The Greenwich Royal Observatory was established by King Charles II in 1675. The initial building, Flamsteed House, was designed by Sir Christopher Wren.
In 1884 most delegates to an international conference agreed that Greenwich should be adopted as the Prime Meridian of the world, Longitude Zero (0° 0' 0"). This line is marked by a metal strip running through the courtyard. By standing over this line, you can be in both the eastern and western hemispheres at the same time.
Every place on the Earth is measured in terms of its angle east or west from this line (longitude), just as the Equator divides the northern and southern hemispheres (latitude). Latitude and Longitude are used on ships to determine where they are.
Latitude was determined by measuring the height of the sun above the horizon. Longtitude was determined by keeping to clocks, one on local time and the other on a standard time (now GMT) and comparing the difference. Given that an error of only a few minutes could result in shipwreck, the creation of an accurate shipboard clock was a matter of vital research for many years.
The Greenwich Observatory is also sometimes described as being at the center of world space and time and was the first place to observe the new millennium. Greenwich was chosen as the site for the UK's Millennium Exhibition, comprising mainly of the Millennium Dome. The building stood empty for years after but is now The O2 entertainment venue.
GMT is mean solar time, with midday defined as the time at which the sun crosses the Greenwich Meridian, 0 degrees longitude.
Watch the Ball Drop
The red ball on top of Flamsteed house drops at 1pm GMT each day (under midday is defined as the time at which the sun crosses the Prime Meridian). Countdowns to the drop are always good with children. Find out more about the Greenwich Prime Meridian.
Other Buildings at the Royal Observatory
The Altazimuth Pavilion and the South Building were built between 1772 and 1897 and now house a collection of historical astronomical instruments and a planetarium. The Peter Harrison Planetarium opened in May 2007 and features Europe's first digital planetarium projector.
Before leaving the observatory grounds, look to the East to see Vanbrugh Castle. This castle, with its fairy-tale towers and turrets, lies just outside the park on Maze Hill. It was designed in 1719 by architect and playwright Sir John Vanbrugh (1664-1726) as his home.