The Prime Meridian is the imaginary line that divides the Earth into two equal parts: the Eastern Hemisphere and the Western Hemisphere. It is the line of longitude that goes through the Royal Observatory, Greenwich in London. It is also known as the International Meridian or Greenwich Meridian. In 1884, the Greenwich Meridian became the Prime Meridian for the whole world. The Prime Meridian is the basis for the world’s time zones.