Your Watch Is Wrong. So Is Every Clock on Earth.

When I was 13 years old, my mother took a photograph of me standing above a brass line in a London courtyard, one foot in each hemisphere. Like countless visitors before and since, I had my picture taken on the Prime Meridian, in front of the Royal Observatory in Greenwich.

Last week I came back and stood on the same line.

"It's for tourists," Dr. Emily Akkermans, Curator of Time at Royal Museums Greenwich, told me as we stood in the courtyard. "It means nothing. It's the telescope beyond that's important."

The telescope beyond the line is the Airy Transit Circle, erected in 1850 for the seventh Astronomer Royal, Sir George Biddell Airy, and in use from January 4, 1851. For decades its crosshairs fixed the historic Prime Meridian, the reference from which the world measured longitude. "We now call it the historic Prime Meridian because it's about a hundred meters from where it should be," Akkermans said.

The line is not the only thing that isn't what it seems. We, as a species, have changed the way we measure time over and over, and each time the old method fell away. The second was once a piece of the day, set by the sun crossing this meridian. Now it is set by the vibration of an atom.

A watch is where the old way survives. It still keeps time on the wrist, by hand, the way a sailor once read it off a chronometer at sea. A GMT watch holds two of those times at once, home and away, in an age that has turned the whole job over to satellites and atomic clocks.

George Bamford still builds watches around GMT, at Bamford London, his own company. "GMT is iconic," he told me. "If you talk about digital watches, we read the time, not tell the time." His GMT watch runs on a Swiss automatic movement, with a 24-hour hand and an internal rotating bezel that tracks a second time zone. It comes in steel, titanium and forged carbon.

The three letters on his dials stand for Greenwich Mean Time, and they come from this hill.

The Observatory was never built for abstract science. "It was founded for navigational purposes," Akkermans said, "to help seafarers find their longitude at sea." Charles II established it in 1675 to solve a problem that was killing sailors.

The idea was simple enough: measure the difference in time between the ship and a fixed reference, and the gap gives longitude. The hard part was building a clock accurate enough to survive the voyage. A self-taught carpenter, John Harrison, spent four decades on one. His last timekeeper, H4, lost only about five seconds on its 1761 trial to Jamaica. On display at the Royal Observatory, H4 looks like an oversized antique pocket watch, though with a 13 cm diameter it is a little too large for most pockets. It also looks like a beautiful piece of technology, enclosed in a case with intricate hand-engraved silver work. The dial features Roman numerals, ornate brass detailing, and delicate dark hands.

An accurate clock at Greenwich solved half the problem. The other half was getting the time to everyone else. A red ball still rides a mast atop Flamsteed House, and at 1 p.m. it still drops, as it first did in 1833. "It was the first public time signal of GMT," Akkermans said. Ships on the Thames watched for it. "Captains observed the exact drop and were able to compare the precise moment of the drop with their onboard chronometers. They no longer had to do long-winded calculations."

For those beyond the river's view, John Henry Belville, an assistant at the Observatory, had a chronometer set to the master clock carried around London, selling accurate time to subscribers. The family kept the business for a century. After Belville's death his widow, Maria, took over, and then their daughter Ruth, who was still walking the London round into the 1930s with an 18th-century chronometer in her handbag. Telegraph wires eventually replaced the Belvilles' traveling chronometer, and radio replaced the telegraph.

It was the railways that forced the issue. Before them, every town kept its own local time, a spread of about 30 minutes across Britain, so a train leaving London on London time might reach Bristol to find the station clock 11 minutes behind. The Great Western Railway put its stations on Greenwich Mean Time in 1840, the other lines followed, and by early 1848 Britain ran on a single clock, the first country to do so.

The need to know the time somewhere else never went away. As Bamford put it, "For me, my thought pattern is that I absolutely love the functionality of a GMT watch...Especially because I'm traveling a lot."

By the 1880s the problem had gone global, and Greenwich already held the advantage. "About 60% of ships, or in tonnage, 72% of shipping tonnage, was using Greenwich charts," Akkermans said. When delegates met in Washington in 1884 to pick one prime meridian for the world, France pushed for a neutral meridian, a line that would cut no great continent. "They spent two days debating whether the Prime Meridian could be neutral," Akkermans recalled. "A meridian has to be measured by a telescope. You can't build a telescope in the ocean." The neutral proposal failed 3 to 21, and Greenwich carried the vote 22 to 1 on October 13, with France and Brazil abstaining. The conference fixed the reference, but it did not hand the world its time zones. "It was up to nations to decide what to do," Akkermans stressed.

The 24-hour grid we now take for granted was largely the vision of Sir Sandford Fleming, a Scottish-Canadian railway engineer who had missed a train in Ireland years earlier, thanks to a timetable error. Fleming proposed dividing the globe into 24 hourly zones, each 15 degrees of longitude wide. Implementation was slow. Nations adopted the system at their own pace, or not at all. France, which had abstained from the Greenwich vote, kept the Paris meridian on its own maps and navigational charts into the twentieth century. French law still defined legal time as Paris Mean Time, retarded by 9 minutes and 21 seconds, until 1978, though in practice France followed GMT from 1911 onwards.

The conference had settled where the world's clocks would begin, but not what a second was. For most of history a second was a slice of the day itself, one part in 86,400 of the mean solar day. The Earth turned out to be an unreliable clock, its rotation wandering by tiny amounts, so in 1960 the General Conference on Weights and Measures redefined the second against the Earth's orbit instead, as a fixed fraction of the length of the year 1900.

Even that lasted only until a better clock made the whole astronomical approach obsolete. In 1955, at the National Physical Laboratory in Teddington, up the Thames from Greenwich, Louis Essen and Jack Parry had built the first working caesium atomic clock, accurate to about a second in 300 years. It measured time by the vibration of an atom, not by anything in the sky. In 1967 the second was redefined again, as 9,192,631,770 oscillations of caesium-133. Essen later described the moment he called his director in to see the clock run as the end of the astronomical second.

Atomic clocks now keep the world's time, coordinated as UTC. The Earth's rotation drifts against them, so since 1972 the two have been reconciled with the leap second, slipped in whenever they fall about a second apart. Twenty-seven have been added, the last at the end of 2016. In 2022 the General Conference voted to phase the practice out, and by 2035 civil time will be allowed to drift away from the Sun.

"Today, Greenwich Mean Time indicates the standard time of the United Kingdom. It's no longer a scientific measurement. We now use satellites, GPS and atomic clocks. GMT is just part of our history now," Dr. Akkermans explained. Airy's meridian, the line the world agreed on in 1884, sits about 100 meters west of the reference satellites use today.

"Today, GMT is basically our cultural heritage," Akkermans said.

For Bamford, GMT is not heritage. It is a tool. "For me, there is something amazing about the functionality of these two hands that are measuring time in two locations with that outer bezel. I think the GMT function is such a great watch."

Standing on the brass line again, one foot in each hemisphere, I thought about what the Curator of Time had told me. The line beneath my feet was a construct for visitors.

Bamford said he rarely thought about the history of GMT. He was convinced, he said, that GMT watches had a place in the 21st century, irrespective of how time was measured in the past. "I absolutely love the functionality of a GMT watch, the idea of having two hands that tell the time, your home time and the time when you're traveling."

I have two photographs now. One of a 13-year-old who didn't understand what he was standing on. And one of a journalist who knows that time is all we really have.