What Is Greenwich Mean Time?
How did the international standard of timekeeping, based on Greenwich Mean Time, come into being?
Time is relative, as any student of Einstein’s theories knows. But for much of history, time was also relative in a very practical sense.
In ancient societies, while there were methods of measuring time, such as water clocks, candle clocks, and sundials, most people lived by the sun, the moon, and the seasons. Time was a local matter, varying from place to place across the planet. This was true even when people believed the Earth was flat, or that the Sun revolved around the Earth.
Early Timekeeping Before Standard Time
The history of standardised timekeeping begins in the 14th century, when mechanical clocks started to replace earlier methods in religious houses, where daily life had previously been regulated by bells.
By the 16th century, the principles of telling time by a clock, and later by portable timepieces, were well established.
Navigation and the Need for Greenwich Mean Time
This period also coincided with the great age of European maritime exploration. It is here that the story of international time standardisation truly begins, driven by the need to synchronise long-distance sea voyages.
Greenwich Mean Time (GMT) is closely linked to the rise and dominance of the English, and later British, navy, and to the practical requirement for sailors to know their precise position while at sea.
Latitude, Longitude, and the Problem of Time
Without accurate timekeeping, it had always been difficult to determine a ship’s east–west position, its longitude, along the imaginary lines running from pole to pole known as meridians.
Latitude, by contrast, had been measurable for centuries using instruments such as quadrants and astrolabes, together with observations of the Sun and the Pole Star. Sophisticated devices for tracking celestial movements, such as the ancient Greek Antikythera mechanism, had existed long before.
Solving Longitude: The Marine Chronometer
Determining longitude remained a major problem until the 18th century. In response, the British government offered a substantial reward to anyone who could solve it, the famous Longitude Prize.
Previously, longitude could only be estimated using complex and infrequent astronomical events, such as lunar eclipses.
In 1773, John Harrison was awarded a significant portion of the prize for his marine chronometer, the result of decades of research and experimentation.
Why Greenwich Became the Centre of Time
By this time, Greenwich was firmly established as both Britain’s maritime and astronomical centre.
King Charles II had founded the Royal Observatory Greenwich in 1675 specifically to improve navigational accuracy at sea. The Astronomer Royal, Nevil Maskelyne, who developed the lunar-distance method of finding longitude, worked and later died there in 1811.
The Prime Meridian and Global Timekeeping
As a result, one of the world’s longitudinal meridians came to pass through Greenwich. However, it was not inevitable that this would become the Prime Meridian at 0° longitude.
That status was only formalised at the International Meridian Conference held in Washington, D.C. in 1884, after strong competition from Paris.
The decision established mean solar time at Greenwich as the basis for global timekeeping, giving rise to Greenwich Mean Time as the world’s reference time.
Greenwich Mean Time in the Modern World
Although Greenwich Mean Time is no longer the technical basis for global civil timekeeping, it remains deeply relevant today.
Modern time standards are based on Coordinated Universal Time (UTC), which uses atomic clocks for extreme accuracy. However, UTC is still aligned with Greenwich, and GMT continues to be used in everyday language, particularly in the UK, in aviation, maritime navigation, weather forecasts, and international scheduling.
For sailors, GMT remains especially important. Nautical charts, tide tables, weather reports, and GPS systems all rely on precise, standardised time. Whether crossing oceans or planning coastal passages, the legacy of Greenwich Mean Time continues to underpin safe and accurate navigation.
Time Well Spent on Lovesail
Time has always mattered at sea, from standing watch to planning a passage. Lovesail brings single sailors together for dating, friendship, and crewing, helping you make the most of your time on and off the water. Join the community and connect with people who share your love of sailing.
You must be logged in to post a comment.