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.
GMT is mean solar time, with midday defined as the time at which the sun crosses the Greenwich Meridian, 0 degrees longitude.
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. See this page for more details.
