Illuminated Meridian Line at night, Royal Observatory, Greenwich, 1993 (photo)

The meridian line in Greenwich represents the Prime Meridian of the world, Longitude Zero (0° 0' 0"). Every place on the Earth is measured in terms of its angle east or west from this line. To stand astride the line is to have one foot in the eastern and one foot in the western hemisphere of the earth - just as the Equator divides the northern and southern hemispheres. The Prime Meridian at Greenwich passes through a massive special telescope called a transit circle. The transit circle was built by Sir George Airy, the seventh Astronomer Royal, in 1850. The cross-hairs seen in the eyepiece of this transit circle define Longitude 0° for the world.