During the 18th century, while the scientific revolution was in full swing and the age of great discoveries was at its peak, the most thorny scientific problem dominating the scientific and political circles of Britain, and beyond, had purely practical significance: it was the calculation of longitude. The inability to measure it had cost thousands of human lives lost at sea for centuries, while posing serious obstacles to the commercial expansion and defensive fortification of the island nation. The leading astronomers of the time tackled this particular problem, but a persistent craftsman, John Harrison, conceived a mechanical solution. Dedicating 40 years of his life, he was able to construct a clock that accurately measured the passage of time at sea, something that the clocks of the time could not achieve even on land. This perfect clock, known today as a chronometer, was the culmination of a heroic effort that faced endless difficulties, doubts, and setbacks, only to finally achieve official recognition and universal acceptance. "Those who were unaware of this unique episode in human history will enjoy a fascinating narrative about an amazing achievement in the fields of time measurement and navigation." Neil Armstrong (2005), the first man to set foot on the Moon (and who prefaces the edition)