Why the prime meridian runs through Greenwich
Longitude needs a starting line, and nothing in nature supplies one. Latitude has the equator, fixed by the earth's rotation. A zero meridian is a matter of choice, and for centuries different people chose differently. Ptolemy measured from the Fortunate Islands, at the western edge of the known world. Later European cartographers often used the island of Ferro, now El Hierro, the westernmost of the Canary Islands.
By the nineteenth century most seafaring nations used the meridian of their own national observatory. British charts counted from Greenwich and French charts from Paris, and charts made elsewhere used Cadiz, Lisbon, Saint Petersburg and other places. A sailor with charts from several countries had to convert between them.
The conference
In October 1884 delegates from twenty-five countries met in Washington, DC, for the International Meridian Conference, called by the US president, Chester Arthur. The conference resolved that a single prime meridian should be adopted and chose the one passing through the transit instrument at the Royal Observatory in Greenwich. The vote was twenty-two in favour, one against (San Domingo, now the Dominican Republic) and two abstentions, France and Brazil.
A large share of the world's shipping already used Greenwich-based charts, which was the main argument for it. France, which had argued for a neutral line, kept using the Paris meridian in its own mapping for some time. French law defined official time by Paris Mean Time until 1911, when it moved to Greenwich, with the legal text carefully worded as Paris time retarded by nine minutes and twenty-one seconds.
The line today
The Greenwich meridian is still marked in the courtyard of the Royal Observatory, and visitors stand astride it. Modern positioning does not use the Greenwich instrument, though. The international reference meridian is defined from satellite measurements, and it lies about a hundred metres east of the brass line in the courtyard.