Based on Dava Sobel's extraordinary novel, this four part TV production is a magnificent drama.

Out of sight of land, Mariners had no reliable way of telling their exact position in the ocean, so in 1714 Parliament offered a £20,000 prize to anyone who could accurately measure longitude at sea.

This is the true story of Harrison;s (Michael Gambon) forty year struggle to win the prize money by proving that his mechanical clock was the answer, not the stars, as most people in the scientific establishment believed.

It also tells the story of Rupert Gould (Jeremy Irons) who two centuries later tracked down the machines that Harrison built and took on the task of restoring the reputation and condition of the clocks enabling future generations to realise the significance of Harrison's vision.