The Complete Guide: The Greenwich Mean Time?

The Greenwich Mean Time (or GMT) is used all around the world by many important entities such as the Royal Navy and BBC World Service. As UTC took over in recent years, new generations lost sight of what GMT means, thus why we made this little guide for everybody who needs a refresher on the subject.

What’s GMT?

The GMT (short for Greenwich Mean Time) was created in 1884 by astronomer Royal George Biddell Airy. Shortly after the timezone system was invented by Sir Sandford Fleming in 1879, it was important to find a way to solve the problem of longitude. The latter, to put it simply, was the problem that traveling cargo ships were facing when they were going from one ocean to the next. They had indeed a hard time determining their longitude, causing many issues and delays during transport.

Determining the latitude was not a problem at the time (in the 1700s). Travelers and transporters could use a sun table that would give them a rather precise idea of the latitude, depending on the sun’s altitude. That being said, it was another story when it came to understanding what was the longitude position of a ship in the middle of the ocean, a solution was urgently needed.

How did GMT solve the longitude problem of the 1700s?

Nowadays, we have very powerful satellites and GPS systems, so finding your position in the middle of the ocean is not a problem anymore. However, it took hundreds of years of evolution and some of the greeted scientific minds around to find a way to precisely determine the longitude position of a ship.

Therefore, the solution was to find a reference line that everybody would use. The latter was named the Prime Meridian or 0° longitude and was found to be located in the small town of Greenwich, United Kingdom. From there on out, it was easier for all the ships crossing the seas to find their longitude position, as they just had to use the Prime Meridian as a reference.

In our current times, you can visit the Royal Observatory of Greenwich, and learn more about the Greenwich Mean Time and how the Prime Meridian came to be.

Mean Time Vs Solar Time?

The Mean Time is a type of time that always stays the same, no matter where you are in the world. It’s basically, in other words, your typical clock time, where the hours, minutes, and seconds are not depending on any external factors. Whereas, the Solar Time, which was greatly used by Egyptians for example, is depending on the position of the sun and the time it takes the sun to cross the meridian.

Since the time that the sun takes to cross the meridian is not always the same, the Solar Time is variable. The Mean Time, on the other hand, is invariable.

Why did Greenwich Mean Time (GMT) get replaced?

There are a couple of reasons why GMT got eventually replaced by UTC (Coordinated Universal Time). Here are the main reasons:

  1. The Greenwich Mean Time is rarely precise, as it entirely depends on the time the sun is crossing the meridian. This is highly variable and needs to take into account the Day Saving Time.
  2. The UTC system uses precise atomic clocks that give a universal time to every timezone in the world.
  3. The UTC system takes into account the imperfections, such as the shape of the Earth, and the rotation of the latter. Additionally, with the UTC time, the Day Saving Time doesn’t need to be taken into account.

Many countries in recent years are thinking about getting rid of the Day Saving Time system. This system forces the clocks of said countries to move one hour ahead of time and to eventually do the opposite. This allows for the sunrise and sunset to happen later during the day, and vice versa.

Why GMT is UK’s time standard?

The United Kingdom still uses the GMT system in some regions, but not all of them. It is a bit complicated to know when and where UTC or GMT is used in the UK. Lucky for us, our digital clocks do the work automatically for us! But if you want to know, most railways and transportation companies adopted the GMT clock system in the past, and a lot of them still use it to this day.

As an extra note, the clocks in the UK go forward 1 hour on the last Sunday of March and goes back 1 hour on the last Sunday of October. When the clocks are 1 hour ahead of time, it is called the British Summer Time or BST, and when the clocks go back 1 hour, it is simply called the Greenwich Mean Time.

What countries still use GMT in 2021?

As said before, the UTC system took over the world, as it is just a more precise system no matter what time zone you live in. That said, many countries still use Greenwich Mean Time as their civil time clock, such as:

  • Malaysia
  • Australia
  • United Kingdom
  • Ireland
  • South Africa
  • New Zealand
  • India
  • Pakistan
  • Iceland
  • Greenland
  • Isle of Man

Please take note that you shouldn’t worry about whether or not the country you’re visiting is using GMT or UTC, as they are sensibly the same. They are divided among the same time zones, and they give almost exactly the same time, it’s just that one is a lot more precise than the other.

How to find 0° longitude?

Now you can find this on pretty much any virtual world map automatically, but if you want to see the Prime Meridian from your own eye, know that it is absolutely possible to do so. In England, you can visit the famous city that gave its name to the GMT system, the city of Greenwich. Near Greenwich’s Royal Observatory, you can actually find a giant line that separates our planet Earth into two hemispheres.

Many tourists are visiting this town to see the Prime Meridian in all its glory. If you ever have the chance to travel to England, the Royal Observatory is certainly one thing you shouldn’t miss!