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Greenwich Time (GMT).

Greenwich Mean Time (GMT) .

It is a time scale based on the Earth’s rotation around itself in one day based on the longitude that passes through the town of Greenwich in Britain, which was adopted as a reference point for timing. Consider it the time scale at zero point. All that lies east of the Greenwich line is counted as time (+) and the time of all that lies west of the Greenwich line counts as time (-).

Greenwich Mean Time (GMT) is the mean solar time at the Royal Observatory in Greenwich, London, calculated from midnight. At various times in the past, it has been calculated in various ways, including counting from the back; As a result, it cannot be used to determine an exact time unless context is given.

English speakers often use GMT as a synonym for Coordinated Universal Time (UTC). For navigation, it is equivalent to UT1 (modern form of mean solar time at 0° longitude); But this meaning can differ from UTC by up to 0.9 seconds. Thus the term GMT should not be used for certain technical purposes that require accuracy.

Because of the Earth’s unequal angular velocity in its elliptical orbit and axial tilt, noon (12:00:00) GMT is rarely the exact moment the sun crosses the Greenwich meridian and reaches the highest point in the sky there. This event may occur up to 16 minutes before or in the afternoon GMT, a discrepancy calculated by the time equation. Noon GMT is the annual mean (ie, “average”) of the moment of this event, which represents the word “mean” in “GMT”.

Originally, astronomers considered the day of Greenwich Mean Time to begin at noon, whereas for almost everyone it begins at midnight. To avoid confusion, the name Universal Time has been introduced to refer to Greenwich Mean Time as calculated from midnight. Astronomers preferred the old convention to simplify their observational data, so that each night was recorded under a single calendar date.

Today, Coordinated Universal Time usually stands for UTC or UT1. The term “GMT” is used in particular by bodies associated with the United Kingdom, such as the BBC World Service, the Royal Navy and the Met Office; And others, especially in Arab countries, such as Middle East Broadcasting Center and OSN. It is a term commonly used in the United Kingdom and Commonwealth countries, including Australia, New Zealand, South Africa, India, Pakistan, Bangladesh and Malaysia; and in many other countries in the Eastern Hemisphere.

How did Greenwich Mean Time begin?

It wasn’t until the invention of the pendulum clock in the 1650s that it was possible to work out the relationship between mean (clock) time and solar time.

John Flamsteed came up with the formula for converting solar time to mean time, and published a set of conversion tables in the early 1670s. Soon after, he was appointed as the first Astronomer Royal and moved into the new Royal Observatory in Greenwich.

Here he had the best pendulum clocks installed and set them to the local time. This was Greenwich Mean Time, or the average time when the Sun crossed the meridian at Greenwich. At first though, Greenwich time was only really important to astronomers.

GMT and the quest for longitude.

In the 1700s, the fifth Astronomer Royal Nevil Maskelyne brought Greenwich Mean Time to a wider audience.

In 1767 Maskelyne introduced the Nautical Almanac as part of the great 18th century quest to determine longitude.

These were tables of ‘lunar distance’ data based on observations at Greenwich and using GMT as the time standard. This data enabled navigators to find their position at sea.

GMT was also crucial to the other great solution to the ‘longitude problem’, represented by John Harrison’s famous timekeepers.

British mariners started keeping at least one chronometer set to GMT. This meant they could calculate their longitude from the Greenwich meridian (longitude 0° by convention).

These two solutions would help pave the way for GMT to become the worldwide time standard a century later.

How did railways lead to GMT becoming the UK time standard?

Until the mid-19th century, almost every town kept its own local time, defined by the Sun. There were no national or international conventions which set how time should be measured.

This meant there was no standard timings for when the day would begin and end, or what length an hour might be. As well as Greenwich Mean Time for example, there was also Bristol Mean Time (10 minutes behind GMT) Cardiff Mean Time (13 minutes behind GMT).

However, the 1850s and 1860s saw the expansion of the railway and communications networks. This meant the need for an national time standard became imperative.

British railway companies started introducing a single standard time across their networks, designed to make their timetables less confusing. It was mostly Greenwich Mean Time that they used. GMT was ultimately adopted across Great Britain by the Railway Clearing House in December 1847. It officially became ‘Railway Time’.

By the mid-1850s, almost all public clocks in Britain were set to Greenwich Mean Time and it finally became Britain’s legal standard time in 1880.

How did Greenwich Mean Time become the international standard?

In 1884 the Greenwich Meridian was recommended as the Prime Meridian of the World.

There were two main reasons for this. The first was that the USA had already chosen Greenwich as the basis for its own national time zone system. The second was that in the late 19th century, 72% of the world’s commerce depended on sea-charts which used Greenwich as the Prime Meridian.

The recommendation was based on the argument that naming Greenwich as Longitude 0º would be of advantage to the largest number of people.

Reference :

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