In 1675 King Charles II commissioned the Royal Observatory. The foundation stone was laid on the 10th August 1675 at Greenwich (prounced GREN-itch).
The purpose of the post was to:
apply himself with the most exact care and diligence to the rectifying of the tables of the motions of the heavens, and the places of the fixed stars, so as to find out the so much desired longitude of places for the perfecting of the art of navigation.
Hence the term Greenwich Meridian or Prime Meridian, the starting point for measuring longitude.