Earlier it was considered as a time reference according to which world time could be calculated. In many countries, still, it is in use and it is being calculated with respect to 0° Meridian which is a north to south longitude that passes through Greenwich. The longitude of this location is 0° 0' whereas the latitude is 51º 28' 38"N. The time calculated with respect to this is called GMT but do you know the GMT full form. In this article, we will be talking about this time zone. We will cover what is the full form of GMT, its history, or how to calculate it with the help of an example along with GMT full form in time context. 

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What is Full Form of GMT?

If you're asked GMT full form in English or GMT long form, it simply refers to Greenwich Mean Time which is calculated at 0° prime meridian which is also called Greenwich meridian. It is called a mean solar time which is calculated with respect to Royal Observatory established at Greenwich, London. The meridian is a geographical reference line to calculate the mean solar time. Usually, it is called GMT and GMT stands for "Greenwich Mean Time". The Sun is said to be above this 0° Greenwich Meridian at 12pm and when it shifts towards the western side or eastern side, the time changes with every 1° and every 1° is equal to 4 minutes. The time of any location which is calculated like this with respect to 0°, is known as GMT. 

15 degrees = 1 hour

1 degree = 4 minutes

1 minute = 4 second

We can expand GMT as Greenwich Mean Time which is related to the mean solar time. There is one more GMT full name which is Greenwich Meridian Time because of its relation to 0° Prime Meridian which is also called Greenwich Meridian. If the location is on the western side of the 0° longitude then it is called it is behind the Greenwich time whereas if it is located on the eastern side, then its time is ahead of the Greenwich.

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History

When the United Kingdom developed as a maritime nation, the Mariners of Britain used to keep at least a chronometer on GMT in order to calculate their longitude with respect to Greenwich Meridian considered being having zero degrees of longitude according to a convention adopted in the 1884 conference i.e International Meridian Conference. The chronometer synchronisation on GMT didn't affect the shipboard time that was still having solar time but the usage of this practice along with other nations' Mariners led to the usage of GMT worldwide and a lot of time zones were being set on the offset of the number of hours with respect to GMT either ahead of GMT or behind GMT. 

The atomic clocks are said to constitute much stable time because the rotation of our planet Earth is irregular and also has a slowing trend. Therefore, GMT was succeeded by another international standard i.e Coordinated Universal Time on 1 January 1972, which is being maintained by ensembling the atomic clocks around the whole world. From 1884 until 1972, GMT was considered the international standard time for civil time. Now GMT has become a time zone and is not a time reference but in some countries, it is still used.

How to Calculate GMT?

The time of any country or location with respect to GMT can be calculated with the help of the following steps:

• To calculate the GMT time, firstly you need to check the number of longitudes between the location you want to find the time of and the GMT which is 0° longitude, and count the number of longitudes between the two locations. 

• Then, with the help of a world map, you need to check whether the location is on the left or right side of Greenwich. If the location is on the western side, then you will have to minus the time and if the location is on the eastern side, then you have to add the time.

• You need to remember the following in order to calculate the time:

15 degrees = 1 hour

1 degree = 4 minutes

1 minute = 4 second

East = Add the time

West = Minus the time

Example

Suppose, location A is on the eastern side at 15° longitude and GMT time right now is 9 pm. How will you calculate the time of location A with respect to GMT?

Solution:

 Given;

GMT = 9pm

We know that, 

1° = 4 minutes

15° = 4 × 15 = 60 minutes

Location A = 15° longitude = 60 minutes 

It is on the eastern side of GMT and thus, need to be added to the time.

 = 9pm + 60 minutes

= 10pm

Thus, the time of location A with respect to GMT is 10 pm which shows that it is 1 hour ahead of GMT.

Full Form of IST and GMT

They are considered as two different time zones. Here, GMT stands for Greenwich Meridian Time whereas IST refers to Indian Standard Time. The former is the mean solar time which is observed at Royal Observatory established in Greenwich, London whereas the latter is the time which is observed in India. The former is considered as equal to modern mean solar time form i.e UT1 at 0° longitude whereas the latter is being calculated based on Mirzapur clock tower. The latter is considered as 5 hours 30 minutes ahead of the former.

Conclusion

In this article, we did a discussion about the GMT time zone and we covered GMT full form, meaning, history, how to calculate GMT time, and examples to understand it better. The time which is being calculated with respect to 0° Meridian which is called Prime Meridian or Greenwich Meridian which is a north to south longitude that passes through Royal Observatory at London, is simply known as GMT. Nowadays, all the time zones of the Earth are being set as per Coordinated Universal Time ( UTC ) which is used to calculate time and set clocks of the world and considered as the successor of GMT.