What is the meridian line?
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.
Since 1884, the Prime Meridian has served as the reference point for Greenwich Mean Time (GMT).
The line runs across the courtyard of the Royal Observatory and was adopted by international agreement to the irritation of the French who continued to use the Paris meridian.