Every place on Earth is measured in terms of its distance east or west from the Greenwich Meridan. The line itself divides the eastern and western hemispheres of the Earth, just as the Equator divides the northern and southern hemispheres.
Please note: Entrance charges apply to the Meridian Courtyard and Flamsteed House, including the Time galleries (ages 5 and under go free). An adult day ticket costs £7 (£5 concessions). You can save money with a 'Big Ticket' (combined Royal Observatory, Cutty Sark and Ansel Adams admission) or an 'Astro Ticket' (Royal Observatory admission + Planetarium). Members go free.
The centre of time
Since the late 19th century, the Prime Meridian at Greenwich has served as the reference line for Greenwich Mean Time. It can now claim to be the centre of world time, and was the official starting point for the new Millennium.
22 October 2009 marked the 125th anniversary of the Greenwich Meridian as the Prime Meridian of the World.