The prime meridian is the zero degree line of longitude that passes near London. The prime meridian was officially established as zero degrees longitude at an international conference in 1884. While latitude has its zero degree line along the equator, an actual physical feature, longitude is not based on any physical feature of the earth and is not impacted by the sun or the earths orbit.
Prior to 1884, some countries utilized local prime meridians to establish their coordinate systems within their country and around the world. In order to have any x,y system of coordinates, as latitude and longitude are, there must be a starting place for both the north-south and the east-west axes.
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Because british was ruling almost the whole world, and it was often decribed as an empire where the sun never set, right from east to west, and Greenwich the imaginary line was established by the british as it was in U K