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.
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.