Time Zone Conversion

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Greenwich Mean Time

 
Greenwich Mean Time (GMT) is recognised all over the globe, by millions of people. But what exactly is GMT, how did it come into existence and do we still need it in today's hi-tech society? Read on to discover the answers! What is GMT?

GMT is the mean solar time at the prime meridian (the line of longitude where east meets west), which runs through Greenwich in the UK. GMT is measured from midnight, so the new day begins at 0000 GMT and the time six hours later would be expressed as 0600 GMT. "Mean solar time" refers to the local time on the prime meridian, using the hypothetical rather than the actual position of the sun. The earth does not orbit the sun evenly, and its axis is tilted in relation to the sun. In the course of one solar day, the earth spins once on its axis and it takes 365 days to complete one full revolution of the sun. However, the relative position of the sun varies throughout the year. At particular times it apparently slows down or speeds up - shortening and lengthening the day by up to 16 minutes. That's why we need to use the average (mean) day length and a hypothetical mean sun when calculating GMT.

Prior to 1925, GMT for sailors and astronomers was measured from midday, while the "civil day" began at midnight. This civil time was also known as GMT, which made for much confusion! In 1925 it was agreed that the two GMTs should be synchronised. Time was thereafter measured from midnight to midnight.

Does Universal Time (UT) differ from GMT?
Universal Time (UT) is really a new name for GMT, and the term UT is officially preferred as it avoids the ambiguity of GMT. From 1928, GMT measured from midnight became known internationally as universal time. By 1972, atomic clocks had been developed which could keep an incredibly accurate standard time, which could be used throughout the world. GMT is not quite in line with this atomic time, because the scientific definition of a single second has changed. A second is no longer defined as a fraction of a mean solar day but by measuring the frequency of radiation of caesium. GMT is thus no longer the world's time standard, nor is it used for scientific purposes. In practise, many people use the terms GMT and UT interchangeably, and GMT does still form the legal basis for UK civil time.

How did GMT come about?
Where we are in the world determines when the sun rises and sets. Locations with different longitudes - i.e. those separated eastward or westward - have different local times. Their distance apart determines the degree of difference. Until the nineteenth century, these relationships were not formalised and clocks were simply set to local time. Midday was simply the point when the sun was as its zenith, thus local time in London was twenty minutes ahead of a clock at Lands End. The local time was more advanced the further west one went. At this stage there was not even consensus on when the day should begin, or the precise length of an hour. Travellers simply adjusted their pocket watch to local time whenever they reached a new town. As the railways burgeoned, long-distance travel increased and telegrams appeared on the scene, "local time" increasingly impeded the smooth running of society. Rail timetables, for example, were impossible to draw up as there was no standard time which did not rely on location.

To ease these logistical and communications problems, 25 nations met in 1884 with the intention of establishing a framework for universal time. The leaders who attended the Washington Meridian Conference agreed that Greenwich should be declared the prime meridian, and GMT was accepted as the global time standard. Following the conference, international time zones were established relative to GMT.

Why Greenwich?
Today Greenwich is busier than ever. Annually, thousands of visitors come to see the Royal Observatory and to enjoy the view over the River Thames as it winds away from central London. For all its charms, you might be forgiven for wondering why the world chose to sets its watch by an unremarkable corner of south-east London! By 1884, the observatory at Greenwich had established itself as an accurate timekeeper and centre of excellence for astronomical research. Since 1833, the "Time Ball" on top of Flamsteed House had been in use as a public time signal - providing a time check for ships on the Thames and for the capital. In 1880, four years before the conference, GMT had been adopted as the single time standard for the UK, and the USA had also chosen to calibrate national time using GMT. In the late 1900s, Greenwich was immensely important for shipping, and almost three quarters of the world's commerce relied on nautical charts which marked Greenwich as the Prime Meridian.

So Greenwich was an obvious choice - but there were those who opposed its promotion to the status of Prime Meridian! At the conference, San Domingo voted against, while France and Brazil abstained. Algeria referred to the new time standard as "Paris Mean Time diminished by 9 mins 21 secs" - this amounted to GMT, but avoided the word Greenwich! France persisted with the Paris Observatory Meridian until 1911.
 
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