What Is Greenwich Mean Time?

In south London, at the Royal Observatory, Greenwich, it’s possible to walk up to a metal strip running along the ground in a courtyard and, stepping over it with one foot, straddle the world. Suddenly, one half of your body is in the Western Hemisphere and the other is in the East. Sort of. More on that later.

This line is the prime meridian, or Greenwich Meridian. In 1851, Sir George Airy established it as 0° longitude. With a fixed line of longitude, or “home meridian,” sailors and explorers were more easily able to nail down their east-west position. All a navigator had to do was compare the time onboard their ship with the local time at the home meridian, and British sailors began keeping a marine chronometer set to the local time at Greenwich.

The practice spread to sailors from other countries, and soon ships all over the world were calculating their positions based on Greenwich time. In 1884, the sailors' custom got legitimized when the International Meridian Conference met in Washington, D.C., and delegates from 25 countries overwhelmingly voted to make the Greenwich Meridian the internationally common point from which to measure time and longitude.

Call it a Day

They also recommended that there should be a Universal Day, counted in 24-hour notation, that would begin at Solar midnight (the point at which the night is equidistant from dusk and dawn) Greenwich Mean Time (the mean being an average that accounts for the uneven speed of the Earth in its rotation).

Of course, not every country at the conference was so gung-ho about adopting a British time and longitude as the world standard. French delegates attempted to convince the others that Paris should be the home of the Prime Meridian, but went ignored. Feeling snubbed, they abstained from the vote and adopted Paris Mean Time as their standard national time and the Paris Observatory meridian as their prime meridian until switching to the Greenwich standard decades later. Even then, some Frenchmen were known to refer to GMT as “the mean time of Paris retarded nine minutes and 21 seconds.”

Just as GMT seemed to have spread over the globe, things began to fall apart. The French reaction demonstrated one big problem with the conference once all the good vibes of international cooperation faded away: the decisions made in Washington had no binding power. They were only recommendations, and it was up to the different national governments to implement them at home.

Progress was slow and confusion rampant. The only nation to do anything concrete within the following decade was Japan, which formally adopted the Greenwich Meridian and a standard national time nine hours in advance of Greenwich (GMT +9) in 1888. Elsewhere, depending on who you were talking to, GMT was used (usually inconsistently) one of two ways — with the hours either numbered starting at midnight, as had been recommended at the conference, or at noon.

To help stem confusion, the International Astronomical Union changed the designation of the standard time of the zero meridian to Universal Time Observed, or UTO, which is more or less equivalent to GMT but more precise and is the mean sidereal time as measured in Greenwich.

In 1972, after the development of super-accurate atomic clocks, Coordinated Universal Time, or UTC, was established. It’s calculated using a weighted average of signals from atomic clocks located in various national laboratories around the world, with leap seconds added at irregular intervals to compensate for the oddities of the Earth’s movement. UTC, like UTO, is synonymous with GMT in common or casual use, but GMT isn’t so precisely defined by the scientific community anymore and isn’t used in technical contexts.

Time Marches On

Even the Greenwich Meridian itself isn’t quite what it used to be. Formerly defined by "the centre of the transit instrument (a specific kind of telescope) at the Observatory at Greenwich,” the line is now defined by a statistical solution resulting from observations of several time-determination stations that the International Bureau of Weights and Measures uses to coordinate the world's time signals. The observatory’s instrument still survives in working order, but is no longer in use, while the actual line in the observatory courtyard, marked by a bronze strip, is actually now a few meters off from the imaginary line of the Prime Meridian.