Explore the history of time and astronomy at the Greenwich Observatory, the home of Greenwich Mean Time. Here the Meridian that divides the earth's eastern and western hemispheres passes right through, where millions of visitors have been photographed standing with a foot on either side. At 1 o'clock every day since 1833, the time-ball, a red ball on a rod on one of the building's turrets, falls so that sailors on ships on the Thames could set their clocks by it.