The Shepherd gate clock can be seen at the gates to the Royal Observatory. The meridian line is marked by the cross-hairs in the Airy Transit Circle eyepiece.įind out more about the Airy Transit Circle The first clock to show GMT to the public Therefore this also became the start of the Universal Day. It was recommended that the meridian line would indicate 0° longitude. Astronomer Royal George Biddell Airy designed it, and it is located at the Royal Observatory Greenwich. The Airy Transit Circle (telescope) became the telescope that would define the Prime Meridian of the World. The recommendation was based on the argument that naming Greenwich as Longitude 0º would be of advantage to the largest number of people.Īs the reference for GMT, the Prime Meridian at Greenwich therefore became the centre of world time and the basis for the global system of time zones. 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 first was that the USA had already chosen Greenwich as the basis for its own national time zone system. In 1884 the Greenwich Meridian was recommended as the Prime Meridian of the World. How did Greenwich Mean Time become the international standard? It officially became 'Railway Time'.īy 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. GMT was ultimately adopted across Great Britain by the Railway Clearing House in December 1847. It was mostly Greenwich Mean Time that they used. This meant the need for an national time standard became imperative.īritish railway companies started introducing a single standard time across their networks, designed to make their timetables less confusing. However, the 1850s and 1860s saw the expansion of the railway and communications networks. 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). There were no national or international conventions which set how time should be measured. Until the mid-19th century, almost every town kept its own local time, defined by the Sun. These two solutions would help pave the way for GMT to become the worldwide time standard a century later.įind out about the longitude problem How did railways lead to GMT becoming the UK time standard? This meant they could calculate their longitude from the Greenwich meridian (longitude 0° by convention). GMT was also crucial to the other great solution to the ‘longitude problem’, represented by John Harrison’s famous timekeepers.īritish mariners started keeping at least one chronometer set to GMT. This data enabled navigators to find their position at sea. These were tables of ‘lunar distance’ data based on observations at Greenwich and using GMT as the time standard. In 1767 Maskelyne introduced the Nautical Almanac as part of the great 18th century quest to determine longitude. In the 1700s, the fifth Astronomer Royal Nevil Maskelyne brought Greenwich Mean Time to a wider audience. At first though, Greenwich time was only really important to astronomers. This was Greenwich Mean Time, or the average time when the Sun crossed the meridian at Greenwich. Here he had the best pendulum clocks installed and set them to the local time. Soon after, he was appointed as the first Astronomer Royal and moved into the new Royal Observatory in Greenwich. 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.

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. Greenwich Mean Time is also the name of the time zone used by some countries in Africa and Western Europe, including in Iceland all year round. Though it has now been replaced by Coordinated Universal Time (UTC), GMT is still the legal time in Britain in the winter, used by the Met Office, Royal Navy and BBC World Service. From 1884 until 1972, GMT was the international standard of civil time. GMT stands for Greenwich Mean Time, the local clock time at Greenwich. Today GMT is reckoned from one midnight to the next.įind out about the Prime Meridian What does GMT stand for? It’s a way of standardising and regularising time so we can all know exactly what time it is for our (or anyone’s) location. Solar time varies throughout the year, as the time interval between the Sun crossing a set meridian line changes.īut each day measured by a clock has the same length, equal to the average (mean) length of a solar day. Essentially, mean time is clock time rather than solar (astronomical) time.

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