GMT: The World's First Universal Way of Telling Time

The world's time is divided into 24 time zones and is run based on offsets from Universal Coordinated Time (UTC). So, for example, if you lived in Central Europe, you would be within the UTC +01:00 time zone. This current basis for telling time around the world was originally based on Greenwich Mean Time, or GMT.

The name of this original standard comes from the location where it was conceived and implemented which is the Royal Observatory in Greenwich, London. This location became the line of zero degrees longitude, otherwise known as the Prime Meridian. All of the world's time zones are measured from this point.

How To Convert From Your Time Zone to GMT

The process of converting from your time zone to GMT isn't difficult at all actually. Since Greenwich Mean Time is the same (give or take .9 seconds) as UTC +00:00, all you need to do is add or subtract hours to your UTC offset to bring it back to zero. Here, let's practice, shall we?

If you want to figure out what it is in GMT now, simply find out your time zone's UTC offset and bring it to zero by adding or subtracting from the current time based on that offset. Of course, you could also use a time zone converter if you'd rather not do the math.

The Difference Between GMT and UTC

The major difference between Greenwich Mean Time and Universal Coordinated Time is the concept of each one. The former is a physical time zone set at zero degrees longitude, while the latter is not a time zone at all, but a time standard.

An easier way to understand it is that Greenwich Mean Time is located at UTC+00:00. Knowing this, the difference becomes clear. Despite the fact that it is no longer used as a universal standard, the time zone is still used by various countries within Europe, Africa, and North America.

This zone is still used as a time standard for astronomical purposes. For this usage, it is listed as UT1. UTC is always kept within .9 seconds of Greenwich Mean Time through the addition or subtraction of leap seconds.

So, in short, Universal Coordinated Time is a concept, a standard if you will. Greenwich Mean Time on the other hand is an actual time zone located at UTC 00:00.

Examining Time Zones Outside of Greenwich Mean Time

Say we move away from this centralized time zone. If we examine the locations within GMT +08:00, the same as UTC +08:00, we find ourselves in a very different part of the world. Here are the placesĀ found within this time zone:

These locations do no observe any kind of Daylight Saving Time or Summer Time. They remain within this offset throughout the year. If we flip the tables and instead examine GMT -08:00, we find ourselves on the opposite side of the Prime Meridian since we are now subtracting time from the center and moving left.

Looking at this offset, the equivalent to UTC -08:00, we find ourselves in these locations:

As you can see, moving further away from the Greenwich Mean Time zone causes us to find other time zones in various parts of the world.

Why Did We Switch to UTC?

The reason Greenwich Mean Time was first employed was out of necessity. With railroads moving people across hundreds of miles, it became apparent that 2:00 pm in one location was not the same time of day in another. This was a result of the Earth's natural rotation. In 1884, this time standard was first adopted by creating time zones that had time added or subtracted to them from the standard which was located at the Royal Greenwich Observatory.

The world switched to UTC as the standard for telling time when a new technology made telling time a far more accurate process. Greenwich Mean Time was based on solar time, or the apparent movement of the sun through the sky. Since the Earth has an uneven speed to its orbit, this caused fluctuations in the accuracy of the standard.

When the atomic clock was first created, scientists were able to create a far more accurate time standard. In 1972 the switch was made, but Greenwich Mean Time is still used as the time zone for the locations within its purview.

What are Leap Second Used For?

Greenwich Mean Time is still used as a standard for astronomical purposes. It is a solar based time after all, but it goes by UT1 to avoid confusion when used by scientists. It is the official time used by the International Space Station as well.

As previously mentioned, the Earth's speed of rotation varies, and this causes UT1 to drift away from atomic time kept by UTC. In order to correct this gap, leap seconds are added or subtracted. This system of keeping the two times within the same realm began in 1972. Since that time, there have been a total of 25 leap seconds inserted.

The most recent leap second was added in June of 2012 and the next one is expected to be added in June of 2015.

Final Thoughts

While GMT was the basis for all time standards, it wasn't something that we could still use today. As a time zone set at UTC 00:00 and a method of calculating astronomical findings, it lives on as a reminder of how far we've come in our methods of telling time.