A brass line set into the ground at the Royal Observatory, Greenwich is the reason your calendar app defaults to the city. It is the Prime Meridian, 0° longitude, and it makes London the literal centre of world time.
The brass line that divided the planet
Before 1884, every major city had its own local time. Noon in London was not noon in Bristol or Paris or New York. This chaos worked for horse-drawn carts but failed for railways, telegraphs, and steamships.
The solution was a single reference line for longitude and time. That line had to be somewhere. In 1884, it was fixed at the Greenwich site.
How London won the 1884 Meridian Conference
Twenty-five nations sent delegates to Washington D.C. in October 1884 for the International Meridian Conference. Their task: choose a single prime meridian for the entire planet.
The vote was not unanimous. France abstained, holding out for the Paris Meridian until 1911. Santo Domingo voted against. But 22 delegates voted for the site in south-east London.
The decision was practical, not sentimental.
72% of the world’s ships already used Greenwich charts
By 1884, 72% of the world’s shipping tonnage used charts that already placed the prime meridian at Greenwich. The British Navy was the largest on earth, and the Nautical Almanac, published by the institution, was the standard navigation reference for merchant fleets from Shanghai to Buenos Aires.
Choosing any other meridian would have required reprinting thousands of charts and almanacs. The site was already the de facto standard. The 1884 conference simply made it official.
The birth of GMT on a hilltop in 1675
The Greenwich institution was founded in 1675 by King Charles II. Its mission was to solve the problem of finding longitude at sea. Astronomer Royal John Flamsteed spent decades mapping the stars from the hilltop, creating the tables that would become the basis for Greenwich Mean Time.
GMT is the mean solar time at the site. When the sun crosses the meridian there, it is noon GMT everywhere on the planet, at least on paper. In practice, the UK only uses GMT in winter. From late March to late October, clocks move forward to British Summer Time (BST), which is one hour ahead.
The IANA time zone identifier Europe/London handles both. A single zone, two offsets, one history.
Does being the ‘centre of time’ still matter?
In the 2020s, your phone gets the time from GPS satellites and atomic clocks. The Coordinated Universal Time (UTC) standard replaced GMT as the scientific reference in 1972. London no longer literally sets the world’s clocks.
But the legacy matters in three ways.
The IERS Reference Meridian, used by GPS for global positioning, sits a short walk east of the historic brass line. The exact line moved when satellite measurements revealed the old line was slightly off. The historic meridian remains the tourist attraction; the real one is invisible.
Financial markets still use London as a hinge. The London Stock Exchange opens at 08:00 local time, a window that overlaps with Asian market closes and precedes US market opens. Traders in New York and Singapore both set alarms for London hours.
The time zone database uses Europe/London as a primary identifier. When you pick “London” in a meeting scheduler, you inherit a century of maritime history.
A tourist attraction with a view of Canary Wharf
The brass line at Greenwich is one of the most photographed patches of ground in the capital. Thousands of visitors stand with one foot in the eastern hemisphere and one in the west. The view from the hilltop across the river to Canary Wharf is a visual summary of the whole story: a 17th-century institution looking at a 21st-century financial district.
The line itself is a copy. The original, used by Flamsteed, sits inside the building. The outdoor meridian was installed in 1850 and became the line that the 1884 delegates voted on.
What this means for you
On Sunday 29 March 2026 at 01:00 GMT, London clocks spring forward to 02:00 BST. The days stretch. Sunset on the June solstice will be at 21:21, giving 16 hours 39 minutes of daylight. Then on Sunday 25 October 2026 at 02:00 BST, the clocks fall back to 01:00 GMT, and the winter darkness returns. On the December solstice, sunrise will be at 08:03 and sunset at 15:53, just 7 hours 50 minutes of light.
The next change you will feel is on Sunday 28 March 2027 at 01:00 GMT, when London shifts back to BST. That date matters if you are planning cross-border calls or travel. The gap between London and New York stays at 5 hours in March 2027, but the gap to Sydney swings from 11 hours in winter to 9 hours in summer. Your calendar app handles the switch. Now you know why it works the way it does.