Sometime in the 1700's ships of the English navy realized they needed to determine where they were in relationship to a known location. In order to accurately locate an island, country or continent to a specific location on the globe, they had to establish a beginning point, somewhere. They could have used the one used by the French, establishing Paris as the Zero mean latitude, but who would want that?

So they choose Greenwich observatory, just down the river from London, as being the Zero mean longitude. With that, they could determine how far, or how many degrees east or west they were from Greenwich, UK.

It was established by Sir George Airy in 1851. By 1884, over two-thirds of all ships and tonnage used it as the reference meridian on their charts and maps. In October of that year, at the behest of US President Chester A. Arthur, 41 delegates from 25 nations met in Washington, D.C., United States, for the International Meridian Conference. This conference selected the meridian passing through Greenwich as the official prime meridian due to its popularity. However, France abstained from the vote, and French maps continued to use the Paris meridian for several decades. In the 18th century, London lexicographer Malachy Postlethwayt published his African maps showing the "Meridian of London" intersecting the Equator a few degrees west of the later meridian and Accra, Ghana.