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

In 1884 the Prime Meridian was defined by the position of the large “Transit Circle” telescope in the Observatory’s Meridian Building. 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.