A best-selling book by Dava Sobel, Longitude tells the story of John Harrison, a humble English carpenter who devoted his entire life to clock-making in an attempt to properly calculate longitude for the first time ever. Drawing on that book, this documentary alternates a narrating voice, interviews and fiction scenes to take us back to XVII century England. When 2,000 skilled sailors of Her Majesty's Navy led by Admiral Shovell died in the Scilly Islands shipwreck, the English Parliament decided to bestow a 20,000 pound reward (appr. 6 million pounds today) on whomever could solve the biggest problem of navigation by developing an accurate method to determine longitude at sea.
Amongst the astronomical methods suggested by astronomers and the unusual and sometimes weird solutions put forth by those allured by the huge reward, Harrison always remained faithful to his original idea – a clock made of top-quality raw materials and equipped with gears that would guarantee its utmost accuracy, capable of withstanding the extreme conditions of navigation: changes in atmospheric pressure, a different gravity at different latitudes, dramatic changes in temperature, plus blows and vibrations. After 25 years of hard work, materials study and gear-making that resulted in three not quite satisfactory prototypes called H1, H2 and H3, Harrison finally realized that a smaller timekeeper was the only possible solution and built H4. Despite the fierce opposition of the most distinguished astronomers of the time led by Nevil Maskelyne (who wanted his "lunar clock" to prevail), the accuracy of H4 and the support of King George III allowed Harrison to finally receive the much-desired prize when he was over seventy years old.