The age of exploration altered the course of human history, but how did early explorers find their way? Calculating their latitude at sea was a relatively straightforward proposition, but calculating longitude presented a vexing problem. Given the big power competition for colonies, conquest, and riches, the search to discover an accurate means of assessing longitude became a scramble. A major piece of the puzzle lay in the accurate assessment of time on a rocking vessel, which tested the ingenuity of the most creative clock makers. With me to discuss the history of the technologies associated with the measurement of longitude is Emily Akkermans. Emily is the Curator of Time at Royal Museums Greenwich, which includes the Royal Observatory Greenwich. The observatory is Britain's oldest purpose-built scientific institution, and it just celebrated its 350th anniversary - which is the impetus for this episode. The observatory is the birthplace of modern astronomy and the home of Greenwich Mean Time and the Prime Meridian.