The Royal Observatory in Greenwich is the home of Greenwich Mean Time (GMT) and the Prime Meridian of the world.

What is a meridian?

A meridian is a north-south line selected as the zero reference line for astronomical observations. By comparing thousands of observations taken from the same meridian it is possible to build up an accurate map of the sky.

Hemispheres

The line in Greenwich represents the Prime Meridian of the World – Longitude 0º. Every place on Earth is measured in terms of its distance east or west from this line. The line itself divides the eastern and western hemispheres of the Earth – just as the Equator divides the northern and southern hemispheres.

Where is the meridian?

In 1884 the Prime Meridian was defined by the position of the large ‘Transit Circle’ telescope in the Observatory’s Meridian Observatory. The transit circle was built by Sir George Biddell Airy, the 7th Astronomer Royal, in 1850. The cross-hairs in the eyepiece of the Transit Circle precisely defined Longitude 0° for the world. As the earth’s crust is moving very slightly all the time the exact position of the Prime Meridian is now moving very slightly too, but the original reference for the prime meridian of the world remains the Airy Transit Circle in the Royal Observatory, even if the exact location of the line may move to either side of Airy’s meridian.