"Longitude" follows John Harrison's quest to find the key to determining longitude. In the 18th century, the problem of measuring longitude confounded scientists, sailors and politicians. In 1707, unable to determine their exact location through a thick fog, 2,000 men of the British fleet perished by accidentally running into the rocks off the Scilly Islands. As a result of this tragedy, in 1714 the British Parliament passed the Act of Longitude to offer an enormous cash prize to the person who could solve the problem of longitude. A carpenter by trade, Harrison believes that the solution lies in finding a way to measure time accurately, going against many of the scientists of the day, who feel that the mystery would be solved through celestial navigation. Harrison designs and creates four increasingly refined timekeepers. This work takes him through 30 years of struggle and determination (not only his but his family's, friends', and allies' as well) to solve what was referred to as the "greatest scientific problem of the time." Along the way, he faces the animosity and interference of many of the celestial navigation proponents.

Minister for the Navy:
[to Parliament] Honorable Members who mourn with us the recent tragic loss off the Scilly Isles of four of Her Majesty's ships, and 2,000 wretched souls therein, under the command of Admiral Sir Cloudisley Shovell, will be pleased to know that Her Majesty's government is to offer a reward -- a prize of twenty thousand pounds -- to any man offering a practicable and useful solution to the problem of finding longitude at sea. A Board of Longitude will be set up, whose sole business will be to investigate any serious suggestions, and finally, it is profoundly to be hoped, to award this prize.

Anachronisms: At the outset of William's voyage home with Capt. Bourke, the command, "Raise upper topsails!" can be clearly heard. Split topsails did not come into use until 100 years later.