It was the best imaginary line I've ever visited and it was an extreme pleasure to be there. The museum at Greenwich is, as of this writing, free for visitors and includes a fantastic exhibition.
At the museum, one can see the various models of John Harrison's Chronometer, the device that allowed for the accurate measurement of longitude at sea. It is incredible to see these works of artistic and mechanic perfection in person.
The museum utilizes a variety of media to communicate its ideas - there are a collection of cabinets at perfect child-height that hide interesting dioramas to explain the process of determining longitude at time throughout the museum.
The exhibit thoroughly covers the development of time - from local sun time which was chaotic and proved utilizable with the development of rapid means of transportation, such as the railroad, to the development of atomic clocks, GPS, and beyond.
The exhibits also cover the International Meridian Conference, held in October 1884, which standardized the prime meridian and zero degrees longitude at Greenwich. Forty-one delegates from twenty-five "nations" met in Washington D.C. for the conference. Greenwich was elected as the prime meridian by a vote of twenty-two in favor, one against (Haiti), and two abstentions (France and Brazil). By the time of the conference, the United Kingdom and its colonies as well as the United States of America had already begun using Greenwich as the prime meridian; this weighed heavily on the selection process.
For several centuries, Greenwich served as the home of the Astronomer Royal and his assistants. The position of Astronomer Royal in the United Kingdom has existed uninterrupted since 1675. The observatory was first opened to the public in 1953.
In 2007, a major new facility is being opened at the Royal Observatory. It will include new galleries, a planetarium, and educational centers.