For many years, different countries measured longitude from different meridians. The French and Algerians, for example, used the Paris Meridian; the Spanish measured from one that passed through Isla del Hierro . By the 1880s, many people could see the advantages of measuring from a single meridian.
By international convention it runs through "the primary transit" instrument (main telescope) at the Royal Observatory in Greenwich.
Why does the Prime Meridian (Zero Longitude) pass through Greenwich?
The International Meridian Conference took place in October 1884 in Washington DC. Twenty-five nations were represented at the conference by 41 delegates. The Greenwich Meridian was chosen to become the Prime Meridian of the World. There were several reasons for this; the main one being that nearly two thirds of the World's ships were already using charts based on it.