World Time Starts Here

What is Greenwich Mean Time (GMT)?

Setting a standard for world time

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Greenwich Mean Time or Meridian Time became a time standard since 1884. It was chosen because the Prime Meridian (the line of 0° Longitude) runs through Greenwich.

Greenwich was a royal park and palace on a hill to the south of the River Thames east of London .

In 1675 the great race to create accurate maps for navigators had begun and Charles II offered the land to The Royal Society for Britain's first national observatory. Christopher Wren was commissioned to design the domed building and John Flamsteed was appointed the Astronomer Royal. British mapmakers began to set Longitude from Greenwich and in 1884 it was adopted as the international meridian or starting point.

Every 15° longitude represents one hour's difference in time: (24 x 15 = 360, the degrees of a circle). You can work out the time at every location on earth if you know how many degrees it is east or west of Greenwich.

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The Greenwich Meridian (Prime Meridian or Longitude Zero degrees) marks the starting point of every time zone in the World. GMT, the abbreviated form of Greenwich Mean (or Meridian) Time, is the mean (average) time that the earth takes to rotate from noon-to-noon.

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The Prime Meridian represents the basis of every world Time Zone of the time zone map. Most time changes are measured by GMT. Although GMT has been replaced by atomic time (UTC) it is still widely regarded as the correct time for every international time zone.

Check out the GMT timestamp to see how accurate your computer time is.

Where is Greenwich , England ?

  • Longitude 0° 0' 0"
  • Latitude 51° 28' 38"N (North of the Equator)

Greenwich Mean Time is international time, the basis of the world time clock. It defines date and time and the exact time.

Greenwich Mean Time in Space

STS-114 Picture of International Space Station (NASA)

Greenwich Mean Time (GMT) is the time used on the International Space Station.

Greenwich Mean Time (GMT) is also known as Zulu Time .

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