What is a Meridian line?
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 First edition of the 'Nautical Almanac and Astronomical Ephemeris'. © NMM | Countries usually defined their prime meridian by the position of an observatory telescope, used to gather the astronomical data for making the navigational charts and tables.
The first Meridian was established when King Charles II ordered an observatory to be built in the Royal Park in 1675.
The first Nautical Almanac
In 1766, the first Nautical Almanac was published. This, used with Harrison’s chronometers, gave the mathematical information mariners required for accurate navigation. It was based on the work of successive Astronomers Royal at Greenwich.
As the use of the Nautical Almanac grew, better charts of the globe were made using its data. Use of the Greenwich as a zero meridian by mariners across the globe also grew.
Too many zero meridians
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| Too many meridian lines. © NMM | As a result of a series of international conferences held in the 1870s, it was agreed that having a multitude of zero meridians complicated the process of chart making and navigation. Countries agreed that a single prime meridian should be fixed upon, to be used by all nations.
On 1 October 1884, the United States hosted the International Meridian Conference in Washington.
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| International Meridian Conference Washington 1884. © NMM |
Representatives of 25 countries attended, and one of the aims was to fix a common prime meridian for time and longitude throughout the world.
The principle was unanimously agreed, but where the prime meridian should actually be fixed was not so easy to resolve.
After considerable argument, the Meridian line at Greenwich was chosen, for two reasons:
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| Captain James Cook. © NMM | The first accurate charts of the world were created by the great 18th century pioneering navigators such as James Cook. These charts were drawn with reference to a zero meridian at Greenwich.
- Also, at that time, 72% of the world’s shipping was using these British-made charts and the Nautical Almanac. Therefore they found their longitude with chronometers set to Greenwich Mean Time.
To this day the prime meridian of the world is still at Greenwich.
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| Greenwich Meridian Line. © NMM |
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