What is meridian?

Meridian lines are imaginary coordinates running from pole to pole that are used to measure distances east and west on the Earth's surface and in the Heavens. Meridian lines are also known as lines of longitude. The word meridian comes from Latin word meridies, meaning "the mid-day", because the Sun crosses a local meridian at noon. Certain meridians became important because astronomers used them in observatories when they set up their telescopes for positional astronomy. This means that all their measurements of the sky and the Earth were made relative to their meridian. Until the end of the 19^th century there were a number of national meridians in observatories in Paris, Cadiz, and Naples.

In 1884 there was an international conference in Washington to agree a single Zero Meridian, or Prime Meridian for the world. The meridian running through the Airy Transit Circle - a telescope mounted so that it rotated in a north/south plane - at the Royal Greenwich Observatory outside London was chosen. This choice was largely a matter of convenience. Most of the shipping charts and all of the American railway system used Greenwich as their longitude zero at the time. South of Greenwich the Prime Meridian crosses through France and Africa, and then runs across the Atlantic Ocean all the way to the South Pole.

In 1850 the 7^th Astronomer Royal of Great Britain, Sir George Biddle Airy (1801-1892), decided he wanted a new telescope. In building it, he moved the previous Prime Meridian for England 5.75 m (19 ft) to the east. The Greenwich Meridian is marked by an illuminated line, which bisects Airy's Transit Circle at the Old Royal Observatory (now a museum). (above)