GMT: Greenwich Mean Time

Greenwich Mean Time (GMT) is a means of telling time based on the sun crossing the Prime Meridian which is located at the Royal Observatory in Greenwich, England.

GMT is the average time it takes the earth to rotate from noon to noon again.

Although GMT tracks time based on the passage of the sun across a certain reference point (Prime Meridian,) it is not considered solar time but rather clock time.

It was the international standard from 1884 to 1972 and was replaced by a new classification for telling time which is now known as Coordinated Universal Time (UTC.)

Displaying קובץ:Greenwich-mean.jpg – ויקיפדיה

Groups That Observe GMT

  • MET (England’s weather authority)
  • Britain’s Royal Navy
  • BBC World Service

The Prime Meridian for GMT is the blue line in the above image.

Countries That Observe GMT

GMT Observed Year RoundGMT Observed During Specific Seasons
Burkina Faso
Ivory Coast
St. Helena
Sao Tome and Principe
lSierra Leone
Isle of Man
Antarctica (Troll Station)   

The Origin Of GMT

In 1646, King Charles II commissioned the building of an observatory (Greenwich Observatory) where he would place astronomers to study the stars and come up with a way for the royal mariners to navigate the seas without getting lost.

John Flamstead was appointed by the king as the first royal astronomer. With only a limited supply of scientific instruments, Flamstead studied and charted the stars.

It wasn’t until 1676 that Thomas Tompion built two regulator clocks that were used by Flamstead to chart the stars positions.

Flamstead’s home had a time-ball where each day at 1:00 p.m. a ball would drop. Many set their chronometers by this time-ball dropping.

The Royal Observatory in Greenwich, England

It was discovered that there was a need to have a prime meridian where calculations of all longitude could be based from.

The world’s first Prime Meridian was developed in 1884 where all of the world’s time/time zones would originate from.

Greenwich Meridian Time became formally recognized worldwide in 1884.

The Prime Meridian served as the point from which every place on Earth was measured in distance both east and west of the line.

The east and west of the line are known as the east and west hemispheres. The Equator divides the north and south hemispheres.

The Royal Observatory

When visiting the Royal Observatory in Greenwich, England, one can have a foot in the eastern hemisphere and the other foot in the western hemisphere with the Prime Meridian being in the middle.

The Prime Meridian serves as the starting point of each time zone for the world.

It is an interesting place to visit and will give one a better understanding visually of the history behind GMT.

Reveling In The Labor

We can appreciate GMT for the extensive labor and brainpower that went into not only inventing it but for how it paved the way for the now used Coordinated Universal Time (UTC.) GMT was replaced with UTC in 1972 and is now the universal standard of time.

Fun Facts About GMT!

  • Zulu time is also known as GMT.
  • When the sun appears at the highest point centered directly above the Prime Meridian, the time at Greenwich is noon.
  • The Prime Meridian is the starting point for every one of the world’s time zones.
  • The U.S. did not adopt and use GMT until November 1, 1884. The International Meridian Conference was held in Washington DC where the International Date Line was designed and produced including the 24 time zones.
  • The International Space Station uses GMT.


(2005 April 21) Greenwich, Longitude, Prime Meridian & GMT, BBC, Retrieved from:

(n.d.) What is Greenwich Mean Time?, Royal Greenwich Museum, Retrieved from: