line

An observatory built to find longitude

King Charles II founded the Royal Observatory in 1675 for one practical reason: ships could not work out their longitude, and lost ships cost lives and money. John Flamsteed, the first Astronomer Royal, set out to map the stars precisely enough that sailors could read the time, and so their position, from the sky. The problem was finally solved in the 18th century by John Harrison's sea clocks and by the Nautical Almanac, first published in 1767, whose tables were calculated for Greenwich. From then on, sailors around the world measured their longitude east or west of Greenwich. For the full story, see the longitude problem.

Giving the time to the world

In 1833 the observatory put a large red time ball on its roof. Every day at 1 p.m. it drops, so ships on the Thames could set their chronometers. In 1852 the Shepherd Gate Clock at the gate began showing Greenwich time to the public, controlled electrically by the observatory's master clock; unusually, its dial shows all 24 hours. The same signals went out by telegraph across Britain, and railways, which needed one time for their timetables, adopted Greenwich time in the 1840s and 1850s. In 1880 an Act of Parliament made GMT the legal time of Great Britain. See railway time.

The line itself: the Airy Transit Circle

There have been several Greenwich meridians, each defined by a different telescope. The one the world adopted belongs to the Airy Transit Circle, built by Astronomer Royal George Airy and first used in 1851. Astronomers timed stars as they crossed its crosshairs, and that line through the telescope became 0° longitude. Today a brass strip, and at night a green laser, marks it across the courtyard.

1884: the world picks Greenwich

By the 1880s countries used dozens of different prime meridians: Paris, Cadiz, Pulkovo, Washington and more. In October 1884, 41 delegates from 25 nations met in Washington, D.C. at the International Meridian Conference. The resolution to use the meridian through Greenwich passed with 22 votes in favour, 1 against (San Domingo, today the Dominican Republic) and 2 abstentions (France and Brazil). Greenwich won mostly because it was already in use: about 70% of the world's shipping relied on Greenwich-based charts. The conference did not create time zones; each country adopted standard time by its own law over the following decades.

France held out longest. It kept Paris time until 1911, when it defined legal time as Paris mean time "retarded by nine minutes and twenty-one seconds", which happens to be GMT. French maps used the Paris meridian into the 20th century.

Why GPS says the meridian is 102 metres away

Visitors with phones often notice that their GPS reads 0° about 102 metres (334 feet) east of the brass line. Neither is wrong. The Airy Transit Circle was aimed using local gravity, which at Greenwich does not point exactly at the Earth's centre. Modern longitude, used by GPS, is measured from the Earth's centre of mass, which shifts the zero line slightly east. Geodesists explained the difference in a 2015 paper in the Journal of Geodesy.

From GMT to UTC

GMT was mean solar time at Greenwich: time kept by the average Sun, not the actual one. As clocks became more precise than the Earth's slightly uneven spin, scientists replaced it with atomic time. Coordinated Universal Time (UTC) took its current form in 1972 and is now the world's standard; time zones are defined as offsets from UTC. GMT lives on as the name of the time zone used by the UK in winter, Ireland's winter time and several West African countries. See UTC vs GMT and GMT time now.

Questions people ask

Why is the prime meridian in Greenwich?

Because by the 1880s most of the world's ships already navigated with charts measured from Greenwich, and the US had based its own time zones on it. At the International Meridian Conference in Washington in October 1884, 22 nations voted for Greenwich, San Domingo (today's Dominican Republic) voted against, and France and Brazil abstained.

Is the prime meridian really at Greenwich?

Almost. The brass line in the Royal Observatory courtyard marks the Airy Transit Circle, the telescope that defined 0° from 1851. The zero longitude used by GPS today runs about 102 metres (334 feet) east of it, because modern longitude is measured from the Earth's centre of mass rather than from local gravity at the telescope.

Is GMT the same as UTC?

On the clock, yes: they show the same time. GMT is the time zone of the UK in winter and some African countries; UTC is the atomic time standard that replaced GMT as the world's scientific reference. See UTC vs GMT.

When did France stop using the Paris meridian?

France kept Paris time after 1884 and only adopted Greenwich-based time in 1911, defining its legal time, with some pride, as "Paris mean time retarded by nine minutes and twenty-one seconds". That was GMT.

Can you visit the prime meridian?

Yes. The Royal Observatory in Greenwich Park, London, is part of Royal Museums Greenwich. You can stand with one foot in each hemisphere on the meridian line, see the Airy Transit Circle, watch the red time ball drop at 1 p.m., and check your watch against the Shepherd Gate Clock by the entrance.

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Last updated October 21, 2026. Found an error? Tell us.