The Greenwich meridian was designated as such by the Convention of 1884.
The parallels of latitude are fixed by the rotation of the Earth. The only human involvement is the use of a 360 degree circle, starting at the equator. The degrees of longitude, although using the same 360 framework, could be commenced from virtually anywhere on the planet – wherever the cartographer so designates, so long as the line runs from the North Pole to the South Pole, or vice versa. Throughout the modern history of map-making, numerous locations have been utilized as the zero or prime meridian, from which all other longitudes on the map (or set of maps) are to be measured. Some of those earlier prime meridians were (in alphabetical order): Alexandria, Berne, Brussels, Copenhagen, Jerusalem, Kyoto, Lisbon, Madrid, Mecca, Paris, Rome, and Washington. By the mid-nineteenth century, the vast majority of nautical charts utilized either Greenwich, England or Paris, France as the prime meridian, with the Greenwich meridian being the clear favorite. It wasn’t so much that one location was better than the other. It was just that the British Admiralty published a more extensive array of charts (and possibly had a better marketing program) than did the French Government. Finally, in 1884, US President Chester A. Arthur convened an international conference in Washington, DC to select an agreed prime meridian. Of the 25 nations represented at the conference, 24 voted in favor of the Greenwich meridian. The Government of France abstained and French maps continued to use Paris as the prime meridian for several decades. Opposite of the prime meridian is the 18oth meridian (at 180° longitude). The international date line generally follows the 180th meridian, but is adjusted to account for national boundaries. There have been four Greenwich meridians. The first Astronomer Royal, John Flamsteed, established oneat the Royal Observatory in 1675. This was refined by the Third Astronomer Royal, Edmund Halley, in 1725. James Bradley, another Astronomer Royal moved the line yet again in the mid-eighteenth century. Finally, yet another Astronomer Royal, Sir George Airy, relocated the line slightly in 1851. The movements were precipitated mostly by the placement of new and more accurate measuring equipment, not by any perverse desire to eclipse the predecessors.