Question

why is zero degree longitude called as the Prime Meridian

Answer 1

Zero degrees latitude is the line designating the Equator and divides the Earth into two equal hemispheres (north and south).  Zero degrees longitude is an imaginary line known as the Prime Meridian.  Therefore, we are looking for what exists at the location where the Equator and the Prime Meridian cross each other.

Answer 2

The Zero degree longitude, also known as Prime Meridian divides the earth into Eastern and Western hemispheres. In other words, it is the point from which we measure 180 degrees to east or west.

 

This imaginary line which passes through the Royal Observatory at Greenwich in London, is the most used reference meridian on charts and maps.