Greenwich Meridian

A meridian is a north – south line that has been selected as a zero point for astronomical observations. Astronomers use this line to compare thousands of observations and therefore creating an accurate map of the sky. The Prime Meridian was defined as the position of the large telescope in the Observatory. The cross-hairs on eyepiece on this ‘Transit Circle’ telescope are defined as 0º Longitude for the world.

The USA originally chose Greenwich as the base for its own time zone system and by 1884 over 70% of the world’s commerce was dependant on sea charts that used Greenwich as the Prime Meridian. Before this, most places kept to their own local time. There was nothing in place as to how time should be measured, when days began or ended or even how long an hour should be. As communication and commerce expanded there needed to be an international system that worked for everybody and Greenwich became the centre for world time now known as GMT or Greenwich Mean Time.

The line that you see on the floor at the top of Greenwich Park divides the eastern and western hemispheres of the Earth, the same way the equator divides the northern and southern hemispheres. Every place on Earth is measured in terms of its distance east or west of this line.