The meridian in Greenwich is the Prime Meridian; a line marking Longitude 0, running from North to South Pole, straight through Greenwich. Every location on Earth is measured in terms of its distance east or west of this line.

The Prime Meridian is also the centre of world time, serving as the reference point for Greenwich Mean Time (GMT), which provides a standard measurement for how time is adjusted in each different time zone.

In the 18th century global trade was dependent on shipping, so it was vital that there was a way to navigate safely at sea. Sailors carried complex instruments like sextants and chronometers to find their way. Astronomers were also developing star charts that enabled sailors to plot their position using the moon and stars. By comparing thousands of observations taken from the same meridian it was possible to build up an accurate map of the sky.

In 1884 the Royal Observatory in Greenwich was chosen as the location for the Prime Meridian at an international conference. The campaign for Greenwich was led by Sir George Biddell Airy, Astronomer Royal from 1835 to 1881. Greenwich was selected because 72% of the world’s commerce depended on sea charts that already used this as their reference point.