Two programmes analysing the concept of the passage of time.

9.30 Longitude
Until the eighteenth century, sailors had no way of pinpointing their exact east-west position on the globe. Precise navigation was impossible and ships were often lost at sea. This special edition of Horizon dramatises the story of clockmaker John Harrison, who spent 30 years working on the conundrum.
See today's choices.
See This Week: page 6

Longitude 9.30pm BBC2
Imagine the disbelief if an unknown amateur came up with a solution to the millennium bug. The reaction was much the same in the 18th century when John Harrison, a clock-maker, claimed his ship's timepiece made it possible to navigate accurately.
This fascinating docu-drama tells the story of the search for a way of determining longitude, ultimately met by Harrison's astonishingly accurate clocks. Patrick Malahide plays the pioneering clock-maker and there is vivid footage of ships and the sea. It is a film on an offbeat yet compelling subject, maintaining the standard set by Dava Sobel's recent bestselling book.