Having an interest in where the stars are at during any given point of time, compelled me to make a journey to Greenwich Observatory, during our visit to London.  I was in search of Ground Zero or the Prime Meridian, also known as the International Meridian or Greenwich Meridian.  It is the meridian at which longitude  is defined to be 0 degrees. The prime meridian, and the opposite 180th meridian (at 180° longitude), which the International Date Line generally follows, separates the eastern and western hemispheres.

Unlike the parallels of latitude, which are defined by the rotational axis of the Earth (the poles being 90° and the Equator, 0°), the prime meridian is arbitrary, and multiple meridians have been used through history as the prime meridians of various mapmaking systems. The Greenwich Meridian was agreed upon as the international standard in October 1884. At the behest of U.S. President Chester A. Arthur, 41 delegates from 25 nations met in Washington, D.C., USA, for the International Meridian Conference. France abstained when the vote was taken, and French maps continued to use the Paris Meridian for several decades.