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Greenwich and the story of time

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The Prime Meridian at Greenwich

What is a Meridian line?

First edition of the 'Nautical Almanac and Astronomical Ephemeris'.
View full size imageFirst 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

Too many meridian lines
View full size imageToo 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.

International Meridian Conference Washington 1884
View full size imageInternational 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:

To this day the prime meridian of the world is still at Greenwich.

Greenwich Meridian Line
View full size imageGreenwich Meridian Line. © NMM
Prime Meridian at Greenwich
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